Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session K18: Rarefied Flows (8:45am  9:30am CST)Interactive On Demand

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K18.00001: Different Approaches to Modeling of Carbon Dioxide Relaxation Alena Kosareva, Olga Kunova, Elena Kustova, Ekaterina Nagnibeda Vibrational relaxation of carbon dioxide is studied using strict stateto state (E. Kustova, E. Nagnibeda. AIP Conf. Proc. 2001, I. Armenise, E. Kustova. Chem. Phys. 2014) and simplified multitemperature approaches (E. Kustova, E. Nagnibeda. Chem. Phys. 2006, A. Kosareva, E. Nagnibeda. J. Phys. Conf. Ser. 2017). The full kinetic scheme considering all vibrational states and different kinds of vibrational energy transitions within and between CO$_2$ modes is formulated and employed for investigation of 0D spatially homogeneous relaxation. The analysis of the contributions of different energy transitions to relaxation is presented, and the most efficient and accurate kinetic scheme is determined in the statetostate approximation. A comparison of the flow parameters obtained using the statetostate and simplified approximations is made for two typical cases corresponding to a compressed gas (excitation regime) and to an expanding flow (deactivation regime). Reducedorder models are evaluated by comparison with the results of full statetostate simulations. Possible sources of errors and differences between approaches are identified and ways of improving multitemperature models are proposed. [Preview Abstract] 

K18.00002: A model of micro lubrication between two walls with an arbitrary temperature distribution based on kinetic theory Toshiyuki Doi Micro lubrication of a gas between two walls with an arbitrary temperature distribution is studied on the basis of the BhatnagarGrossKrookWelander model of the Boltzmann equation. Applying the slowly varying approximation, the kinetic equation is studied analytically when the Knudsen number based on the gap size is large. The leading order approximation, which ought to be the solution of the nonlinear heat transfer problem, is replaced by its free molecular solution. Due to this crude approximation, a macroscopic lubrication model of Reynolds type equation is derived in a closed form. For an assessment of the model, direct numerical analyses of lubrication flows between nonuniformly heated or cooled walls are conducted. The lift calculated using our model approximates that of the direct numerical analysis within the error of 5 \% over the range of the Knudsen number between 0.1 and 10 when the highest temperature of the wall is twice the lowest one. [Preview Abstract] 

K18.00003: Datadriven approach for the RosenbluthFokkerPlanck Equation Kyoungseoun Chung, Fei Fei, Hossein Gorji, Patrick Jenny In this study, we demonstrate a datadriven technique to efficiently obtain an accurate approximation of the transport coefficients in the RosenbluthFokkerPlanck equation. Our approach is based on an endtoend mapping between the statistical moments of plasma particles and the highdimensional transport coefficient fields in the velocity space. The probability density functions are approximated based on the maximumentropy closure and then parameterized. This allows us to compute the transport coefficients from a physically realizable set of moments without assuming thermodynamic equilibrium. In order to obtain training data, Direct Simulation Monte Carlo (DSMC) was performed starting with specified initial conditions, which resulted in temporal information of moments. We adopted an artificial neural network model to accurately approximate the training data set. In order to utilize the trained model in the existing particle simulation framework, the evaluated transport coefficient fields have to be interpolated to the particle locations, and the evolution of particle positions and velocities are based on a Langevin equation. Our approach successfully demonstrates correct anisotropic relaxation behavior and shows improved computational efficiency. [Preview Abstract] 

K18.00004: DSMC simulation of collision process in ArgonNitrogen mixed gaseous thermal plasma. Sahadev Pradhan, A. K. Kalburgi The collision process in ArgonNitrogen mixed gaseous thermal plasma consist of electrons and heavy particles is studied using Direct Simulation Monte Carlo (DSMC) simulations to understand the effect of large mass ratio (electron and heavy particles) on collision rate when each species specified as a separate collision group as well as all the species in a single group for number of simulated particles per cell ($F_{N})$ in the range 2 \textless $F_{N\thinspace }$\textit{\textless 200, }with eight subcells per cell. \quad By including the separate collision group for each species the collision rates between heavy particles as well as among electrons and heavy particles with $F_{N} \quad =$ 200 and 20 are in excellent agreement with the theoretical value, to within 5{\%}. However, the mean spacing between collision pair is increased and the selection is forced beyond the subcell. This also leads to an increase in the overall acceptance rate of collision pairs The comparison reveals that the inclusion of all the species in a single group become overwhelming when electrons are present. The very low $F_{N\thinspace \thinspace }$value$ (F_{N\thinspace }=$\textit{2) }results in an excessive increase \quad in mean spacing between collision pairs, and the error in the collision rate turn out to be very significant /a [Preview Abstract] 

K18.00005: Investigation of Rarefied Open Cavity Flows in all Rarefaction Regimes using DSMC Method Deepak Nabapure, Ram Chandra Murthy K The hypersonic reentry vehicles often encounter different rarefaction regimes during their flight.The flow field around them is generally investigated using the Direct Simulation Monte Carlo (DSMC) method.Surface discontinuities in the form of protuberances, notches, steps, cavities or gaps occur on various surfaces of the hypersonic reentry vehicles.These surface anomalies can in turn increase the thermal and aerodynamic loads and hence are a major focus of the aerospace industry.Thus, there is a need to accurately estimate these loads which help in the safe design of the reentry vehicles. The flow over an open cavity is one such anomaly with a simple geometry.The present work analyses the rarefied flow of nonreacting air over an open cavity using the DSMC method.Twodimensional simulations are carried out for various Mach numbers ($Ma=5,10,25,30$), dimensionless cavity wall temperatures ($T_w/T_\infty$=1,2,4,8) and rarefaction regimes ($Kn=0.05,0.1,1,10,20$) using \textit{dsmcFoam} solver, based on the framework of OpenFOAM.The effects of Mach number, cavity wall temperature and rarefaction on the physics of the problem are illustrated. The flow and aerodynamic properties are found to depend strongly on $Ma$ and $Kn$ and rather weakly on $T_w/T_\infty$. [Preview Abstract] 

K18.00006: Modelling of Unsteady Ionized Flows Using a VelocitySpace Hybridization of DSMC and a QuasiParticle Solver Georgii Oblapenko, David Goldstein, Philip Varghese, Christopher Moore A recently developed Boltzmann equation solver which combines DSMC and QUIPS – a discretevelocity based quasiparticle representations of the velocity distribution function in velocity space (Oblapenko et al., J. Comp. Phys 2020109302, AIAA 20201063) has been shown to provide better accuracy in modelling highvelocity tails of the distribution function compared to pure DSMC, while improving upon discretevelocity based quasiparticle methods in terms of computational cost. In the present work, the method is expanded to incorporate ionizing collisions, and numerical studies are performed to assess its accuracy and efficiency in modelling of an unsteady flow of ionized argon with a constant external electric field. Validation is performed by comparing with the Bolsig+ solver, and comparisons with pure DSMC and pure discretevelocity methods are carried out. The influence of various hybridizationrelated parameters on the accuracy and efficiency of the computations is also studied [Preview Abstract] 
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