On simulating the turbulent flow around the Ahmed body: A French–German collaborative evaluation of LES and DES

Assessment of RANS and DES methods for realistic automotive models

“…However at 25 o (where the complex interaction between the counter-rotating vortices and the separated flow results in a shorter separation region) the majority of RANS models,regardless of mesh refinement or wall treatment, failed to predict the flow correctly. In general, they either failed to predict separation completely, or even when they did, they did not predict the correct separation point and thus were unable to capture the correct size of the recirculation region due to an under-prediction of the turbulent stresses [22].…”

“…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.…”

“…The Ahmed car body [1][2] represents a generic car geometry with a slanted back and a flat front and has been extensively tested in the literature [20,21,22,23,24]. While it is a much simplified version of a real car, it nevertheless provides many of flow features found in real-life cars such as the complex vortex interactions that occur in its wake and the large 3D separation region behind the car body itself.…”

“…This case has been the focus of several CFD investigations [20,21,22,25,26,27,19] where a range of RANS, LES and hybrid RANS-LES models were investigated. The general conclusions from the studies involving RANS models [20,21,25,27,19] were dependant on the slant angle.…”

Dealing with numerical noise in CFD-based design optimization

“…[29,30] by the slant angle, , which is kept constant at 30º. Other steady-state numerical studies cite poor results for smaller slant angles; when the slant angle is reduced to 25º for example, partial detachment of the flow behind the slant provides small scale structures which are inherently unsteady and these cannot be captured by steadystate approaches which often lead to poor results [31]. In order to simulate the flow field around the vehicle, a solid model was generated using Ansys Design Modeler (version 13.0) [32] based on the dimensions shown.…”