Hochschulbibilographie

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Du Puits, Ronald;
Thermal boundary layers in turbulent Rayleigh-Bénard convection with rough and smooth plates: a one-to-one comparison. - In: Physical review fluids, ISSN 2469-990X, Bd. 9 (2024), 2, 023501, S. 023501-1-023501-18

Turbulent convection at rough surfaces covers a large variety of heat transfer processes in nature and engineering. However, in particular the transport of heat near the rough surface is still not well understood. We present measurements of the near-wall temperature field in turbulent Rayleigh-Bénard convection with rough walls, and for reference also with smooth walls. The measurements have been undertaken in a large-scale convection experiment, the “Barrel of Ilmenau.” Our experiments covered Rayleigh numbers in a domain 5.4 × 10^9 < Ra < 9.6 × 10^11 and two different aspect ratios Γ = 1.1 and 2.9. The working medium was air with a Prandtl number Pr = 0.7. Using very tiny micro-thermistors of 150 µm diameter and 350 µm length, we conducted highly resolved measurements of the temperature field near the heated bottom plate. Our measurements show that, as was already observed in high Prandtl number fluids, the ratio between the thickness of the thermal boundary layer δth = H/(2 Nu) and the roughness height h plays a crucial role for the near-wall temperature field, and thus for the convective heat transport. If δth/h > 1, the temperature field at the rough surface does not differ from that at the smooth one. If this ratio falls below δth/h = 1, both the mean temperature field and the temperature fluctuations start to change and to differ from that at the smooth surface. These variations are virtually independent of the Rayleigh number in the parameter domain we investigated.



https://doi.org/10.1103/PhysRevFluids.9.023501
Webner, Florian; Shishkin, Andrei; Schmeling, Daniel; Wagner, Claus
A direct infection risk model for CFD predictions and its application to SARS-CoV-2 aircraft cabin transmission. - In: Indoor air, ISSN 1600-0668, Bd. 2024 (2024), e9927275, S. 1-18

Current models to determine the risk of airborne disease infection are typically based on a backward quantification of observed infections, leading to uncertainties, e.g., due to the lack of knowledge whether the index person was a superspreader. In contrast, the present work presents a forward infection risk model that calculates the inhaled dose of infectious virus based on the virus emission rate of an emitter and a prediction of Lagrangian particle trajectories using CFD, taking both the residence time of individual particles and the biodegradation rate into account. The estimation of the dose-response is then based on data from human challenge studies. Considering the available data for SARS-CoV-2 from the literature, it is shown that the model can be used to estimate the risk of infection with SARS-CoV-2 in the cabin of a Do728 single-aisle aircraft. However, the virus emission rate during normal breathing varies between different studies and also by about two orders of magnitude within one and the same study. A sensitivity analysis shows that the uncertainty in the input parameters leads to uncertainty in the prediction of the infection risk, which is between 0 and 12 infections among 70 passengers. This highlights the importance and challenges in terms of superspreaders for risk prediction, which are difficult to capture using standard backward calculations. Further, biological inactivation was found to have no significant impact on the risk of infection for SARS-CoV-2 in the considered aircraft cabin.



https://doi.org/10.1155/2024/9927275
Bahavar, Philipp; Wagner, Claus
Sessile super-droplets as quasi-static wall deformations in direct numerical simulation of turbulent channel flow. - In: Computers & fluids, Bd. 269 (2024), 106135, S. 1-11

Condensation is an important aspect of many flow applications due to the ubiquitous presence of humidity in the air at ambient conditions. For direct numerical simulations of such flows, simulating the gas phase as a mixture characterized by temperature and humidity coupled by the latent heat release and absorption has been shown to yield results consistent with multiphase direct numerical simulations at reduced cost. In the case of surface condensation, the deposition of condensate droplets represents an additional mechanism for flow modification. Extending the single-phase approach by tracking the mean deposition rates and consolidating the condensate mass into static super-droplets reintroduces the effects of surface droplets on the flow while retaining the computational advantages of simulating only the gas phase. Results of simulations of turbulent flow through a cooled, vertical channel with and without such droplets illustrate the additional effects captured compared to the original approach. In the immediate vicinity of a super-droplet, turbulent heat and vapor transport towards the cooled wall is enhanced. Direct impingement and deflection of the flow on the super-droplet cause a qualitative change in the distribution of the condensation rates, increasing on the surface of the super-droplets and decreasing in the surrounding regions. This modification of the near-wall transport leads to increased global cooling and drying efficiency compared to a smooth channel.



https://doi.org/10.1016/j.compfluid.2023.106135
Müller, Max; Ehrenfried, Klaus; Wagner, Claus
Low-order modeling of bistable side forces on a sphere measured for a transient inflow in a wind tunnel. - In: New results in numerical and experimental fluid mechanics XIV, (2024), S. 88-98

The unsteady forces acting on a sphere laterally mounted in a wind tunnel with transient sinusoidal inflow are investigated. The study shows that the low-frequency dynamic is characterized by a spontaneous switching between two partially stable states, also known as bistability. Additionally, oscillatory forces exist, which, on the one hand, are induced by the sinusoidal transient inflow and, on the other hand, originate from natural vortex shedding. By separating the bistability and the oscillatory forces using proper orthogonal decomposition, a low-order model based on the Duffing equation is developed both for the bistability and the oscillatory multi-scale induced forces. The bistability is represented by a chaotic bistable Duffing equation, whose parameters are determined by manual adjustment. In contrast, the parameters of the oscillatory multi-scale forcing are determined by expanding the forcing of the Duffing equation to three terms and preserving the phase angles between the three characteristic frequencies, the flapping frequency, its multiples and the natural vortex shedding.



Batmaz, Ege; Bahavar, Philipp; Schmeling, Daniel; Wagner, Claus
DNS of aerosol particle spreading emitted by coughing and breathing in a simplified room. - In: New results in numerical and experimental fluid mechanics XIV, (2024), S. 509-518

Besides coughing and sneezing, breathing is the most frequent particle emission event of aerosol droplets carrying the SARS-COV-2 virus or viruses of other airborne diseases. Direct Numerical Simulations (DNS) of ‘jet-like’ emissions of particle clouds through the mouth caused by coughing and breathing are performed in a cuboidal simplified room to study the spreading of respiratory droplets with different momentum and size. Contrary to coughing, we found that no droplet follows a ballistic trajectory after a breathing event since all the droplets are trapped in the humid puff of air. The detailed analysis and the comparison of the predictions obtained for respiratory droplets emitted by single breathing and coughing events are further discussed. Despite the major difference between the maximum exhalation speeds reached during coughing and breathing, the horizontal propagation distance differs by less than 30%. Additionally, a comparison of the results of the present DNS neglecting aerosol evaporation and considering buoyancy forces with the results of an earlier DNS study from the literature taking evaporation into account but neglecting buoyancy, revealed that buoyancy damps the horizontal propagation of the humid puff and enhances the upward motion.



https://doi.org/10.1007/978-3-031-40482-5_48
Webner, Florian; Kohl, Andreas; Schmeling, Daniel; Wagner, Claus
Aerosol spread in a generic train entrance: comparison between experiment and numerical simulation. - In: New results in numerical and experimental fluid mechanics XIV, (2024), S. 590-600

The global COVID-19 outbreak in 2020 has made understanding pathogen-laden aerosol transport and the associated transmission routes more relevant than ever. To determine how aerosol particles generated by continuous breathing accumulate in confined spaces, the particle concentrations in a small room resembling a train entrance are investigated. The room is ventilated and equipped with two heated manikins, one of which is continuously exhaling aerosol through the mouth for 30 min. For this setup we conducted local particle measurements in the center plane and a RANS simulation including the prediction of the transient particle transport. It is shown that the particle concentration increases logarithmically and attains a nearly steady state. The resulting local particle concentrations normalized to the source concentrations are subsequently compared. We find good agreement with the experiment in the exhalation zone of the breathing manikin and larger differences for the sensor positions beneath the ventilation inlet.



https://doi.org/10.1007/978-3-031-40482-5_56
Volk, Marie-Christine; Niehaus, Konstantin; Westhoff, Andreas; Wagner, Claus
An approach to automated detection of sessile droplets in mixed convection. - In: New results in numerical and experimental fluid mechanics XIV, (2024), S. 579-589

A method for the measurement of position and size of sessile water droplets is presented. Droplets originate from condensation on a plane vertical surface in a vented cuboidal cavity with a mixed convective flow with humid air as the working fluid. Condensation is observed through a transparent cooling device coated with polyvinyl chloride with an average contact angle of 80.0(3)&ring;. The implemented detection algorithm is based on the circle Hough Transform together with sophisticated pre- and post-processing steps, which are detailed in this work. Validation experiments yield a detection of over 97% of the area covered by droplets by detecting a minimal radius of 13.8 μm. Additionally, first experimental results of droplet size distributions are presented.



https://doi.org/10.1007/978-3-031-40482-5_55
Bahavar, Philipp;
Direct numerical simulation of turbulent channel flow with condensation. - Ilmenau : Universitätsbibliothek, 2023. - 1 Online-Ressource (128 Seiten)
Technische Universität Ilmenau, Dissertation 2023

Diese Arbeit präsentiert direkte numerische Simulationen von turbulenter Strömung feuchter Luft durch einen gekühlten, vertikalen Kanal. Die Kombination von Feuchtigkeit, Temperatur und Mischkonvektion tritt in der Belüftung von Fahrgasträumen auf. In dieser Anwendung stellt unerwünschte Kondensation an kühlen Oberflächen wie Fenstern und Windschutzscheibe ein Problem dar, das die Nutzung des Fahrzeugs kurz- oder langfristig beeinträchtigt. Die Wechselwirkung zwischen Auftrieb, Konvektion und Phasenübergängen berührt gleichermaßen die Thermodynamik und die Fluidmechanik. Für die Rahmenbedingungen, die für die Belüftung in Automobilen relevant sind, kann die Strömung von flüssigem Wasser vernachlässigt werden. Die direkte numerische Simulation betrachtet deshalb nur die Gasphase und modelliert den Einfluss des Phasenübergangs nur im Hinblick auf die feuchte Luft. Flüssiges Wasser wird entweder komplett vernachlässigt oder als Kondensattropfen nachempfundene Wandverformung behandelt. Mithilfe von Simulationen mit und ohne Phasenübergang und mit und ohne Wandverformung wird der Einfluss der unterschiedlichen Faktoren voneinander getrennt untersucht. Die entgegengesetzte Wirkung von Auftrieb, der direkt aus dem Abkühlen und Trocknen der feuchten Luft an der Wand resultiert, und dem Auftrieb, der durch die freiwerdende Kondensationswärme zustande kommt, dämpft den Einfluss der gekühlten Wand auf die Strömung im Vergleich zu gekühlter Kanalströmung ohne Kondensation. In den Simulationen mit Wandverformung durch angelagertes Kondensat verursachen diese Verformungen einen positiven Feedback-Loop, der die Kondensationsraten an der Oberfläche von bereits existierenden Tropfen verstärkt.



https://doi.org/10.22032/dbt.57712
Lange, Sven;
Entwicklung von Maßnahmen zur Reduktion des aerodynamischen Kühlluftwiderstandes eines Personenkraftwagens mittels numerischer Strömungssimulation und Windkanaluntersuchungen. - Ilmenau : Universitätsbibliothek, 2023. - 1 Online-Ressource (xi, 191 Seiten)
Technische Universität Ilmenau, Dissertation 2023

In der vorliegenden Arbeit werden die Auswirkungen der Motorkühlluft auf den aerodynamischen Widerstand von Kraftfahrzeugen am Beispiel eines Golf 7 analysiert. Hierzu werden Untersuchungen im Windkanal der Volkswagen AG sowie numerische Strömungssimulationen basierend auf der Lattice-Boltzmann-Methode (LBM) durchgeführt. Im Vorfeld ist es erforderlich, das eingesetzte Simulationsprogramm (PowerFlow) sowie das Simulations-Set-up dahingehend zu überprüfen und zu ertüchtigen, dass es die im Windkanal gemessenen Werte mit der geforderten Genauigkeit wiedergibt. Zum Abgleich der Simulationsergebnisse werden neben Druck- und Kühlluftmassenstrommessungen auch umfangreiche Untersuchungen mittels Particle Image Velocimetry (PIV) durchgeführt. Neben den Windkanalversuchen mit dem Versuchsfahrzeug wird auch das Strömungsfeld der leeren Messstrecke mittels PIV untersucht. Des Weiteren wird der Vorderwagen inklusive Motorraum eingescannt und mit dem Simulationsmodell verglichen, um die Übereinstimmung der Simulationsergebnisse mit den Messwerten aus den Windkanalversuchen zu verbessern. Auf Grundlage von Mess- und Scandaten des Fahrzeugs wird das Simulations-Set-up so angepasst, dass die geforderte Übereinstimmung der experimentellen und numerischen Ergebnisse gegeben ist. Durch die Analyse der Fahrzeugströmung kann bei verschlossenen Kühlluftöffnungen ein Rückströmungseffekt ermittelt werden, der die Bestimmung des Kühlluftwiderstandes basierend auf der Differenz der Luftwiderstände mit und ohne Kühlluft infrage stellt. Im Weiteren werden die Hauptströmungspfade der Kühlluft lokalisiert und eine theoretische Betrachtung der Kühlluftauswirkungen durchgeführt, um zu bestimmen, an welchen Stellen der Kühlluftwiderstand beeinflusst werden kann. Resultierend daraus werden die drei Bereiche Kühllufteinlass, Kühlluftauslass in die Radhäuser und in den Unterboden identifiziert, in denen eine positive Beeinflussung des Kühlluftwiderstandes möglich ist. Als Ergebnis dieser Analysen wird eine leckagefreie Kühlluftführung mit reduzierten Kühllufteinlassflächen umgesetzt. Des Weiteren wird die Kühlluft über neu entwickelte Luftauslässe im Radhaus und Unterboden der Umströmung zugeführt. Mit diesen Maßnahmen kann im Windkanalversuch der cW-Wert des Golf 7 in Summe um ΔcW = 0,025 und der Kühlluftwiderstand um 60 % gesenkt werden.



