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), 9927275, 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.



https://doi.org/10.1007/978-3-031-40482-5_9
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