Numerical Simulation of the Flow in the Wake of Ahmed Body Using Detached Eddy Simulation and URANS Modeling
The flow around Ahmed car body is studied for a 35° slant angle case and 25° slant angle case. For the steady 35° case, RANS modelling have shown good results but is not efficient in the 25° configuration, beeing unable to capture the detachment of the fl
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1 Institut de mécanique des fluides de Toulouse, France Institut de mécanique des fluides et solides de Strasbourg, France
Abstract The flow around Ahmed car body is studied for a 35° slant angle case and 25° slant angle case. For the steady 35° case, RANS modelling have shown good results but is not efficient in the 25° configuration, beeing unable to capture the detachment of the flow over the slant of the body. Then, DES and DDES modelling are performed on the 25° configuration showing better results.
1 Introduction
Drag Coefficient
Because of the complexity of cars aerodynamics and in order to simplify studies, Ahmed car body has become reference geometry. Past experimental studies have
Fig. 1. Flow in the wake of the Ahmed car body (left, courtesy of S. Becker and H. Lienhart, LSTM Erlangen), and drag coefficient versus slant angle φ (right; from Ahmed and al., 1984) S.-H. Peng and W. Haase (Eds.): Adv. in Hybrid RANS-LES Modelling, NNFM 97, pp. 125–131, 2008. springerlink.com © Springer-Verlag Berlin Heidelberg 2008
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G. Martinat et al.
shown that the topology of this complex fully three dimensional flow is dependant from the slant angle of the geometry considered. When the slant angle is below 30 degrees, the flow has an unsteady topology. Two vortices are created on the side edges of the slant. Over the slant, the flow separates and reattaches later on the slant. Two counter-rotating vortices are created on the rear face of the body. When the slant angle is above 30 degrees, the vortex over the slant doesn't reattach and is then more intense as the sides vortices. For this case, the flow remains steady. Figure 1 shows the evolution of average drag coefficient with the slant angle. We can notice the discontinuity in drag variation for slant angle above 30°. In this study, we will concentrate on the 25° and 35° slant angle geometries which are well documented DESIDER and Ercoftac test cases. 35° case will be studied with URANS modelling where for 25° case, 3 URANS turbulence modelling will be compared to DES and DDES modelling. In both cases, Reynolds number is 768000, based on the body height and is the same as for the experimental study from Lienhardt et al, 2000.
2 DES Modelling As said in Travin and al, 2000, “A Detached-Eddy Simulation is a three-dimensional numerical simulation using a single turbulence model, which functions as a sub-grid scale model in regions where the grid density is fine enough for a Large-Eddy Simulation, and as a Reynolds-Average model in regions where it is not”. The DES length scale is chosen according to the following equation:
lDES = min ( lRANS , CDES × Δ ) where CDES is the DES constant calibrated by means of homogeneous, isotropic turbulence spectrum, and Δ is the largest dimension of the elementary control volume cell, Δ=max(Δx,Δy,Δz). For the one equation Spalart-Allmaras model (Spalart and Allmaras, 1993), it gives:
lDES = min ( dω , CDES × Δ ) where dω is the distance from the wall. The consequence of the length scale modification is an increase of the di
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