https://doi.org/10.22032/dbt.57493
Mommert, Michael; Niehaus, Konstantin; Schiepel, Daniel; Schmeling, Daniel; Wagner, Claus
Measurement of the turbulent heat fluxes in mixed convection using combined stereoscopic PIV and PIT. - In: Experiments in fluids, ISSN 1432-1114, Bd. 64 (2023), 6, 111, S. 1-13

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.



https://doi.org/10.1007/s00348-023-03645-4
Schmeling, Daniel; Shishkin, Andrei; Schiepel, Daniel; Wagner, Claus
Numerical and experimental study of aerosol dispersion in the Do728 aircraft cabin. - In: CEAS Aeronautical Journal, ISSN 1869-5590, Bd. 14 (2023), 2, S. 509-526

The dispersion of aerosols originating from one source, the 'index' passenger, within the cabin of the aircraft Do728 is studied experimentally using an aerosol-exhaling thermal manikin and in Reynolds-averaged Navier-Stokes simulations (RANS). The overall aim of the present study is the experimental determination of the aerosol spreading for the state-of-the-art mixing ventilation (MV) and to evaluate the potential of alternative ventilation concepts for controlling the aerosol spreading in RANS. For MV, the experiments showed that the ratio of inhaled to exhaled aerosol particles drops below 0.06% (volume ratio) for distances larger than two seat rows from the source. However, within a single row, the observed ratio is higher. Further, the dispersion is much weaker for a standing than for a seated index passenger. High air exchange rates and a well-guided flow prevent a dispersion of the aerosols in high concentrations over larger distances. Additionally, the positive effect of a mask and an increased air flow rate, and especially their combination are shown. In the complementary conducted RANS, the advantages of floor-based cabin displacement ventilation (CDV) which is alternative ventilation concept to MV, regarding spreading lengths and the dwell time of the aerosols in the cabin were determined. The obtained results also underline the importance of the flow field for the aerosol dispersion. Further, additional unsteady RANS (URANS) simulations of the short-term process of the initial aerosol cloud formation highlighted that the momentum decay of the breathing and the evaporation processes take place within a few seconds only.



https://doi.org/10.1007/s13272-023-00644-3
Niehaus, Konstantin; Westhoff, Andreas; Wagner, Claus
A semi-empirical model for the prediction of heat and mass transfer of humid air in a vented cavity. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 205 (2023), 123926, S. 1-14

A semi-empirical model to predict the mass transfer rate of water from humid air in mixed convection together with the global heat transfer in a novel experimental set-up is presented. The cuboidal sample consists of isothermally cooled and heated plates with ventilation channels driving a mixed convective flow with inlet channel Reynolds numbers between 210 and 1270, Grashof numbers up to 8.46 ×10^7, and with relative humidities from 29% to 83% (at 25 &ring;C). The volumetric velocity field was measured by means of tomographic particle image velocimetry together with the fluid temperature and humidity. The measurement results are used to develop a one-dimensional model to predict the global heat and mass transfer by quantifying the dependency of the Nusselt and Sherwood number on the experimental boundary conditions. A relative deviation between the measurement results and the model prediction below 1% for the sensible heat transfer is reported, while the prediction of the vapor-mass transfer rate exhibits an average relative deviation below 6%.



https://doi.org/10.1016/j.ijheatmasstransfer.2023.123926
Konstantinov, Mikhail; Schmeling, Daniel; Wagner, Claus
Numerical simulation of the aerosol formation and spreading in an air-conditioned train compartment. - In: Journal of aerosol science, ISSN 1879-1964, Bd. 170 (2023), 106139, S. 1-19

This paper presents results of the unsteady aerosol formation and transmission process in a train compartment under ventilation conditions obtained by Computational Fluid Dynamics (CFD) methods. The latter include various models to simulate unsteady flows including the transient behaviour of a two-phase atomization process and thermal air flow. The obtained aerosol distributions predicted for the four cases coughing, speaking and breathing (with and without mask) in a ventilated cabin compartment are discussed analysing the dispersion of the exhaled droplets for a double cough, 10s of speaking and continuous breathing of one source passenger. The results show that the aerosol particles propagate two times deeper in the cabin for the coughing scenario than for speaking, 2.5 times deeper than for free breathing and 17 times deeper than for breathing with a mask. Further, the results reveal that 2min after the end of the coughing, only about 6% of active aerosol particles remain in the compartment.



https://doi.org/10.1016/j.jaerosci.2023.106139
Kästner, Christian; Schneider, Julien David; Du Puits, Ronald
Evolution and features of dust devil-like vortices in turbulent Rayleigh-Bénard convection - an experimental study. - In: JGR, ISSN 2169-8996, Bd. 128 (2023), 2, e2022JD037466, S. 1-20

We present an experimental study simulating atmospheric dust devils in a controlled laboratory experiment. The experimental facility, called the “Barrel of Ilmenau” (www.ilmenauer-fass.de) represents a classical Rayleigh-Bénard set-up and is believed to model the phenomena in a convective atmospheric boundary layer fairly well. Our work complements and extends the numerical work of Giersch and Raasch (2021) https//doi.org/10.1029/2020jd034334 by experiments. Dust devils are thermal convective vortices with a vertical axis of rotation visualized by entrained soil particles. They evolve in the convective atmospheric boundary layer and are believed to substantially contribute to the aerosol transport into the atmosphere. Thus, their evolution, size, lifetime, and frequency of occurrence are of particular research interest. Extensive experimental studies have been conducted by field measurements and laboratory experiments so far. Beyond that, our study is the first attempt of Rayleigh-Bénard convection (RBC) in air to investigate dust devil-like vortices in a laboratory experiment. Up to now, this set-up mimics the natural process of dust devil evolution as closest to reality. The flow measurement was carried out by particle tracking velocimetry using neutrally buoyant soap bubbles. We initially identified dust devil-like vortices by eye from the Lagrangian velocity field, and in a later, more sophisticated analysis by a specific algorithm from the corresponding Eulerian velocity field. We analyzed their frequency of occurrence, observation time, and size. With our work, we could demonstrate that turbulent RBC is an appropriate model to mimic the natural process of the evolution of dust devils in the convective atmospheric boundary layer without artificial stimulation.



https://doi.org/10.1029/2022JD037466
Siegel, Lars;
Aeroakustische Untersuchungen an stationären Stäben und einem oszillierenden Flügel mittels synchroner Particle-Image Velocimetry und Mikrofonmessungen. - Ilmenau : Universitätsbibliothek, 2022. - 1 Online-Ressource (x, 121 Seiten)
Technische Universität Ilmenau, Dissertation 2022

Die kombinierte, experimentelle Erfassung der Strukturen in einer Strömung und der dadurch verursachten Schallabstrahlung ist Gegenstand der vorliegenden Arbeit. Als Messmethoden kamen einerseits die Particle Image Velocimetry (PIV), mit Hilfe derer Schwankungsgrößen im Strömungsfeld innerhalb einer Lichtschnittebene aufgezeichnet werden, und andererseits Mikrofone, die die Druckfluktuationen im akustischen Fernfeld erfassen, zum Einsatz. Durch die synchrone Erfassung lässt sich die Kreuzkorrelation zwischen diesen beiden Größen berechnen, welche dabei Einblicke in den Mechanismus der Schallentstehung und -ausbreitung sowie den Zusammenhang mit den auftretenden Strömungsstrukturen liefert. Der Schwerpunkt der vorliegenden Arbeit liegt in der Erweiterung des Erkenntnisraums dieses experimentellen Verfahrens hinsichtlich zweier Aspekte. Zum einen wird untersucht, inwieweit die Korrelationsergebnisse und davon abgeleitete Größen verwendet werden können, um Rückschlüsse auf die Region der Schallquellen zu schließen, das Ausbreitungsverhalten der Schallschnelle von der Quellregion bis ins Fernfeld nachzuverfolgen und die Verteilung der akustischen Intensität zu quantifizieren. Dafür wurden Experimente an umströmten Stäben in einem aeroakustischen Windkanal durchgeführt, wobei das Beobachtungsfenster der PIV-Messungen sequentiell vom Nahfeld der Stäbe bis ins akustische Fernfeld traversiert werden konnte. Um die erzielten Kreuzkorrelationsergebnisse abzugleichen und die skalierte Kreuzkorrelations-funktion als Ersatzgröße der Schallschnelle zu validieren, wurden analytische Modelle verwendet. Es konnte gezeigt werden, dass in denjenigen Strömungsbereichen, in denen fluiddynamische Prozesse eine untergeordnete Rolle spielen, die skalierte Kreuzkorrelationsfunktion tatsächlich das Ausbreitungsverhalten der Schallschnelle widerspiegelt. In Bereichen mit starken turbulenten Teilchenbewegungen dominieren hingegen die fluiddynamischen Schwankungen um teils mehrere Größenordnungen, so dass eine klare Trennung der rein akustischen Prozesse nicht möglich ist. Jedoch konnte eine eindeutige Verknüpfung der kohärenten, periodisch ablösenden Strömungsstrukturen im Nachlauf der Stäbe mit der Schallemission und dem Transport bzw. der Konvektion der akustischen Informationen sowohl experimentell als auch im Vergleich mit den analytischen Modellen nachgewiesen werden. Mittels einer generalisierten Intensitätsanalyse auf Basis der skalierten Kreuzkorrelationsfunktion konnte darüber hinaus die Quellregion der akustischen Emissionen identifiziert werden. Zum anderen wird untersucht, welchen Einfluss der zusätzliche Freiheitsgrad der periodischen Bewegung eines oszillierenden Tragflügels in einer Strömung auf die Kreuzkorrelationsergebnisse hat. Mit diesem Experiment sollte geklärt werden, ob man mit der synchronen Messtechnik in der Lage ist, trotz sich permanent ändernden Strömungszuständen kohärente Strömungsstrukturen zu identifizieren, die mit der Schallentstehung und -ausbreitung zusammenhängen. Hierbei konnte gezeigt werden, dass zu unterschiedliche Flügelstellungen bzw. Phasenwinkeln charakteristische Strömungsstrukturen auftreten, die verschiedenen Schallquellmechanismen zugeordnet werden können. Im Verlauf des Zyklusses des dynamischen Strömungsabrisses kommt es sowohl zu periodischen Wirbelablösungen an der Flügelhinterkante als auch zu großflächigen Ablösungen entlang der gesamten Profilsehne des Flügels, welche jeweils mit charakteristischen Eigengeräuschmechanismen im Zusammenhang stehen. Darüber hinaus konnten durch Variation der Konfigurationsparameter auffällige Veränderungen in der Geräuschemission identifizieren und quantifizieren werden.



https://doi.org/10.22032/dbt.55499
Müller, Max; Ehrenfried, Klaus; Wagner, Claus
Druckkraftantwort auf eine Kugel bei instationärer Anströmung mit sinusartigem Seitenwind. - In: Experimentelle Strömungsmechanik - 29. Fachtagung, 6.-8. September 2022, Ilmenau, (2022), 23

Mommert, Michael;
Untersuchung der Zirkulationsbewegung und des Wärmetransports in turbulenter Mischkonvektion mittels optischer Messverfahren. - Ilmenau : Universitätsverlag Ilmenau, 2022. - 1 Online-Ressource (xxi, 188 Seiten)
Technische Universität Ilmenau, Dissertation 2022

Diese Arbeit befasst sich mit dem dynamischen Verhalten von Strömungsstrukturen in turbulenter Mischkonvektion. Zu deren Untersuchung wurden Experimente in einer quaderförmigen Rayleigh-Bénard-Zelle, welche durch Luftein- und -auslässe an der oberen und unteren langen Kante auf einer Seite der Zelle erweitert wurde, durchgeführt. Um die Mechanik hinter den verschiedenen Dynamiken der Strukturen zu verstehen, wurden die Geschwindigkeitsfelder für zwei Typen unterschiedlichen dynamischen Verhaltens mit der tomographischen Particle Image Velocimetry (PIV) für den linken Teil der Zelle erfasst. Da das dynamische Verhalten, welches mit niedrigeren Einströmgeschwindigkeiten verbunden ist, selten und nicht periodisch auftrat, wurde eine automatisierte Methode entwickelt, um die Messungen mit dem Eintreten der Dynamik zu starten. Die Analyse der Geschwindigkeitsfelder und ihre Zerlegung in orthogonale Moden ergaben, dass unterschiedliche Mechanismen das dynamische Verhalten der beiden Fälle antreiben: Für den niedrigen Luftdurchsatz dominieren thermische Prozesse und das dynamische Verhalten hängt mit einer Akkumulation warmer Luft innerhalb des vorliegenden Wirbelsystems zusammen, die in unregelmäßigen Abständen zur Freisetzung von Plumes führt. Im Gegensatz dazu dominieren bei höheren Luftdurchsätzen trägheitsgetriebene Prozesse. Insbesondere werden die dynamischen Prozesse in diesem Fall durch Taylor-Görtler-artige Wirbel angetrieben. Der initiale Widerspruch des für beide Fälle beobachteten Umklappens der Rollenstrukturen mit dem Translationsmechanismus aus früheren Untersuchungen wurde durch stereoskopische PIV-Messungen über die gesamte Zelllänge aufgelöst: Sie zeigen, dass der Klappmechanismus primär das Verhalten der Strömung im Seitenwandbereich beschreibt. Um den Beitrag Taylor-Görtler-artiger Wirbel zum Wärmetransport zu untersuchen, wurde die Methode der kombinierten stereoskopischen Particle Image Velocimetry und Thermometrie angewandt, um die Berechnung messebenennormaler Wärmeströme zu ermöglichen. Die Ergebnisse daraus zeigen, dass die Temperatur und die vertikale Geschwindigkeitskomponente korreliert sind, obwohl die Taylor-Görtler-artigen Wirbel durch Trägheitskräfte erzeugt werden. Weitere Analysen zeigen, dass der statistische Fußabdruck dieser Strukturen es erlaubt, sie von z.B. Plumes an der Frontwand zu unterscheiden.



https://doi.org/10.22032/dbt.52357
Wagner, Claus; Wetzel, Tim
Coherent structures in turbulent mixed convection flows through channels with differentially heated walls. - In: GAMM-Mitteilungen, ISSN 1522-2608, Bd. 45 (2022), 2, e202200006, S. 1-18

