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.
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
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
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)˚. 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
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
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
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
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
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 ˚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
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