Yong-Jun Jang
The Journal of Mechanical Science and Technology, vol. 22, no. 5, pp.1019-1029, 2008

Abstract : The ability of non-linear eddy-viscosity and second-model-closure models to predict the flow around a simplified three-dimensional car body, known as the “Ahmed body,” is investigated with a steady RANS scheme. The principal challenge is to predict the separation from and reattachment onto the slanted rear roof portion at the slant angle 25o, which is close to the critical value at which separation is just provoked from the roof surface. At these conditions, it has been conjectured that separation is intermittent, with periodic flapping being a highly influential process. This is thus an exceptionally challenging case, especially for low-Re models, as the geometrical complexity occurs together with high- Re conditions (Re = 768,000) and highly complex flow features in the wake of the body. A 1.89M-node mesh containing 44-blocks was employed for one half of the spanwise symmetric body. The results demonstrate that the Reynoldsstress- transport model employed is able to reproduce, in contrast to all other models, the reattachment of the flow on the slanted rear surface. As a consequence, the strong streamwise vortices emanating from the sides of the body and associated with lift and circulation are also reproduced in good agreement with experimental data. The physical processes at play and the reasons for the predictive differences are discussed.

Keyword : Turbulence; Separation; Non-linear eddy-viscosity modelling; Reynolds-stress modelling