The occurrence and shape of turbulent structures in mixed convection flows through a differently heated vertical channel are investigated in terms of thermally induced attenuation and amplification of turbulent velocity, pressure, and temperature fluctuations using direct numerical simulations. It is shown that the wall-normal momentum transport is decreased and increased near the heated and cooled wall, respectively, and that this leads to a reduced and elevated production of turbulent velocity fluctuations in the streamwise velocity component in the aiding and opposing flow, respectively. The corresponding flow structures are smoother, faster and warmer in the aiding flow and aligned along the main flow, while the colder structures in the opposing flow are more frayed and less directed. The warmer flow structures in the aiding flow are overall more stable than the colder structures in the opposing flow. Besides, the study reveals that the position of the maximum temperature fluctuations moves toward the heated wall, so that the sweeps produced at the two walls are affected differently by the former. As a consequence, the distance and time period over which the fluctuations develop in the aiding flow are shorter than in the opposing flow. It is further shown that vortex structures oriented in the streamwise direction usually arise with an offset to the right or left above a sweep or an ejection, whereby the decreasing values of the correlation coefficients with increasing Grashof number indicate a weakening of the vortex structures. Since none of the evaluated vortex criteria, that is, the distributions of the vorticity, λ2- value or Rortex-value correlate well with the evaluated minima of the pressure fluctuations, they do not allow a clear identification of the vortex structures. Finally, analyzing the budget of the turbulent kinetic energy it is confirmed that the velocity fluctuations are only indirectly influenced by the buoyancy force. Thus, the attenuation and amplification of the turbulent velocity fluctuations is reflected in the reduction and exaggeration of the Reynolds shear stresses in the aiding and opposing flow, respectively.



https://doi.org/10.1002/gamm.202200006
Du Puits, Ronald;
Time-resolved measurements of the local wall heat flux in turbulent Rayleigh-Bénard convection. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 188 (2022), 122649, S. 1-6

We report direct and time-resolved measurements of the local wall heat flux in turbulent Rayleigh-Bénard convection in air. The measurements have been performed in a cylindrical test section with a diameter of 7.1 m and a diameter-to-height ratio of 8. They cover Rayleigh numbers 8.5×10^5<Ra<2.6×10^9, while the Prandtl number was fixed at Pr=0.7. In order to measure the local wall heat flux, we use heat flux plates that have been flush mounted into the hot/cold surfaces of the convection experiment. The results of our measurements show that the local wall heat flux in turbulent Rayleigh-Bénard convection strongly fluctuates. With increasing Rayleigh number, the variance of the fluctuations goes down with Ra^-0.5 and the probability of large excursions from the mean decreases. The probability density of the fluctuations can be well-described by a distribution according to the Generalized Extreme Value Theory. We also analysed the typical time scales, and we found that the power of the fluctuations becomes more weighted towards higher frequencies, if the Rayleigh number increases.



https://doi.org/10.1016/j.ijheatmasstransfer.2022.122649
Bauer, Christian; Schiepel, Daniel; Wagner, Claus
Assimilation and extension of particle image velocimetry data of turbulent Rayleigh-Bénard convection using direct numerical simulations. - In: Experiments in fluids, ISSN 1432-1114, Bd. 63 (2022), 1, 22, S. 1-17

https://doi.org/10.1007/s00348-021-03369-3
Schmeling, Daniel; Shishkin, Andrey; Schiepel, Daniel; Wagner, Claus
Aerosolausbreitung in der DO-728-Passagierkabine. - In: Luft- und Raumfahrt, ISSN 0173-6264, Bd. 42 (2021), 2, S. 20-23

Am Deutschen Zentrum für Luft- und Raumfahrt in Göttingen wird am Institut für Aerodynamik und Strömungstechnik mit modernsten numerischen und experimentellen Methoden die Ausbreitung von Aerosolen in Flugzeugkabinen ausgehend von einer ausgewählten Quellposition untersucht. Ziele der vorliegenden Studie sind einerseits die experimentelle Bestimmung des Aerosoltransports, um die Grundlagen für die weitere Analyse möglicher Ansteckungen für Mediziner und Virologen zu legen, und andererseits durch numerische Studien das Potential alternativer Belüftungskonzepte aufzuzeigen. Mithilfe eines Aerosolausatmenden thermischen Menschmodells wurde experimentell die Ausbreitung von Aerosolen in dessen naher Umgebung bei klassischer Mischlüftung, d.h. die frische Luft wird im Deckenbereich zugeführt, nachgewiesen. Bereits in einer Entfernung von zwei Reihen beträgt die gemittelte Aerosolkonzentration weniger als 0.1 %. Innerhalb einer Reihe und insbesondere auf einer Gangseite findet dagegen ein erhöhter Aerosoltransport statt, was auch in den numerischen Strömungssimulationen bestätigt wird. Hohe Luftwechselraten und eine gerichtete Strömung verhindern, dass eine Ausbreitung der Aerosole über größere Entfernungen hinweg stattfindet. Wie in [5] dargestellt ist, reicht das derzeitige etablierte wissenschaftliche Verständnis der SARS-CoV-2-Übertragungsdynamik nicht aus, um aus den im Folgenden diskutierten Messungen des Aerosoltransports ein definitives SARS-CoV-2-Übertragungsrisiko zu berechnen. In den numerischen Studien wird nachgewiesen, dass Änderungen in den durch das Belüftungssystem induzierten Luftströmen in der Passagierkabine einen großen Einfluss auf die Aerosolverteilung haben. Es wird gezeigt, dass eine Belüftung nach dem Prinzip der bodenseitigen Verdrängungslüftung sowohl die räumliche Ausbreitung der Aerosole reduziert als auch die Verunreinigungsausfuhreffizienz deutlich steigern kann: nach 90 s sind nur noch 16 % der Partikel in der Kabinenluft.



Bell, James R.; Wilhelmi, Henning; Heine, Daniela; Jessing, Christoph; Wagner, Andreas; Wiedemann, Jochen; Wagner, Claus
Aerodynamic characterization of a full-scale compact car exposed to transient crosswind. - In: SAE International journal of passenger cars, ISSN 1946-4002, Bd. 14 (2021), 1, 06-14-01-0001P1-18

The transient surface pressure over a full-scale, operational compact automotive vehicle - a Volkswagen Golf 7 - exposed to transient crosswinds with relative yaw angles of β = 22-45&ring; has been characterized. Experiments were performed at the BMW side-wind facility in Aschheim, Germany. Measurements of the incoming flow in front of the car were taken with eleven five-hole dynamic pressure probes, and separately, time-resolved surface pressure measurements at 188 locations were performed. Unsteady characteristics (not able to be identified in quasi-steady modelling) have been identified: the flow in separated regions on the vehicle's leeward side takes longer to develop than at the windward side, and spatially, the vehicle experiences local crosswind as it gradually enters the crosswind. The total effects of these localized unsteady pressure characteristics have also been predicted to affect the vehicle globally, with the evaluation of the transient forces and moments - through pressure integration - exhibiting unsteady characteristics of overshoots, undershoots, and temporal lag; relative to steady crosswind. These results provide new insight into both the characteristics and the causes of the unsteady crosswind response of a real, operational vehicle operating in real-world conditions. This insight can be used in the design of future vehicles to make them less sensitive to unsteady gusts. Further these results can be used to develop and validate numerical and reduced-scale experimental methods that can test and assess new vehicle geometry designs during the early stages of vehicle development.



https://doi.org/10.4271/06-14-01-0001
Weinman, Keith A.; Wilhelmi, Henning; Bell, James R.; Heine, Daniela; Wagner, Claus
On the simulation of a heavy vehicle wake in OpenFOAM with real-world data. - In: New results in numerical and experimental fluid mechanics XIII, (2021), S. 504-513

Two computational approaches are taken to characterize the drag of a car driven behind a heavy vehicle under real conditions. The on-road approach uses velocity measurements, obtained from an array of static five-hole probes, to construct on-flow boundary conditions replicating the atmospheric dynamics encountered during an on-road measurement. The wind tunnel approach uses an oscillating flap system to control flow time and length scales upstream of a wind tunnel model. The amplitude and frequency of the flap motion is calibrated to reproduce length and time scales at on-road conditions under respective Reynolds and Strouhal number scaling. These approaches are evaluated against experimental measurements using Computational Fluid Dynamics (CFD) and demonstrates that they reproduce the aerodynamic drag and a significant part of the measured onflow condition for the car.



Niehaus, Konstantin; Westhoff, Andreas; Wagner, Claus
Characterization of a mixed convection cell designed for phase transition studies in moist air. - In: New results in numerical and experimental fluid mechanics XIII, (2021), S. 483-493

A detailed description of an experimental set-up designed for upcoming investigations of latent and sensible heat transfer in a cuboid sample with air in- and outlets is given. The container is heated at the rear and cooled at the transparent front wall. Temperature measurements reveal that both sides exhibit a mean temperature deviation below 2% relative to the temperature difference between the mean plate temperature and the ambient temperature. This is a suitable temperature distribution for such measurements. Tomographic particle image velocimetry covering the entire volume of the mixed convection cell exhibits a large-scale circulation due to forced convection with a buoyancy-induced flow close to the temperature controlled surfaces. Forced convection origins from a flow between the inlet and the outlet channel with a mean deviation of 1% from the mean velocity and a maximum absolute deviation of 0.04 m/s. Measurements were performed for Reynolds numbers ranging from 300<Re<2000 and Grashof numbers Gr<1.2×10^8. A representative flow field obtained at Re=620 and Gr=1.1×10^8 is presented as an example.



Brückner, Christian; Westhoff, Andreas; Wagner, Claus
Modelling of heat transfer for droplet condensation in mixed convective duct flow. - In: New results in numerical and experimental fluid mechanics XIII, (2021), S. 461-471

Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with condensation. The latent and total heat transfer during the experiment are determined through the thermal balance for inlet temperatures from 27.5 &ring;C to 35.5 &ring;C, relative humidities from 30% to 55% and at four Reynolds numbers (2000-8000). The experimental results are compared with a heat transfer model from the literature. Adjusted in terms of the geometry and surface properties, the model shows partial agreement for the cases with forced convection but has to be further adjusted regarding the influence of thermal convection.



Bauer, Christian; Müller, Max; Ehrenfried, Klaus; Wagner, Claus
Bi-stability detection in the flow around a sphere by means of experiments and lattice Boltzmann simulations. - In: New results in numerical and experimental fluid mechanics XIII, (2021), S. 441-450

With the aim to detect bi-stability in the highly separated flow around a sphere mounted on a cross-stream rod, wind tunnel measurements and numerical simulations with a Lattice Boltzmann method (LBM) are performed and compared. In both, LBM simulation and experiment, a bi-stable flow behaviour is detected at a Reynolds number of Re=10^5 based on the sphere diameter, the inflow velocity and the kinematic viscosity, whereas such behaviour is absent at Reynolds numbers as high as Re=3×10^5. The bi-stable behaviour detected in the time series of the aerodynamic side force acting on the sphere for Re=10^5 is reflected by a bi-modal probability density function. For Re=3×10^5, on the contrary, the probability density function is nearly Gaussian. Further, in agreement with the experiment, two counter-rotating vortex tubes originating from the intersection of the leeward and cross-stream rod-ward side of the sphere are generated.



Wilhelmi, Henning; Jessing, Christoph; Bell, James R.; Heine, Daniela; Wiedemann, Joachim; Wagner, Andreas; Wagner, Claus
Aerodynamic characterisation of a compact car driving behind a heavy vehicle. - In: New results in numerical and experimental fluid mechanics XIII, (2021), S. 514-524

The transient incoming flow a compact car experiences whilst driving 10 m - 100 m behind a heavy vehicle on a runway has been characterised. The incoming flow was measured using a 2D array of 11 five-hole probes mounted 1 m in front of an operational, full-scale compact car. Additionally, 188 surface pressure taps were used to measure the effect of the incoming flow conditions on the compact car. The experiments were performed under ideal conditions on a 2.9 km long runway in Faßberg near the DLR in Trauen, Germany.



Schiepel, Daniel; Herzog, Sebastian; Wagner, Claus
Time-resolved high-density particle tracking velocimetry of turbulent Rayleigh-Bénard convection in a cubic sample. - In: Turbulence and interactions, (2021), S. 176-182

A newly developed high-density particle tracking velocimetry (HD-PTV) technique is introduced and validated on a synthetic data set. Further, Lagrangian velocities in turbulent Rayleigh-Bénard Convection (RBC) are determined based on particle images measured in a cubic sample filled with water with a Prandtl number Pr=6.9 and a Rayleigh number of Ra=1.0⋅10^10. It is shown that the new technique allows to resolve not only a three-dimensional (3D) large-scale circulation in a diagonal plane of the sample but also the secondary flow structures developing in the corners of the perpendicular plane.



Bauer, Christian; Wagner, Claus
Effect of very-large-scale motions on one- and two-point statistics in turbulent pipe flow investigated by direct numerical simulations. - In: Turbulence and interactions, (2021), S. 117-122

Very-large-scale motions appear in the bulk region of turbulent pipe flow. They become increasingly energetic with the Reynolds number and interact with the near-wall turbulence. These structures appear either in the shape of positive (high-speed) or negative (low-speed) streamwise velocity fluctuation. The impact of the sign of the structures on the pipe flow turbulence is analysed in this study by means of conditionally averaged one- and two-point statistics, using data from direct numerical simulations of turbulent pipe flow in a flow domain of length L=42R and friction Reynolds numbers of 180≤Reτ≤1500. Conditionally averaged two-point velocity correlations reveal that low-speed motions are longer and more energetic than their high-speed counterparts. The latter are predominately responsible for the Reynolds number dependency of turbulence statistics in the vicinity of the wall, which is in good agreement with observations of the so-called amplitude modulation in wall-bounded turbulence.



