Vorticity Confinement method applied to flow around an Ahmed body and comparison with experiments

In this paper, the Vorticity Confinement method (VC) is presented for the calculation of flow field around a bluff body. The VC method was originally proposed by Fan et al. ("Computing blunt body flows on coarse grids using voracity confinement", J. Fluids Engineering, 124(4):1-18, Dec 2002) for the computation of thin vortical regions in the high Reynolds number incompressible flows. Steinhoff et al. (Large Eddy Simulation: Computing Turbulent Flow Dynamics, Chapter 12: Turbulent Flow Simulations Using Vorticity Confinement: Cambridge University Press, 2006) claim that VC is capable of capturing most of the main features of high-Re turbulent flows without the massive mesh resolution needed for Reynolds-Averaged Navier-Stokes (RANS) solutions or Large Eddy Simulation (LES). The aim of the method is to capture small-scale vortical structures efficiently. The presented study assesses the VC method (namely the first formulation, referred to as VC-1, which involves first derivatives of velocity), for external aerodynamics presented here on a well documented simplified car - Ahmed bluff body with different slant angles, namely 25 degrees and 35 degrees. These slant angles were chosen because the 35 slant angle supports the boundary layer separation and therefore stronger wake (also stronger oscillations of the wake), while at the 25 degrees slant angle no separation takes part and the wake is much weaker and also more stable. The VC method was applied using different values for the diffusion parameter and confinement parameter, respectively. In addition, different a mesh size was tested. The comparison results are presented for RANS models. The RANS simulations were carried out using the k -omega SST model run in transience with second-order differencing in time and space. OpenFOAM code with wall functions was used. The experimental data used for validation were taken from Lienhart et al. (Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), Measurements MOVA- project, online database: http://cfd.mace. manchester.ac.uk/ercoftac/) from the ERCOFTAC database. Compared are velocity profiles in different sections along the Ahmed body for both 25 degrees and 35 degrees slant angles, and forces acting on the body and drag coefficient.