High-order large-eddy simulation of flow over the “Ahmed body” car model

Abstract

The structure of the turbulent flow over a simplified automotive model, the Ahmed body (S. R. Ahmed and G. Ramm, SAE Paper No. 8403001, 1984) with a 25° slanted back face, is investigated using high-order large-eddy simulations (LESs) at Reynolds number Re=768000. The numerical approach is carried out with a multidomain spectral Chebyshev–Fourier solver and the bluff body is modeled with a pseudopenalization method. The LES capability is implemented thanks to a spectral vanishing viscosity (SVV) technique, with particular attention to the near wall region. Such a SVV-LES approach is extended for the first time to an industrial three-dimensional turbulent flow over a complex geometry. In order to better understand the interactions between flow separations and the dynamic behavior of the released vortex wake, a detailed analysis of the flow structures is provided. The topology of the flow is well captured showing a partial separation of the boundary layer over the slanted face and the occurrence of two strong contrarotating trailing vortices expanding farther in the wake. The interactions of these large vortices with smaller structures reminiscent of horseshoe vortices, within the shear layer over the slanted face, form large helical structures providing strong unsteady phenomena in the wake. Mean velocity fields and turbulence statistics show a global agreement with the reference experiments of Lienhart et al. (DGLR Fach Symposium der AG SRAB, Stuttgart University, 15–17 November 2000). In order to provide a deeper insight into the nature of turbulence, the flow is analyzed using power spectra and the invariant theory of turbulence of Lumley [Adv. Appl. Mech. 18, 123 (1978)].

Fluid-structure interaction analysis of rear spoiler vibration for energy harvesting potential

“…The Ahmed body and rear spoiler is enclosed in a computational domain of size 8L × 5H × 5W in the streamwise, transverse and spanwise directions, respectively (where L = length, H = height and W = width of the Ahmed body). The size of the computational domain is consistent with the previous studies (see for example, [18][19][20]), where the blockage effects are minimized. The standard dimensions for the Ahmed body with the rear slant angle  = 35 o were used, as shown in Figure 1(b), which may be found in [17,18].…”

“…In order to model the flow and to facilitate the fluid grid movement as the fluid boundary changes with the interfacing rear spoiler motion, the unsteady ReynoldsAveraged Navier-Stokes (RANS) equation is solved together with the conservation of mass, in an Arbitrary Lagrangian-Eulerian description [21]. Previous studies have shown that RANS-based models are still appropriate in comparison with the experimental results, for a rear slant angle of 35 o [20]. In the present study, we employ a k- SST (shear stress transport) turbulence model in order to simulate the turbulent flow.…”

Assessment of RANS and DES methods for realistic automotive models

“…The most prominent unified hybrid approach is Detached Eddy Simulation (DES) proposed by Spalart et al [39]. Indeed, the DES of Menter and Kuntz [29,30] and Kapadia et al [20] show globally much better results than the ones provided by RANS approaches. They resolve in particular for u = 25°the vortical structures in the wake as well as in the shear layer between the freestream and the recirculation but do not recover the partial detachment on the slant.…”

“…More recently, a number of LES & DES studies have been performed on this case, mainly at the more challenging 25 o slant angle [22,28,29,30,26,31,32,27,19]. These studies were performed with a range of sub-grid scale models and wall-treatments and, while some were more successful than others, many failed to capture fully the correct recirculation region.…”

“…Several of these employed Reynolds-Averaged Navier-Stokes (RANS) models such as [10,6,4,29,12]. A review on this topic by F. Menter can be found in [25].…”