Köthe, Thomas;
Optimierung von Luftauslass- und Kanalgeometrien mit adjungierten Verfahren. - Ilmenau : Universitätsbibliothek, 2021. - 1 Online-Ressource (xxi, 166 Seiten)
Technische Universität Ilmenau, Dissertation 2021

Die Auslegung von Belüftungssystemen in Flugzeugen, Zügen und Autos ist in erster Linie von der Erfahrung und dem Fachwissen der Entwicklungsingenieure sowie intensiven Tests abhängig. Mit numerischen Optimierungsverfahren ist es jedoch möglich, die Systeme noch weiter zu verbessern. In der vorliegenden Arbeit werde ich eine Prozesskette entwickeln, die mit Hilfe der adjungierten Formoptimierung auf Grundlage numerischer Strömungsmechanik den Kabinenluftauslass des Dornier 728 Flugzeugs optimiert. Dabei soll in der Netzverformung eine hohe Zahl an Freiheitsgraden verwendet werden, die durch die adjungierte Formoptimierung zur Verfügung steht. Zusätzlich wird die vorgestellte Prozesskette auf ein vektrowertiges Optimierungsproblem angewendet, wobei die Pareto-Front zweier Zielfunktionen für den Kabinenluftauslass bestimmt wird. Laut Stand der Technik wird für solche Problemstellungen die Methode der gewichteten Summe angewendet. Diese bedeutet zugleich einen enormen Rechenaufwand, wenn man komplexe Geometrien untersucht. Um diesen Aufwand zu reduzieren, präsentiere ich ein Verfahren, bei dem die Sensitivitätsfelder aus skalarwertigen Optimierungsproblemen kombiniert werden. Die Variation dieser Skalierungsfaktoren erfolgt mit einem genetischen Algorithmus. Im nächsten Schritt wird das Problem noch einmal umformuliert und die Variation der Wichtungsfaktoren wird mit einem E-Schranken-Verfahren untersucht. Im Vergleich zur Kombination der Zielfunktionen liefert das präsentierte Verfahren bei gleichbleibender Rechenzeit eine Verbesserung der Zielfunktion von ca. 70% bzw. ca. 40% Verbesserung bei einer Einsparung von 50% der Rechenzeit. Zusätzlich zeigt auch die bestimmte Pareto-Front geringere Werte für die untersuchten Zielfunktionen.



https://nbn-resolving.org/urn:nbn:de:gbv:ilm1-2021000301
Herzog, Sebastian; Schiepel, Daniel; Guido, Isabella; Barta, Robin; Wagner, Claus
A probabilistic particle tracking framework for guided and Brownian motion systems with high particle densities. - In: SN Computer Science, ISSN 2661-8907, Bd. 2 (2021), 6, 485, insges. 20 S.

This paper presents a new framework for particle tracking based on a Gaussian Mixture Model (GMM). It is an extension of the state-of-the-art iterative reconstruction of individual particles by a continuous modeling of the particle trajectories considering the position and velocity as coupled quantities. The proposed approach includes an initialization and a processing step. In the first step, the velocities at the initial points are determined after iterative reconstruction of individual particles of the first four images to be able to generate the tracks between these initial points. From there on, the tracks are extended in the processing step by searching for and including new points obtained from consecutive images based on continuous modeling of the particle trajectories with a Gaussian Mixture Model. The presented tracking procedure allows to extend existing trajectories interactively with low computing effort and to store them in a compact representation using little memory space. To demonstrate the performance and the functionality of this new particle tracking approach, it is successfully applied to a synthetic turbulent pipe flow, to the problem of observing particles corresponding to a Brownian motion (e.g., motion of cells), as well as to problems where the motion is guided by boundary forces, e.g., in the case of particle tracking velocimetry of turbulent Rayleigh-Bénard convection.



https://doi.org/10.1007/s42979-021-00879-z
Schiepel, Daniel; Schmeling, Daniel; Wagner, Claus
Simultaneous tomographic particle image velocimetry and thermometry of turbulent Rayleigh-Bénard convection. - In: Measurement science and technology, ISSN 1361-6501, Bd. 32 (2021), 9, 095201, insges. 17 S.

https://doi.org/10.1088/1361-6501/abf095
Dillmann, Andreas; Heller, Gerd; Krämer, Ewald; Wagner, Claus
New results in numerical and experimental fluid mechanics XIII : contributions to the 22nd STAB/DGLR Symposium. - Cham, Switzerland : Springer, 2021. - xiv, 787 Seiten. - (Notes on numerical fluid mechanics and multidisciplinary design ; volume 151) ISBN 978-3-030-79560-3
"In 2020, the 22nd DGLR/STAB Symposium was planned to be held in Berlin. Unfortunately, due to the corona pandemie, the STAB Board had to cancel the conference to protect the health of the participants. However, it was the Board's strong ambition to give researchers who had planned a presentation for the symposium the opportunity to publish their latest work in an approprialte setting. Thus, the present volume contains the papers originally envisaged for the symposium after having successfully undergone a thorough peer review process..." (s. "Preface" Seite V)

Lösch, Alice; Du Puits, Ronald
The Barrel of Ilmenau: a large-scale convection experiment to study dust devil-like flow structures. - In: Meteorologische Zeitschrift, ISSN 1610-1227, Bd. 30 (2021), 1, S. 89-97

We present an experimental facility for the validation of numerical simulations on atmospheric dust devils in a controlled laboratory experiment. Dust devils are atmospheric air vortices with a vertical axis, and are formed by intense solar radiation and the resulting vertical temperature gradient. The structure of a typical dust devil is dominated by a radial inflow near the surface and a vertical upward flow within the vortex. These vortices have been studied in recent years using field observations, in situ measurements, and large-eddy simulation (LES). Field tests suffer from the limited area and their unpredictable behavior, while the LES approach cannot resolve the dust devils well enough. Dust devil-like structures may also occur in direct numerical simulation (DNS) with a Rayleigh number of at least Ra = 10^7 in Rayleigh-Bénard convection, with the advantage that the structures can be resolved more precisely. In order to validate the DNS approach and provide measurement data, the airflow is measured inside of a large-scale Rayleigh-Bénard cell of similar geometry (i.e. inside the Barrel of Ilmenau) to the DNS set-up for Rayleigh numbers from Ra = 10^6 to Ra = 10^12. For the measurement of the flow in a large volume, an optical measurement method is used to obtain the trajectories of single particles. Since there are no commercial systems that are suitable for such a large measurement volume, we developed our own system.



https://doi.org/10.1127/metz/2020/1046
Hertlein, Anna; Du Puits, Ronald
Direct measurements of the thermal dissipation rate in turbulent Rayleigh-Bénard convection. - In: Physics of fluids, ISSN 1089-7666, Bd. 33 (2021), 3, 035139, S. 035139-1-035139-11

https://doi.org/10.1063/5.0033746
Bauer, Christian;
Direct numerical simulation of very-large-scale motions in turbulent pipe flow
1. Auflage. - Göttingen : Cuvillier Verlag, 2021. - xxxi, 208 Seiten
Technische Universität Ilmenau, Dissertation 2020

ISBN 978-3-7369-7369-5

Die turbulente Rohrströmung ist nicht nur von Bedeutung für Anwendungen im Ingenieurbereich, sondern auch für die Grundlagenforschung von wandnaher Turbulenz. Auf letzterem Gebiet wurden, neben dem gut untersuchten Wandzyklus kleinskaliger turbulenter Schwankungen, in jüngerer Zeit sogenannte "sehr großskalige Strömungsbewegungen" (engl.: very-large-scale motions, VLSMs) entdeckt. Letztere treten bei hohen Reynoldszahlen auf und interagieren mit den kleinskaligen Strömungsstrukturen in der Nähe der Wand. Obwohl sich eine Vielzahl von Studien mit der Charakterisierung von VLSMs beschäftigt, sind die Prozesse, welche deren Interaktion mit kleinskaligen Strukturen beherrschen weitgehend unbekannt. Die vorliegende Arbeit widmet sich daher dem Studium von VLSMs in der turbulenten Rohrströmung mittels direkter numerischer Simulation bei Schubspannungs-Reynoldszahlen bis ReT=2880. Nachdem die virtuelle Rohrlänge, welche benötigt wird, damit VLSMs realistisch simuliert werden können, auf L≥42R festgelegt wurde, wird das Konvergenz- und Skalierungsverhalten verschiedener statistischer Momente der Geschwindigkeitsverteilung untersucht und in Bezug auf turbulente kohärente Strukturen diskutiert. Neben der Abhängigkeit des Maximums der axialen Reynolds'schen Spannung von der Reynoldszahl, sind die logarithmischen Reynoldszahlabhängigkeiten der Schiefe der axialen Geschwindigkeitsverteilung sowie der Flachheit der wandnormalen Geschwindigkeitsverteilung festzustellen. Diese Reynoldszahlabhängigkeiten sind auf die Interaktion der VLSMs mit dem wandnahen Strömungsbereich zurückzuführen. Die folgende Analyse der axialen Energietransportgleichung des gefilterten Geschwindigkeitsfeldes legt offen, dass VLSMs Energie von der mittleren Strömung zugeführt bekommen, ähnlich den kleinskaligen Strukturen durch den turbulenten Produktionsmechanismus. Im Gegensatz zu den kleinskaligen Fluktuationen im wandnahen Zyklus, wird für VLSMs keine inverse Energiekaskade im statistischen Mittel detektiert. Skalen, welche kleiner sind als VLSMs sind jedoch verantwortlich für den physikalischen Transport von VLSM Energie in Richtung Wand und Hauptströmungsbereich. Zudem findet sich eine inverse Energiekaskade in Momentaufnahmen der Strömung in Regionen langsamer, sich von der Wand wegbewegender VLSMs. Dieses Verhalten unterscheidet sich grundsätzlich von dem kleinskaliger Fluktuationen, wo die inverse Energiekaskade mit schnellen, sich zur Wand hinbewegenden Strukturen zusammenhängt. In beiden Fällen ist der Energietransport schneller Strömungsbewegungen stärker, als jener der langsamen. Im Allgemeinen tritt die rückwärtige Energiekaskade vor allem in Regionen hoher turbulenter kinetischer Energie auf. Zusammengefasst gewährt die Forschungsarbeit neue Einblicke in die Interaktion von VLSMs mit dem turbulenten Wandzyklus und trägt zum besseren Verständnis der turbulenten Rohrströmung bei.



Siegel, Lars; Henning, Arne; Ehrenfried, Klaus; Wagner, Claus
Investigation of the acoustic particle velocity using synchronized non-invasive measurements in the near and far field of a turbulent flow. - In: Journal of sound and vibration, ISSN 0022-460X, Bd. 493 (2021), 115820, insges. 14 S.

The sound generation of an aeroacoustic source with dominating tonal components and the propagation into the acoustic far field is investigated based on measurements of the flow around a square rod - mounted horizontally in an open-jet closed-circuit anechoic wind-tunnel - at a Reynolds number of 6×104. Pressure time series are measured with microphones in the far field while velocity field measurements are performed simultaneously with the non-invasive particle image velocimetry (PIV) technique in the flow field. It is shown that the propagation of acoustic waves from the source region to the far field can be resolved analyzing additionally flow fields recorded in vertically shifted fields of view, if the time evolution is synchronized properly with the far-field pressure signal. Furthermore, a comparison with two analytical solutions shows that the cross-correlation function of far-field pressure and near-field velocity fluctuations, normalized by the standard deviation of the pressure signal, corresponds to the acoustic particle velocity. However, this only applies to areas where the acoustic fluctuations dominate, i.e. not in the wake or in the free shear layer between the wind tunnel flow and the quiescent air where acoustic fluctuations are superimposed by hydrodynamic ones.



https://doi.org/10.1016/j.jsv.2020.115820
Bahavar, Philipp; Wagner, Claus
Direct numerical simulation of turbulent channel flow with condensation using a cluster-based droplet deposition model. - In: Direct and large eddy simulation XII, (2020), S. 111-117

Wetzel, Tim;
Analyse der Strukturbildung in turbulenten Mischkonvektionsströmungen auf Basis direkter numerischer Simulationen. - Ilmenau : Universitätsbibliothek, 2020. - 1 Online-Ressource (xxvii, 131 Seiten)
Technische Universität Ilmenau, Dissertation 2020

In der vorliegenden Arbeit wurden die Interaktionen zwischen Strömungsstrukturen in turbulenten Mischkonvektionsströmungen untersucht. Anhand von direkten numerischen Simulationen eines differentiell beheizten vertikalen Kanals wurde die Strömung im Hinblick auf die thermisch induzierte Dämpfung und Anfachung der turbulenten Geschwindigkeits-, Druck- und Temperaturfluktuationen analysiert. Es wurde eine umfassende physikalische Erklärung der Dämpfung und Anfachung erarbeitet, die bisher in der Literatur nicht vorhanden war. Dabei konnte gezeigt werden, dass die Relativgeschwindigkeiten der turbulenten Strömungsstrukturen aufgrund ihrer unterschiedlichen Temperaturen in der Nähe der beheizten Kanalwand reduziert und in der Nähe der gekühlten Wand erhöht werden. Die Veränderungen der Relativgeschwindigkeiten zwischen diesen Strukturen wirken sich unmittelbar auf die internen Scherschichten der Strömung aus und verringern und erhöhen die Umverteilung von fluktuierender turbulenter Energie von der Hauptströmungsrichtung in die anderen beiden Richtungen. Dadurch wird auch der wandnormale Energietransport verringert und erhöht, was zu entsprechenden Veränderungen der Produktion fluktuierender turbulenter Energie führt. Der Kreislauf des Energietransportes wird insgesamt so verändert, dass die turbulenten Geschwindigkeitsfluktuationen an der beheizten Wand verringert und an der gekühlten Wand erhöht werden. Damit einhergehend werden die turbulenten Strömungsstrukturen in der Nähe der beheizten Wand länger und in der Nähe der gekühlten Wand kürzer während ihre Breite in allen betrachteten Fällen nahezu konstant ist. Anhand von Simulationen einer belüfteten Mischkonvektionszelle wurde die Strömung analysiert und die Struktur der großskaligen Strömungsstrukturen im gesamten Rechengebiet diskutiert. Dadurch konnte gezeigt werden, dass die direkten numerischen Simulationen in der Lage sind, die Strömungsstrukturen im Vergleich mit Ergebnissen experimenteller Untersuchungen sowohl qualitativ als auch quantitativ gut abzubilden. Damit ist zum einen sichergestellt, dass das verwendete Simulationsverfahren physikalisch richtige Ergebnisse liefert und dass diese zum anderen zukünftig für die detaillierte Analyse der physikalischen Vorgänge in der Mischkonvektionszelle verwendet werden können.



