Reynolds Averaged Navier-Stokes (RANS) and Large Eddy simulations (LES) are performed in conjunction with wind tunnel experiments on a simplified fifth scale mock-up of a Citroen C5. Aerodynamic force and torque components, wall pressure coefficient and mean velocity profiles are compared. It is well known that current RANS based methodologies are able to correctly predict drag force components for attached flow and mild separation on realistic car geometries under uniform wind conditions but may fail in case of crosswind. During this study, it was shown that the LES can be a suitable tool for studying vehicle aerodynamics in crosswind conditions, as it provided more accurate aerodynamic force components, a better prediction of the wall pressure coefficient distribution and more accurate mean velocity profiles in the leeward region and in the wake of the vehicles. In addition, it gave insight into the unsteady behavior of the flow, a necessary feature for both vehicle stability studies and, further down the line, aero-acoustic applications. Finally, the turnaround time of the LES simulations on current Linux clusters were shown to be within a time that can be considered reasonable for production use. Animation and instantaneous flow visualizations demonstrated the highly three-dimensional, unsteady nature of the vortices that form in the vehicle vicinity. The predominant peak frequencies calculated from a Fourier transform of the pressure signal at several locations on the leeward side is equal to 110 Hz, a value very close to peak frequencies measured from current experimental investigations in progress. Finally, it should be recalled that, in the present study, only an un-detailed macro-shape of a vehicle was considered. Besides, its 1:5 scale reduced size resulted in a lower Reynolds number, and thus larger turbulence eddy scales and less constraints on the volume mesh. In order to address more realistic situations such as full scale models, more resolution will be required for future simulations. In addition, more detailed geometries can be addressed by current advanced hybrid RANS/LES modeling approaches such as Detached Eddy Simulation. In the long run, one might expect further improvements in terms of mesh-dependency and robustness from more general global RANS/LES modeling approaches currently under study.
Large eddy simulation of a semi realistic vehicle shape under crosswind conditions
Großwirbelsimulation einer halbrealistischen Fahrzeugform unter Seitenwindbedingungen
2007
14 Seiten, 17 Bilder, 1 Tabelle, 8 Quellen
Conference paper
English
Large Eddy Simulation of a Semi Realistic Vehicle Shape under Crosswind Conditions
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