https://nbn-resolving.org/urn:nbn:de:gbv:ilm1-2020000706
Wei, Tie; Du Puits, Ronald
Inner and outer scales in turbulent Rayleigh-Bénard convection. - In: Physics of fluids, ISSN 1089-7666, Bd. 32 (2020), 11, 115115, S. 115115-1-115115-15

https://doi.org/10.1063/5.0029853
Mommert, Michael; Schiepel, Daniel; Schmeling, Daniel; Wagner, Claus
Reversals of coherent structures in turbulent mixed convection. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 904 (2020), S. A33-1-A33-33

Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and forced flow contributions, at the two different Richardson numbers and and similar Rayleigh numbers of are investigated with tomographic particle image velocimetry in combination with local temperature measurements. For both cases, the three-dimensional velocity fields reflect diagonally aligned large-scale circulations (LSC) switching their alignment by rotating their axes around a pivot located at the centre of the LSC, while the temperatures perform a translation movement of the structures in agreement with earlier temperature-based investigations. For the high case, the switching process of the observed spontaneous reconfigurations is induced by a reversing thermal flow contribution while the forced flow contribution is constant. Furthermore, it is shown that a secondary roll structure, which drives the reconfiguration process in Rayleigh-Bénard convection, also exists in mixed convection. However, in the latter, the flow reversals are triggered by different structures which accumulate and release their kinetic energy according to a proper orthogonal decomposition analysis. In contrast, for the low case, the structure formation during continuous reconfigurations is governed by a Taylor- or Görtler-type instability. This means that the forced convection substantially affects the reconfiguration mechanism of these structures. Therefore, the reconfigurations cannot be described by a simple superposition of structures associated with the two flow contributions as for the high.



https://doi.org/10.1017/jfm.2020.705
Du Puits, Ronald; Bruecker, Christoph
Fluctuations of the wall shear stress vector in a large-scale natural convection cell. - In: AIP Advances, ISSN 2158-3226, Bd. 10 (2020), 7, 075105, insges. 10 S.

https://doi.org/10.1063/5.0006610
Westhoff, Andreas; Wagner, Claus
Experimental study of moist air flow in the gap between the aircraft's fuselage and its cabin wall. - In: CEAS Aeronautical Journal, ISSN 1869-5590, Bd. 11 (2020), 3, S. 591-607

We carried out an experimental study on the moisture transfer and the heat transport in warm and humid air flows between the cabin lining and the fuselage skin. The measurements were performed in a rectangular gap channel, representing the space between fuselage and cabin wall. Long-term measurements were performed for three configurations: without insulation, with fibreglass blanket and with melamine resin foam blanket. To simulate realistic flight conditions in a laboratory setup, we applied a concept of scaling. This concept is intended to guarantee similitude between the real flight conditions and the laboratory experiment. The results reveal that without insulation, the moisture transfer rate is much higher compared to the configurations with insulation blankets. With insulation, most of the water evaporates during ground conditions and just a small amount is entrapped in the insulation. Without insulation, just a small part of the frozen water evaporates on the ground. When comparing the two insulation blankets, it is found that they both have a similar heat transmittance coefficient. However, the condensation rate of the water and the resulting accumulation of water are significant, higher for the fibreglass blanket.



https://doi.org/10.1007/s13272-020-00440-3
Bahavar, Philipp; Wagner, Claus
Condensation-induced flow structure modifications in turbulent channel flow investigated in direct numerical simulations. - In: Physics of fluids, ISSN 1089-7666, Bd. 32 (2020), 1, 015115, S. 015115-1-015115-11

The turbulent flow of a fluid carrying trace amounts of a condensable species through a differentially cooled vertical channel geometry is simulated using single-phase direct numerical simulations. The release of latent heat during condensation is modeled by interdependent temperature and vapor concentration source terms governing the relation between the removal of excess vapor from the system and the associated local increase in fluid temperature. A coupling between condensation and turbulence is implemented via solutal and thermal buoyancy. When compared to simulations of an identical system without phase transition modeling, the modifications of the subcooled boundary layer due to the transient and highly localized release of latent heat could be observed. A separate analysis of fluid before and after phase transition events shows a clear increase in post-interaction streak spacing, with the release of latent heat during condensation events opposing the cooling effect of the channel wall and the associated damping of turbulence.



https://doi.org/10.1063/1.5128976
Wei, Tie; Du Puits, Ronald
Mean temperature profile and thermal displacement thickness in turbulent Rayleigh-Bénard convection. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 148 (2020), 119021, insges. 7 S.

The shape of the mean temperature profile in turbulent RayleighBénard Convection (RBC) plays an important role in the understanding and prediction of heat transfer. Based on the characteristics of the mean temperature profile, we propose a four-layer structure for each half of a turbulent RBC cell. Layer I, a thin layer adjacent to the bottom or top plate, is the traditional thermal diffusional sub-layer, in which the mean temperature is dominated by linear growth. It is found, empirically, that there exists a layer in which the mean temperature deficit can be approximated by a power-law decay, and this layer is called Layer III. Between Layer I and Layer III there exists a transitional or buffer layer, which is designated Layer II. Outside Layer III is a well-mixed outer layer, Layer IV. In a fully turbulent RBC, Layers I, II, and III occupy a small fraction of the RBC cell, but these layers contain nearly all the temperature increment. Layer IV occupies a large fraction of the RBC cell, but with negligible temperature increment. One parameter that quantifies the shape of the mean temperature is the thermal displacement thickness [delta theta]d. The four-layer model is used to clarify the contribution of different layers to the thermal displacement thickness in a turbulent RBC. It is found that the integral in the calculation of the thermal displacement thickness mainly comes from Layer III.



https://doi.org/10.1016/j.ijheatmasstransfer.2019.119021
Ullrich, Sebastian; Buder, Ricardo; Boughanmi, Nesrine; Friebe, Christian; Wagner, Claus
Numerical study of the airflow distribution in a passenger car cabin validated with PIV. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 457-467

Öngüner, Emir; Henning, Arne; Fey, Uwe; Wagner, Claus
Towards aerodynamically optimized freight wagons: an experimental study on container designs. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 437-446

Herzog, Sebastian; Wagner, Claus
Development of artificial neural networks with integrated conditional random fields capable of predicting non-linear dynamics of the flow around cylinders. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 71-79

Brückner, Christian; Bahavar, Philipp; Westhoff, Andreas; Wagner, Claus
Reynolds number dependency of the heat and mass transfer in mixed convective duct flow with condensation at a cooled wall. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 523-532

Niehaus, Konstantin; Mommert, Michael; Schiepel, Daniel; Schmeling, Daniel; Wagner, Claus
Comparison of two unstable flow states in turbulent mixed convection. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 543-552

Wilhelmi, Henning; Jessing, Christoph; Bell, James; Heine, Daniela; Wagner, Claus; Wiedemann, Jochen
Aerodynamic characterisation of a compact car overtaking a heavy vehicle. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 794-804

Westhoff, Andreas; Wagner, Claus
Experimental study on the Richardson number dependence of large-scale flow structures and their dynamics in a miniaturised aircraft cabin. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 468-477

Müller, Max; Ehrenfried, Klaus; Bell, James; Wagner, Claus
Analysis of model mount configurations with regard to force measurements with transient inflow. - In: New results in numerical and experimental fluid mechanics XII, (2020), S. 684-694

Dillmann, Andreas; Heller, Gerd; Krämer, Ewald; Wagner, Claus; Tropea, Cameron; Jakirliâc, Suad
New results in numerical and experimental fluid mechanics XII : contributions to the 21st STAB/DGLR Symposium, Darmstadt, Germany, 2018. - Cham, Switzerland : Springer, 2020. - xvii, 867 Seiten. - (Notes on numerical fluid mechanics and multidisciplinary design ; volume 142) ISBN 978-3-030-25252-6

Bauer, Christian; Wagner, Claus
Analysis of the energy budget of the largest scales in turbulent pipe flow. - In: Progress in turbulence VIII, (2019), S. 113-118

Bauer, Christian; Kameke, Alexandra; Wagner, Claus
Kinetic energy budget of the largest scales in turbulent pipe flow. - In: Physical review fluids, ISSN 2469-990X, Bd. 4 (2019), 6, 064607, insges. 22 S.

https://doi.org/10.1103/PhysRevFluids.4.064607
Wetzel, Tim; Wagner, Claus
Momentum and buoyancy repartition in turbulent mixed convection. - In: Direct and large-eddy simulation XI, (2019), S. 291-297

https://doi.org/10.1007/978-3-030-04915-7_39
Bahavar, Philipp; Wagner, Claus
Direct numerical simulation of convective turbulent channel flow of fluid mixtures. - In: Direct and large-eddy simulation XI, (2019), S. 285-290

https://doi.org/10.1007/978-3-030-04915-7_38
Bauer, Christian; Wagner, Claus
Scaling of high-order statistics in turbulent pipe flow. - In: Direct and large-eddy simulation XI, (2019), S. 537-543

https://doi.org/10.1007/978-3-030-04915-7_71
Mommert, Michael; Schiepel, Daniel; Schmerling, Daniel; Wagner, Claus
A flow-intrinsic trigger for capturing reconfigurations in buoyancy-driven flows in automated PIV. - In: Measurement science and technology, ISSN 1361-6501, Volume 30 (2019), 4, 045301, Seite 1-6

In order to acquire flow fields of rare reconfiguration events in buoyancy-driven flows, an automated flow-intrinsic trigger method for 3D particle image velocimetry (PIV) was developed. The trigger condition for initiating PIV is based on continuous multi-probe temperature measurements. The autonomously started PIV was realized with LED illumination and a controlled helium-filled soap bubble generator. The measured velocity fields reveal the transition between two different flow states in a mixed convection sample as proof of concept for this method. It is further proposed to consider the time series of all temperature probes for the trigger condition using proper orthogonal decomposition (POD). It is demonstrated that the use of this POD-based trigger condition allows one to detect flow events much earlier than the trigger based on single raw temperature signals.



https://doi.org/10.1088/1361-6501/ab0619
Wessels, Malte; Schmeling, Daniel; Bosbach, Johannes; Wagner, Claus
On the impact of the aspect ratio on the formation of large-Scale structures in turbulent mixed convection in a cuboidal sample. - In: International journal of heat and fluid flow, ISSN 1879-2278, Bd. 76 (2019), S. 231-241

https://doi.org/10.1016/j.ijheatfluidflow.2019.01.004
Wetzel, Tim; Wagner, Claus
Buoyancy-induced effects on large-scale motions in differentially heated vertical channel flows studied in direct numerical simulations. - In: International journal of heat and fluid flow, ISSN 1879-2278, Bd. 75 (2019), S. 14-26

Direct numerical simulations of turbulent convection in a differentially heated vertical channel flow with Prandtl number Pr = 0.71 are conducted with a fourth-order accurate finite volume method for a bulk Reynolds number Reb = 4328 and three Grashof numbers Gr [Element] {0, 6.4 &hahog; 105, 9.5 &hahog; 105}. The discussion of instantaneous flow field snapshots, first- and second-order moments, budget equations, power density spectra and quadrant analyses shows that the turbulent velocity fluctuations are attenuated in the aiding flow and enhanced in the opposing flow. In contrast, temperature fluctuations are attenuated in the opposing flow and enhanced in the aiding flow. The analyses further reveal that the low-momentum flow structures in the aiding flow are warmer than the highmomentum flow structures and vice versa in the opposing flow. Due to their different temperatures, buoyancy accelerates and decelerates the flow structures differently, which leads to reduced and increased pressure and shear fluctuations in the aiding and opposing flow. Thus, the redistribution of turbulent velocity fluctuations is lower in the aiding flow and higher in the opposing flow. The Reynolds shear stresses are consequently decreased in the former and increased in the latter, influencing the production of streamwise velocity fluctuations accordingly. In summary, the discussed physical mechanisms underline the indirect effect of buoyancy on the turbulent velocity fluctuations.



https://doi.org/10.1016/j.ijheatfluidflow.2018.09.005
Brückner, Christian; Westhoff, Andreas; Wagner, Claus
Temperature dependence of heat and mass transfer in a forced convective duct flow with and without condensation on one wall. - In: 5th International Conference on Experimental Fluid Mechanics, (2018), S. 39-44

https://doi.org/10.18726/2018_2
Dillmann, Andreas; Heller, Gerd; Krämer, Ewald; Wagner, Claus; Bansmer, Stephan; Radespiel, Rolf; Semaan, Richard
New results in numerical and experimental fluid mechanics XI : contributions to the 20th STAB/DGLR Symposium Braunschweig, Germany, 2016. - Cham : Springer, 2018. - Online-Ressource (xv, 767 Seiten). - (Notes on numerical fluid mechanics and multidisciplinary design ; 136) ISBN 978-3-319-64519-3

https://doi.org/10.1007/978-3-319-64519-3
Konstantinov, Mikhail; Wagner, Claus
Numerical simulation of the sound generation and the sound propagation from air intakes in an aircraft cabin. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 623-633

https://doi.org/10.1007/978-3-319-64519-3_56
Bahavar, Philipp; Wagner, Claus
Direct numerical simulation of convective channel flow with temperature and concentration gradients. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 445-453

https://doi.org/10.1007/978-3-319-64519-3_40
Schuster, Roland; Berkefeld, Tobias; Henning, Arne; Ehrenfried, Klaus; Wagner, Claus
Simulating pass-by noise of vehicles - comparison of a ray tracing model using simplified geometrical building models with measurement. - In: Taming noise and moving quiet, (2018), S. 135-145

Drahotský, Jakub; Hanzelka, Pavel; Musilová, Věra; Macek, Michal; Du Puits, Ronald; Urban, Pavel
Temperature profiles measurements in turbulent Rayleigh-Bénard convection by optical fibre system at the Barrel of IImenau. - In: EFM17 - Experimental Fluid Mechanics 2017, (2018), 02020, S. 1-6

https://doi.org/10.1051/epjconf/201818002020
Horstmann, Gerrit Maik; Schiepel, Daniel; Wagner, Claus
Experimental study of the global flow-state transformation in a rectangular Rayleigh-Bénard sample. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 126 (2018), Part A (Nov.), Seite 1333-1346

Large-scale flow structures were measured in a rectangular Rayleigh-Bénard sample of equal length and height of 10 cm and a depth of 2.5 cm. The working fluid was water at a Prandtl number of Pr = 6.9. Aiming to capture flow structures to their full extent and throughout the entire sample, the threedimensional (3D) Lagrangian measurement technique 3D-Particle Tracking Velocimetry (3D-PTV) was employed. The study provided direct confirmation that the global mean flow field changes significantly in dependence of the Rayleigh number Ra within the investigated range of 2.1 &hahog; 10^6 ≤ Ra ≤ 4.5 &hahog; 10^8. Several distinguishable flow states were observed in the laminar-turbulent transition regime complementing the well-known mean wind. A two-dimensional (2D)-mode decomposition revealed a breakdown of the mean wind during the transition phase and its new formation in the turbulent regime. Further, the global distribution of magnitudes of the instantaneous velocity fields was used to extract characteristic velocities in the flow field. It was shown, that the Reynolds-number-dependent scaling law Re ˜ Ra^y can be unambiguously determined by means of directly measured Lagrangian velocities, since the same scaling behavior holds true for all chosen reference velocities. In this context, using the product of a polynomial and an exponential function, the global velocity distribution was described analytically in dependence of Ra and four independent fit parameters in the sub-range 2.8 &hahog; 10^7 ≤ Ra ≤ 4.5 &hahog; 10^8.



https://doi.org/10.1016/j.ijheatmasstransfer.2018.05.097
Weinman, Keith A.; Fragner, Moritz; Deiterding, Ralf; Heine, Daniela; Fey, Uwe; Braenstroem, F.; Schultz, Bert; Wagner, Claus
Assessment of the mesh refinement influence on the computed flow-fields about a model train in comparison with wind tunnel measurements. - In: Journal of wind engineering and industrial aerodynamics, Bd. 179 (2018), S. 102-117

A consistent mesh refinement study, relating to the prediction of aerodynamic forces about an experimentally validated reference train geometry, is presented in this paper. The flow about a high-speed train has a multi-scale character which poses challenges for the design of computationally effective meshes. The purpose of this study is to assist in the development of guidelines for effective drag prediction of high-speed trains using numerical simulation. These guidelines should assist CFD practitioners by identifying the regions of the mesh that are critical for the correct estimation of drag as well as providing information on appropriate mesh characteristics, such as volume and surface element length scales. Numerical assessments are validated against an experimental drag measurement program and the extent to which RANS is sufficiently predictive for industrial design is discussed. The results obtained in the work suggest that the mesh about the train nose is essential for the proper assessment of the aerodynamic drag acting on the vehicle.



https://doi.org/10.1016/j.jweia.2018.05.005
Siegel, Lars; Ehrenfried, Klaus; Wagner, Claus; Mulleners, Karen; Henning, Arne
Cross-correlation analysis of synchronized PIV and microphone measurements of an oscillating airfoil. - In: Journal of visualization, ISSN 1875-8975, Bd. 21 (2018), 3, S. 381-395

The present study focuses on the correlation between the flow structures evolving during the dynamic stall processes of a two-dimensional NACA64-618 airfoil, which performs a sinusoidal movement about its quarter chord axis, and their aeroacoustic response in the far field. Experiments are conducted in an anechoic wind tunnel at a Reynolds number of 8 × 10^5 based on the chord length and include simultaneous velocity field measurements in the vicinity of the airfoil and microphone measurements in the acoustic far field. A causality correlation method based on phase locked snapshots of the velocity field allows for the identification of specific structures at different phases of the dynamic stall life cycle that contribute to the sound generation process. The sound emission during the stall development and flow reattachment phases is attributed to coherent structures evolving downstream of the trailing edge. When the flow is fully stalled, the region that contributes to the sound emission increases. The position of the sound emitting coherent structures also fluctuates stronger between oscillation cycles during full stall.



https://doi.org/10.1007/s12650-018-0473-7
Feldmann, Daniel; Bauer, Christian; Wagner, Claus
Computational domain length and Reynolds number effects on large-scale coherent motions in turbulent pipe flow. - In: Journal of turbulence, ISSN 1468-5248, Bd. 19 (2018), 3, S. 274-295

We present results from direct numerical simulations (DNS) of turbulent pipe flow at shear Reynolds numbers up to Re T = 1500 using different computational domains with lengths up to L/R = 42. The objectives are to analyse the effect of the finite size of the periodic pipe domain on large flow structures in dependency of Re T and to assess a minimum L/R required for relevant turbulent scales to be captured and a minimum Re T for very large-scale motions (VLSM) to be analysed. Analysing one-point statistics revealed that the mean velocity profile is invariant for L/R ≥ 7. The wall-normal location at which deviations occur in shorter domains changes strongly with increasing Re T from the near-wall region to the outer layer, where VLSM are believed to live. The root mean square velocity profiles exhibit domain length dependencies for pipes shorter than 14R and 7R depending on Re T. For all Re T, the higher-order statistical moments show only weak dependencies and only for the shortest domain considered here. However, the analysis of one- and two-dimensional pre-multiplied energy spectra revealed that even for larger L/R, not all physically relevant scales are fully captured, even though the aforementioned statistics are in good agreement with the literature. We found to be sufficiently large to capture VLSM-relevant turbulent scales in the considered range of Re T based on our definition of an integral energy threshold of 10%. The requirement to capture at least 1/10 of the global maximum energy level is justified by a 14% increase of the streamwise turbulence intensity in the outer region between Re T = 720 and 1500, which can be related to VLSM-relevant length scales. Based on this scaling anomaly, we found Re T 1500 to be a necessary minimum requirement to investigate VLSM-related effects in pipe flow, even though the streamwise energy spectra does not yet indicate sufficient scale separation between the most energetic and the very long motions.



https://doi.org/10.1080/14685248.2017.1418086
Salort, Julien; Rusaouen, Éléonore; Robert, Laurent; du Puits, Ronald; Loesch, Alice; Pirotte, Olivier; Roche, Philippe-E.; Castaing, Bernard; Chillà, Francesca
A local sensor for joint temperature and velocity measurements in turbulent flows. - In: Review of scientific instruments, ISSN 1089-7623, Bd. 89 (2018), 1, S. 015005, insges. 12 S.

We present the principle for a micro-sensor aimed at measuring local correlations of turbulent velocity and temperature. The operating principle is versatile and can be adapted for various types of flow. It is based on a micro-machined cantilever, on the tip of which a platinum resistor is patterned. The deflection of the cantilever yields an estimate for the local velocity, and the impedance of the platinum yields an estimate for the local temperature. The velocity measurement is tested in two turbulent jets: one with air at room temperature which allows us to compare with well-known calibrated reference anemometers, and another one in the GReC jet at CERN with cryogenic gaseous helium which allows a much larger range of resolved turbulent scales. The recording of temperature fluctuations is tested in the Barrel of Ilmenau which provides a controlled turbulent thermal flow in air. Measurements in the wake of a heated or cooled cylinder demonstrate the capability of the sensor to display the cross correlation between temperature and velocity correctly.



https://doi.org/10.1063/1.4989430
Wetzel, Tim; Wagner, Claus
Subgrid scale modelling of relaminarization effects in a differentially heated channel. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 499-508

https://doi.org/10.1007/978-3-319-64519-3_45
Wilhelmi, Henning; Thieme, Thomas; Henning, Arne; Wagner, Claus
Aerodynamic investigations of the effects of virtual coupling on two next generation trains. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 695-704

https://doi.org/10.1007/978-3-319-64519-3_62
Bauer, Christian; Feldmann, Daniel; Wagner, Claus
Revisiting the higher-order statistical moments in turbulent pipe flow using direct numerical simulations. - In: New results in numerical and experimental fluid mechanics XI, (2018), S. 75-84

https://doi.org/10.1007/978-3-319-64519-3_7
Schiepel, Daniel; Herzog, Sebastian; Wagner, Claus
Experimental study of turbulent Rayleigh-Bénard convection using large-scale Tomo-PIV and high-density PTV. - In: Turbulence and interactions, (2018), S. 225-231

https://doi.org/10.1007/978-3-319-60387-2_24
Bauer, Christian; Feldmann, Daniel; Wagner, Claus
On the convergence and scaling of high-order statistical moments in turbulent pipe flow using direct numerical simulations. - In: Physics of fluids, ISSN 1089-7666, Bd. 29 (2017), 12, 125105, insges. 13 S.

https://doi.org/10.1063/1.4996882
Siegel, Lars; Ehrenfried, Klaus; Henning, Arne; Lauenroth, Gerrit; Wagner, Claus
Acoustic particle velocity investigations in aeroacoustics synchronizing PIV and microphone measurements. - In: Towards a quieter future, (2016), S. 5845-5855

Kath, Christian; Wagner, Claus
Highly resolved simulations of turbulent mixed convection in a vertical plane channel. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 515-524

Czarnota, Tomasz; Wetzel, Tim; Wagner, Claus
Large Eddy simulation of turbulent thermal convection using different subgrid-scale models. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 505-514

Kin, Natalia; Deiterding, Ralf; Wagner, Claus
Large Eddy simulations of side flow past a generic model of a high-speed train. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 421-431

Jakubek, Daria; Wagner, Claus
Adjoint-based, CAD-free aerodynamic shape optimization of high-speed trains. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 409-419

Feldmann, Daniel; Wagner, Claus
On phase asymmetries in oscillatory pipe flow. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 113-122

Kin, Natalia; Deiterding, Ralf; Wagner, Claus
Large Eddy simulations of side flows past a generic model of a high-speed-train using a finite volume and a lattice Boltzmann method. - In: Proceedings of the Third International Conference on Railway Technology: Research, Development and Maintenance, (2016), Paper 60

Jakubek, Daria; Wagner, Claus
Aerodynamic shape optimization of train heads using adjoint-based computational fluid dynamics with different objective functions. - In: Proceedings of the Third International Conference on Railway Technology: Research, Development and Maintenance, (2016), Paper 56

Willert, Christian E.; Du Puits, Ronald; Resagk, Christian
Statistical and temporal characterization of turbulent Rayleigh-Bénard convection boundary layers using time-resolved PIV measurements. - In: Progress in wall turbulence 2, (2016), S. 317-333

Konstantinov, Mikhail; Wagner, Claus
Flow and thermal comfort simulations for double decker train cabins with passengers. - In: Proceedings of the Third International Conference on Railway Technology: Research, Development and Maintenance, (2016), Paper 42

Czarnota, Tomasz;
Interactions between turbulent convection and thermal radiation in direct numerical simulations. - Ilmenau : Universitätsbibliothek, 2016. - 1 Online-Ressource (xxiii, 150 Seiten, 10.95 MB)
Technische Universität Ilmenau, Dissertation 2016

Das Ziel der vorliegenden Arbeit ist die Untersuchung der Interaktionen zwischen turbulenter Konvektion und Oberflächenstrahlung durch direkte numerische Simulationen. Bei dem untersuchten Bereich handelt es sich um eine quaderförmige Konvektionszelle, die durch horizontale Platten endlicher Dicke begrenzt ist. Die Simulationen werden für die Prandtl-Zahl Pr=0.7, Rayleigh-Zahlen im Bereich von Ra=3.5×10^5 bis Ra=6.3×10^7, Strahlungszahlen zwischen Nr=0,0008 und Nr=0,0026 und Temperaturverhältnissen zwischen [Theta]=29 und [Theta]=45 durchgeführt. Zusätzlich ist das Verhältnis der Temperaturleitfähigkeit auf [lambda]_K=0.003 und [lambda]_K =3.8 eingestellt. Darüber hinaus wird die Schwarzkörperstrahlung betrachtet; dabei geht man davon aus, dass die Seitenwände für die Strahlung durchlässig sind, nicht jedoch für die Wärmeleitung. Die großskaligen Konvektionsrollen sind in Gegenwart von Strahlung größer und sie machen den Bulk gleichförmiger. Überdies ist die durch die Strahlung induzierte Senkung der Turbulezintesität im Bulk von der Ra und der Plattenleitfähigkeit abhängig. Bei [lambda]_K =0.003 beeinflusst die Strahlung die Temperatur der Wärmequellen sichtbar: die Heizplatten werden gekühlt und die Kühlplatten erwärmt. Die Intensität dieser Veränderungen ist jedoch ortsabhängig. Dadurch werden strahlungsbedingt die Temperaturen der kalten Plumes gleichmäßig beeinflusst, während die Temperaturen der heißen Plumes unterschiedlich betroffen sind, je nach ihrer Lage. Bei [lambda] K =3.8 sind die heißen und kalten Plumes sowohl in Gegenwart als auch in Abwesenheit von Strahlung ähnlich. Die Auswertung der Nusselt-Zahl zeigt, dass die Wärmestromdichte der Strahlung wesentlich zur Gesamtwärmestromdichte beiträgt, besonders bei den Platten mit geringer Leitfähigkeit und niedrigen Ra. Darüber hinaus erhöht sich der Strahlungsaustausch mit der Nr schneller als mit [Theta]. Außerdem wird bei [lambda]_K =0.003 der konvektive Abfall festgestellt. Die Untersuchungen ergaben, dass in Regionen, in denen die Plumes entstehen, nur die Wärmestromdichte der Strahlung und ihre Intensität für diesen Abfall verantwortlich sind, während in Regionen, wo die Plumes ankommen, die durch die ankommenden Plumes verursachte konvektive Wärme ebenfalls eine Rolle spielt. Wenn man das Komplexitätsniveau der Randbedingungen reduziert, so dass die Strahlung oder die Temperaturabweichungen vernachlässigt sind, beginnt die Entwicklung der thermischen Plumes bei ähnlichen Durchschnittstemperaturen. Jedoch sind die vorhergesagten flächigen Plumes kälter und größer. Dieser Temperaturabfall wird durch die Überproduktion der mäßig warmen Plumes auf Kosten der Unterproduktion der wärmsten Plumes in der Mitte und bei den Seitenwänden verursacht.



http://nbn-resolving.de/urn:nbn:de:gbv:ilm1-2016000777
Feldhusen, Kai; Deiterding, Ralf; Wagner, Claus
A dynamically adaptive lattice Boltzmann method for thermal convection problems. - In: International journal of applied mathematics and computer science, ISSN 2083-8492, Bd. 26 (2016), 4, S. 735-747

https://doi.org/10.1515/amcs-2016-0051
Feldmann, Daniel; Wagner, Claus
On the influence of computational domain length on turbulence in oscillatory pipe flow. - In: International journal of heat and fluid flow, ISSN 1879-2278, Bd. 57 (2016) Part A (Okt.), Seite 229-244

http://dx.doi.org/10.1016/j.ijheatfluidflow.2016.09.005
du Puits, Ronald; Willert, Christian E.
Boundary layers in turbulent Rayleigh-Bénard convection. - In: 80th Annual Meeting of the DPG and DPG-Frühjahrstagung (Spring Meeting) of the Condensed Matter Section (SKM), (2016), DY 57.3, insges. 1 S.

Kaiser, Robert; du Puits, Ronald
Global flow modes in turbulent Rayleigh-Bénard convection. - In: 80th Annual Meeting of the DPG and DPG-Frühjahrstagung (Spring Meeting) of the Condensed Matter Section (SKM), (2016), DY 57.13, insges. 1 S.

Schuster, Roland; Ehrenfried, Klaus; Henning, Arne; Wagner, Claus
Validation of a geometric diffraction model with respect to level-time history prediction. - In: Fortschritte der Akustik, ISBN 978-3-939296-10-2, (2016), S. 868-871

Du Puits, Ronald; Willert, Christian
The evolution of the boundary layer in turbulent Rayleigh-Bénard convection in air. - In: Physics of fluids, ISSN 1089-7666, Bd. 28 (2016), 4, 044108, insges. 15 S.

We report measurements of the near-wall flow field in turbulent Rayleigh-Bénard convection in air (Pr = 0.7) using particle imagevelocimetry. The measurements were performed in a thin, rectangular sample at fixed Rayleigh number Ra = 1.45 × 10^10. In particular, we focus on the evolution of the boundary layer that a single convection roll generates along its path at the lower horizontal plate. We identify three specific flow regions along this path: (i) a region of wall-normal impingement of the down flow close to one corner of the sample, (ii) a region where a shear layer with almost constant thickness evolves, and (iii) a region in which this boundary layer grows and eventually detaches from the plate surface at the opposite corner of the sample. Our measurements with a spatial resolution better than 1/500 of the total thickness of the boundary layer show that the typical velocity field as well as its statistics qualitatively varies between the three flow regions. In particular, it could be verified that the shear layer region covering about 75% of the total area of the plate is in transition to turbulence at the Rayleigh number as low as investigated in the present work.



https://doi.org/10.1063/1.4947261
Konstantinov, Mikhail; Wagner, Claus
Numerical simulation of the thermal comfort in a model of a passenger car cabin. - In: New results in numerical and experimental fluid mechanics X, (2016), S. 383-393

Dillmann, Andreas; Heller, Gerd; Krämer, Ewald; Wagner, Claus; Breitsamter, Christian
New results in numerical and experimental fluid mechanics X : contributions to the 19th STAB/DGLR Symposium Munich, Germany, 2014. - [Cham] : Springer, 2016. - xvi, 897 Seiten. - (Notes on numerical fluid mechanics and multidisciplinary design ; Volume 132) ISBN 978-3-319-27278-8
19th DGLR/STAB-Symposium held in Munich, Germany, 4-5 November 2014. - Mit Literaturangaben und Index

"This book presents contributions to the 19th biannual symposium of the German Aerospace Aerodynamics Association (STAB) and the German Society for Aeronautics and Astronautics (DGLR). The individual chapters reflect ongoing research conducted by the STAB members in the field of numerical and experimental fluid mechanics and aerodynamics, mainly for (but not limited to) aerospace applications, and cover both nationally and EC-funded projects. Special emphasis is given to collaborative research projects conducted by German scientists and engineers from universities, research-establishments and industries. By addressing a number of cutting-edge applications, together with the relevant physical and mathematics fundamentals, the book provides readers with a comprehensive overview of the current research work in the field. Though the book's primary emphasis is on the aerospace context, it also addresses further important applications, e.g. in ground transportation and energy."--



Czarnota, Tomasz; Wagner, Claus
Turbulent convection and thermal radiation in a cuboidal Rayleigh-Bénard cell with conductive plates. - In: International journal of heat and fluid flow, ISSN 1879-2278, Bd. 57 (2016), S. 150-172

The interactions between turbulent convection and surface-to-surface radiation are investigated by means of direct numerical simulations (DNS) performed for a cuboidal Rayleigh-Bénard cell bounded by horizontal plates of finite thickness and with high and low conductivities. The simulations are conducted for the Prandtl number Pr = 0.7, Rayleigh numbers ranging from Ra = 3.5 × 105 to Ra = 6.3 × 107, radiation numbers between Nr = 0.0008 and Nr = 0.0026, and temperature ratios between = 29 and = 45. Additionally, blackbody radiation is considered, and the side walls are assumed to be transparent to radiation but adiabatic for conduction. The DNS are performed with a well validated fourth order accurate finite volume method. Analysing the DNS data it is shown that radiation renders the bulk more uniform by enlarging the large scale circulations and damps turbulence intensities by decreasing the buoyancy force. Howthese effects depend on Ra and the plates' diffusivity is also discussed. The temperatures of the cold plumes are affected equally by radiation, while those of the hot plumes are affected differently, depending on their location. Radiation affects the temperature of the heat sources by cooling the heating plates and heating the cooling plates. The spatial fluctuations at the interfaces are decreased or increased by radiation, depending on the plates' diffusivity. The temporal fluctuations are not affected by radiation when high conductivity plates are employed. The radiative heat flux contributes considerably to the total one, particularly for low conductivity plates and low Ra. Heat radiation also initiates a so-called convective drop. It is found that in regions where the plumes emerge, only the radiative heat flux and its intensity are responsible for this drop, while in regions where the plumes arrive, the convective heat transported by the arriving plumes contributes as well. Small variations in the temperature distribution at the interfaces do not affect irradiation, which implies the possibility of further simplifications and a decrease in the computational cost. The convective and radiative heat fluxes are anticorrelated for low conductivity plates and uncorrelated in space for high conductivity plates. Furthermore, it is shown that the radiative heat transfer increases with the control parameters for radiation, yet the increase is more sensitive to variations of the radiation number than the temperature ratio. On the other hand, the convective heat transfer and the effective temperature difference decrease whenever any of the control parameters for radiation increase. The meaning of the control parameters for radiation is discussed and the dependencies are expressed with scaling functions.



http://dx.doi.org/10.1016/j.ijheatfluidflow.2015.10.006
Liot, Olivier; Salort, Julien; Kaiser, Robert; du Puits, Ronald; Chillà, Francesca
Boundary layer structure in a rough Rayleigh-Bénard cell filled with air. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 786 (2016), S. 275-293

In this experimental work, the aim is to understand how turbulent thermal flows are enhanced by the estabilization of the boundary layers. Square-stud roughness elements have been added on the bottom plate of a rectangular Rayleigh-Bénard cell in air, to trigger instabilities in the boundary layers. The top plate is kept smooth. The cell proportions are identical to those of the water cell previously operated and described by Salort et al. (Phys. Fluids, vol. 26, 2014, 015112), but six times larger. The very large size of the Barrel of Ilmenau allows detailed velocity fields to be obtained using particle image velocimetry very close to the roughness elements. We found that the flow is quite different at low Rayleigh numbers, where there is no heat-transfer enhancement, and at high Rayleigh numbers where there is a heat-transfer enhancement due to the roughness. Below the transition, the fluid inside the notch, i.e. between the studs, is essentially at rest, though it is slowly recirculating. The velocity profiles on the top of obstacles and in grooves are fairly compatible with those obtained in the smooth case. Above the transition, on the other hand, we observe large incursions of the bulk inside the notch, and the velocity profiles on the top of obstacles are closer to the logarithmic profiles expected in the case of turbulent boundary layers.



http://dx.doi.org/10.1017/jfm.2015.649
Köthe, Thomas; Herzog, Sebastian; Wagner, Claus
Shape optimization of aircraft cabin ventilation components using adjoint CFD. - In: Engineering optimization IV, (2015), S. 675-680

Feldhusen, Kai; Deiterding, Ralf; Wagner, Claus
Validation of a dynamically adaptive lattice Boltzmann method for 2D thermal convection simulations. - In: 2015 Second International Conference on Mathematics and Computers in Sciences and in Industry, ISBN 978-1-4799-8673-6, (2015), S. 199-206

https://doi.org/10.1109/MCSI.2015.60
Wagner, Claus; Feldmann, Daniel
Turbulent kinetic energy transport in oscillatory pipe flow. - In: Direct and large-eddy simulation IX, (2015), S. 351-357

Horn, Susanne; Wagner, Claus
Rotating Rayleigh-Bénard convection of SF6 in a slender cylinder. - In: Direct and large-eddy simulation IX, (2015), S. 353-359
Im Titel ist "6" tiefgestellt

Siegel, Lars; Ehrenfried, Klaus; Henning, Arne; Wagner, Claus
Intensity-analysis of cross-correlated synchronized PIV and microphone measurements. - In: Jahresbericht 2015 zum 17. STAB-Workshop, 10.-11. November 2015, Göttingen, (2015), S. 132-133

Konstantinov, Mikhail; Wagner, Claus
Numerical simulation of the thermal comfort in a double decker train cabin. - In: New challenges for the greening of transport, ISBN 978-951-39-6219-7, (2015), S. 35-39

Fragner, Moritz; Deiterding, Ralf; Wagner, Claus
Investigating side-wind stability of high speed trains with CFD methods that resolve turbulent dynamics on decreasing scales. - In: New challenges for the greening of transport, ISBN 978-951-39-6219-7, (2015), S. 31-34

Konstantinov, Mikhail; Wagner, Claus
Numerical simulation of the thermal comfort in a train cabin. - In: The international journal of railway technology, ISSN 2049-5358, Bd. 4 (2015), 3, S. 69-88

Results of numerical simulations of the air flow, including the heat transport, the thermal radiation and the thermal comfort of passengers in a train cabin are presented. The computations have been performed by coupling flow simulations conducted with the computational fluid dynamics (CFD) code OpenFOAM with finite-element simulations of the heat transport within the passengers using the code THESEUS-FE. With the latter, the bodies of passengers were modelled based on various layers with different heat transport characteristics to account for effects like blood flow, heat transfer through the skin and the clothing, as well as activity levels and ambient humidity. Based on these computations we simulated and analysed the thermal comfort of passengers in the cabin of German Aerospace Centres concept train called Next Generation Train (NGT).



Feldmann, Daniel;
Eine numerische Studie zur turbulenten Bewegungsform in der oszillierenden Rohrströmung. - Ilmenau : Universitätsbibliothek, 2015. - 1 Online-Ressource (xx, 237 Seiten)
Technische Universität Ilmenau, Dissertation 2015

In der vorliegenden Arbeit habe ich die turbulente Bewegungsform in der oszillierenden Rohrströmung (OR) mittels direkter numerischer Simulation(DNS) untersucht. Neben der allgemeinen wissenschaftlichen Relevanz der Thematik liegt dieser Studie vorrangig der Modellcharakter der OR für die Untersuchung der Atemgasströmung im menschlichen Bronchialbaum als Motivation zu Grunde. Dabei steht die mechanische Beatmung mittels Hochfrequenzoszillationsventilation (HFOV) im Fokus. Die Modellierung dieser Strömung unter Verwendung klinischer Daten aus der Literatur hat ergeben, dass die für die HFOV tatsächlich relevanten Reynolds-Zahlen Werte von bis zu Re = 37000 annehmen. Die HFOV ist demnach durch bis zu 26 mal größere Re charakterisiert, als sie für die ruhige Spontanatmung typisch sind. Für die praktisch relevanten Womersley-Zahlen liefert das aufgestellte Modell Werte im Bereich 0.06 < Wo < 20. In dem soidentifizierten Kennzahlbereich habe ich 21 DNS der OR durchgeführt, die Folgendes gezeigt haben: In Abhängigkeit von Re und Wo stellt sich entweder eine laminare oder eine bedingt turbulente Bewegungsform ein. In einem speziellen Fall resultiert aufgrund einer Hysterese eine asymmetrische Mischform aus beiden Strömungen, welche sich durch eine Variation der verwendeten Randbedingung unterdrücken lässt. Die turbulente Bewegungsform ist in der OR im Allgemeinen durch eine wiederholte Dämpfung, Anfachung und Umverteilung von lokalen Fluktuationen in der Strömung gekennzeichnet. Zu welcher Phase und an welchem Ort diese Vorgänge stattfinden, hängt ebenfalls von der Wahl der charakteristischen Kennzahlen und teilweise von deren Quotienten ab. Die Analyse der phasengemittelten Strömungsfelder hat gezeigt, dass die turbulente Bewegung in der OR sowohl Ähnlichkeiten als auch deutliche Unterschiede zur voll entwickelten turbulenten Strömung aufweist. Der Vergleich der DNS-Daten mit theoretischen Vorhersagen für die laminare OR hat ergeben, dass die einsetzende Turbulenz deutliche Einflüsse auf die oszillierende Strömung ausübt. Typische Effekte der laminaren OR treten in den phasengemittelten Größen aufgrund der turbulenten Durchmischung nur in abgeschwächter Form auf. Die Auswertung integraler Größen hat gezeigt, dass die einsetzende Turbulenz in einigen Fällen eine Erhöhung des Durchflusses um bis zu 5% bei gleichzeitig bis zu 14% geringeren Schubspannungen bewirkt.



http://www.db-thueringen.de/servlets/DocumentServlet?id=27407
Köthe, Thomas; Herzog, Sebastian; Wagner, Claus
Multi-objective shape optimization of aircraft cabin ventilation components using adjoint CFD. - In: 16th AIAA/ISSMO Multidisciplinary Analysis and Optimization Conference 2015, (2015), S. 774-786

Fragner, Moritz M.; Weinman, Keith A.; Deiterding, Ralf; Fey, Uwe; Wagner, Claus
Comparison of industrial and scientific CFD approaches for predicting cross wind stability of the NGT2 model train geometry. - In: The international journal of railway technology, ISSN 2049-5358, Bd. 4 (2015), 1, S. 1-28

Schuster, Roland; Henning, Arne; Wagner, Claus
Towards the assessment of the noise level time-history for ground based transportation. - In: Implementing noise control technology, (2015), S. 1012-1023

Karcher, Christian; Du Puits, Ronald; Schumacher, Jörg; Wagner, Claus
Das Institut für Thermo- und Fluiddynamik. - In: Jenaer Jahrbuch zur Technik- und Industriegeschichte, ISSN 2198-6746, Bd. 18 (2015), S. 147-174

Kaiser, Robert;
Wärmestromverteilung in turbulenter Rayleigh-Bénard Konvektion, 2015. - Online-Ressource (PDF-Datei: XIV, 110 S., 79,06 MB) Ilmenau : Techn. Univ., Diss., 2015

In der vorliegenden Dissertation wurde der lokale Wärmestrom an der Heizplatte mit einer neuartigen Messtechnik im Bereich der Rayleigh-Bénard Konvektion für verschiedene thermische Randbedingungen in unterschiedlichen Geometrien erforscht. Unter Verwendung einer Wärmebildkamera, mit einer hohen Datenrate (30 fps) und einer hohen Auflösung (640 px x 480 px) konnte die Wandwärmestromdichte an der Grenzfläche zwischen einem Festkörper und einem Fluid bestimmt werden. In einer schlanken rechteckigen Konvektionszelle (Höhe 2.5 m, Länge 2.5 m, Breit 0.65 m) wurde der lokale konvektive Wärmetransport für 1.36e10≤Ra≤5.45e10 untersucht. Innerhalb dieser Konvektionszelle ist die großskalige Wirbelstruktur in eine Raumrichtung fixiert. Die Messungen ergaben eine inhomogene Verteilung der mittleren Wandwärmestromdichte, wobei eine Variation von bis zu 37% bezüglich des globalen Wärmestromes festgestellt wurde. Auf Grundlage dessen wurde der lokale Wandwärmestrom in drei Teilregionen (Prallströmung, Zentrum und Eckströmung) untergliedert und die lokale Skalierung der Wandwärmestromdichte in Abhängigkeit von der Rayleigh-Zahl berechnet. In einer zylindrischen Geometrie mit [Gamma]=1.13 wurde für 1e11≤Ra≤8e11 die Dynamik einer dreidimensionalen Konvektionsströmung erforscht. In Übereinstimmung mit den Ergebnissen des quasi-zweidimensionalen Falls variiert der zeitgemittelte lokale Wandwärmestrom um bis zu 30% im Vergleich zu dem globalen Wärmestrom. Die Hauptebene der großskaligen Wirbelstruktur oszillierte dabei um ±90&ring;, wobei eine Homogenisierung des lokalen Wandwärmestromes im Zentrum zu beobachten war. Weiterhin wurde eine Erhöhung des Wandwärmestromes im Bereich der Seitenwand festgestellt, die eine Verteilung in azimutaler Richtung aufweist. Der Einfluss der Seitenwand auf den globalen Wärmetransport wurde in einem Bereich von 1.13<[Gamma]<4 untersucht. Die Analyse bei einer konstanten Rayleigh-Zahl offenbarte eine signifikante Änderung des globalen Wärmestromes von bis zu 35%. Ein entscheidender Mechanismus stellt der Zerfall der großskaligen Wirbelstruktur bei [Gamma]=1.65 dar. Diese Tatsache verdeutlicht, dass ein Konvektionsexperiment mit [Gamma]=1 nicht die theoretischen Wärmetransportmodelle für eine horizontal unendlich ausgedehnte Fluidschicht verifizieren kann.



http://www.db-thueringen.de/servlets/DocumentServlet?id=26753
Schmeling, Daniel; Bosbach, Johannes; Wagner, Claus;
Measurements of the dynamics of thermal plumes in turbulent mixed convection based on combined PIT and PIV. - In: Experiments in fluids, ISSN 1432-1114, Bd. 56 (2015), 6, 134, insges. 20 S.

https://doi.org/10.1007/s00348-015-1981-z
Haff, Johannes;
A comparative study of engineering tools in heavy vehicle aerodynamics, 2015. - Online-Ressource (PDF-Datei: XIV, 132 S., 339,78 MB) Ilmenau : Techn. Univ., Diss., 2015

Die Sattelzugkonfiguration ist insbesondere aufgrund der hohen Flexibilität und Verfügbarkeit der Hauptverkehrsträger für den Warengütertransport in Europa. Gekennzeichnet durch einen hohen spezifischen Energieverbrauch, bieten Maßnahmen zur Reduzierung des aerodynamischen Widerstands ein erhebliches Potential zur Steigerung der Leistungsfähigkeit von Sattelzügen. Untersuchungen im Bereich der Nutzfahrzeugaerodynamik machen allerdings die Anwendung anspruchsvoller experimenteller und numerischer Methoden erforderlich. Das wesentliche Ziel dieser Arbeit ist es, einige dieser Methoden hinsichtlich ihrer Praktikabilität und Vorhersagbarkeit für die Entwicklung widerstandsreduzierender Maßnahmen im Nutzfahrzeugbereich zu verifizieren. Als Grundlage für die Untersuchungen dient eine im Rahmen von Windkanalmessungen eruierte Modellkonfiguration mit einer Unterbodenvollverkleidung und einem Heckdiffusor. Neben einer gemessenen Reduzierung des aerodynamischen Gesamtwiderstands von bis zu 3,3%, liefern die Experimente unter Anwendung modernster Strömungsmesstechniken wesentliche Erkenntnisse über die induzierten strömungsphysikalischen Effekte, welche eine Widerstandsreduktion bewirken. Die Berechnung des Gesamtströmungsfeldes, durch Lösen der Reynolds-gemittelten Navier-Stokes Gleichungen unter Vorgabe der Randbedingungen im Windkanalexperiment, liefert zusätzliche Erkenntnisse zur Klärung der genauen Wirkungsweise der untersuchten Modellmodifikationen. Dadurch kann die aus den Experimenten gewonnene Erkenntnis, dass diffusorinduzierte Strömungsstrukturen den Basisdruck am Heck des Aufliegers erhöhen, bestätigt werden. Ein weiterer wesentlicher Aspekt im Bereich der Fahrzeugaerodynamik ist die Berücksichtigung der unter realen Strömungsbedingungen vorhandenen Relativbewegung zwischen Fahrzeug und Fahrbahn, welcher im Rahmen der durchgeführten Simulationen ebenfalls untersucht wurde. Dabei wird eine verringerte Wirkung der Unterbodenmodifikationen unter realen Bodenrandbedingungen nachgewiesen. Zur Ermittlung des Einflusses der Randbedingungen von Windkanalmessungen auf die Strömungsvorgänge eines Sattelzugs unter realen Anströmbedingungen und zur Identifizierung der Unterschiede in den jeweiligen berechneten und gemessenen Strömungsfeldern dienen Messergebnisse aus einem Fahrversuch. Insgesamt leistet die vorliegende Arbeit einen Beitrag zum besseren Verständnis der instationären Strömungsvorgänge im Ablösegebiet hinter stumpfen Körpern und liefert einen genaueren Einblick in die Übertragbarkeit von Strömungsfelddaten aus skalierten Modellmessungen und numerischen Simulationen auf die Realität.



http://www.db-thueringen.de/servlets/DocumentServlet?id=26088
Feldmann, Daniel; Wagner, Claus
Über Asymmetrien in oszillierender Rohrströmung. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 46-47

Kin, Natalia; Deiterding, Ralf; Wagner, Claus
Hochauflösende Simulation der seitlichen Umströmung eines generischen Modells eines Hochgeschwindigkeitszuges. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 210-211

Jakubek, Daria; Herzog, Sebastian; Wagner, Claus
Numerische Formoptimierung eines generischen Hochgeschwindigkeitszuges mit Hilfe adjungierter Methoden. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 208-209

Kath, Christian; Wagner, Claus
Highly resolved simulations of turbulent mixed convection in a vertical plane channel. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 136-137

Czarnota, Tomasz; Wetzel, Tim; Wagner, Claus
Large eddy simulation of turbulent thermal convection using different subgrid scale models. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 132-133

Konstantinov, Mikhail; Wagner, Claus
Numerische Berechnung des thermischen Komforts in einem Modell der PKW-Kabine. - In: Jahresbericht 2014 zum 19. DGLR-Fach-Symposium der STAB, 4. - 5. November 2014, München, (2014), S. 120-121

Czarnota, Tomasz; Wagner, Claus
Direct numerical simulations of turbulent convection and thermal radiation in a Rayleigh-Bénard cell with solid plates. - In: Turbulence and interactions, (2014), S. 39-46

Wagner, Claus; Horstmann, Gerrit; Herzog, Sebastian; Jakubek, Daria; Rutschmann, Sabrina
Shape optimization of train heads with respect to the aerodynamic loads on track side objects. - In: The international journal of railway technology, ISSN 2049-5358, Bd. 3 (2014), 1, S. 83-104

Shishkina, Olga; Wagner, Claus
Highly-resolved numerical simulations of high Rayleigh and Reynolds number indoor ventilation in a generic room. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 303-311

Kath, Christian; Wagner, Claus
DNS and LES of turbulent mixed convection in the minimal flow unit. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 123-131

Linke, Anne; Lauenroth, Gerrit; Rung, Thomas; Wagner, Claus
Fluid-dynamic optimization of the cabin air outlet Do728-KLA with adjoint sensitivity analysis. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 75-82

Shishkin, Andrei; Wagner, Claus
Numerical investigation of the aerodynamic forces induced by the flow around free flying fruit fly. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 671-679

Haff, Johannes; Tschech, Joachim; Richard, Hugues; Loose, Sigfried; Wagner, Claus
An experimental and numerical investigation of the near wake field of a tractor-trailer configuration. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 325-333

Wagner, Sebastian; Shishkina, Olga; Wagner, Claus
Influence of the geometry on Rayleigh-Bénard convection. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 313-321

Obermeier, Frank; Konstantinov, Mikhail; Shishkin, Andrei; Wagner, Claus
Sound generation by low Mach number flow through pipes with diaphragm orifices. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 629-637

Westhoff, Andreas; Bosbach, Johannes; Wagner, Claus
Large-scale coherent structures in turbulent mixed convective air flow. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 285-292

Konstantinov, Mikhail; Lautenschlager, Waldemar; Shishkin, Andrei; Wagner, Claus
Numerical simulation of the air flow and thermal comfort in aircraft cabins. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 293-301

Konstantinov, Mikhail; Wagner, Claus
Numerical simulation of the air flow and thermal comfort in a train cabin. - In: Proceedings of the Second International Conference on Railway Technology: Research, Development and Maintenance, ISBN 978-1-905088-59-1, (2014), Paper 328

Fey, Uwe; Haff, Johannes; Jönsson, Mattias; Loose, Sigfried; Wagner, Claus
Experimental investigation of topological changes in the flow field around high-speed trains with respect to Reynolds number scaling effects. - In: Proceedings of the Second International Conference on Railway Technology: Research, Development and Maintenance, ISBN 978-1-905088-59-1, (2014), Paper 32

Fragner, Moritz M.; Weinman, Keith A.; Deiterding, Ralf; Fey, Uwe; Wagner, Claus
Numerical and experimental studies of train geometries subject to cross winds. - In: Proceedings of the Second International Conference on Railway Technology: Research, Development and Maintenance, ISBN 978-1-905088-59-1, (2014), Paper 24

Schiepel, Daniel; Bosbach, Johannes; Wagner, Claus
Large scale tomographic particle image velocimetry of turbulent Rayleigh-Bénard convection. - In: New results in numerical and experimental fluid mechanics IX, (2014), S. 525-533

Du Puits, Ronald; Resagk, Christian; Willert, Christian
Boundary layer heat transport in turbulent Rayleigh-Bénard convection in air. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 14 (2014), 1, S. 657-658

http://dx.doi.org/10.1002/pamm.201410312
Du Puits, Ronald; Li, Ling; Resagk, Christian; Thess, André; Willert, Christian
Turbulent boundary layer in high Rayleigh number convection in air. - In: Physical review letters, ISSN 1079-7114, Bd. 112 (2014), 12, 124301, insges. 5 S.

Flow visualizations and particle image velocimetry measurements in the boundary layer of a Rayleigh-Bénard experiment are presented for the Rayleigh number Ra 1/4 1.4 × 1010. Our visualizations indicate that the appearance of the flow structures is similar to ordinary (isothermal) turbulent boundary layers. Our particle image velocimetry measurements show that vorticity with both positive and negative sign is generated and that the smallest flow structures are 1 order of magnitude smaller than the boundary layer thickness. Additional local measurements using laser Doppler velocimetry yield turbulence intensities up to I 1/4 0.4 as in turbulent atmospheric boundary layers. From our observations, we conclude that the convective boundary layer becomes turbulent locally and temporarily although its Reynolds number Re [approximately] 200 is considerably smaller than the value 420 underlying existing phenomenological theories. We think that, in turbulent Rayleigh-Bénard convection, the transition of the boundary layer towards turbulence depends on subtle details of the flow field and is therefore not universal.



https://doi.org/10.1103/PhysRevLett.112.124301
Kaiser, Robert; Du Puits, Ronald;
Local wall heat flux in confined thermal convection. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 73 (2014), S. 752-760

We report highly resolved measurements of the local wall heat flux in turbulent Rayleigh-Bénard convection using an infrared camera. The measurements have been undertaken in a Rayleigh-Bénard cell with rectangular base of 2.50 m x 0.65 m and a height of 2.5 m which is filled with air. First of all, it could be demonstrated that in a Rayleigh-Bénard cell with rectangular cross-section the time-averaged wall heat flux locally deviates by 30% from its mean. Furthermore, a strong correlation between the global flow structure inside the cell and the distribution of the local wall heat flux could be identified.



https://doi.org/10.1016/j.ijheatmasstransfer.2014.02.033
Schmerling, Daniel; Bosbach, Johannes; Wagner, Claus
Simultaneous measurement of temperature and velocity fields in convective air flows. - In: Measurement science and technology, ISSN 1361-6501, Bd. 25 (2014), 3, S. 035302, insges. 16 S.

http://dx.doi.org/10.1088/0957-0233/25/3/035302
du Puits, Ronald; Kaiser, Robert; Rilk, Johannes; Thess, André
Turbulent Rayleigh-Bénard convection in air: how uniform is the local wall heat flux at finite aspect ratio?. - In: Progress in turbulence V, (2014), S. 149-152

Schiepel, Daniel; Bosbach, Johannes; Wagner, Claus
Tomographic particle image velocimetry of turbulent Rayleigh-Benard convection in a cubic sample. - In: Journal of flow visualization and image processing, ISSN 1940-4336, Bd. 20 (2013), 1/2, S. 3-23

https://doi.org/10.1615/JFlowVisImageProc.2014010441