On the steady and unsteady turbulence modeling in ground vehicle aerodynamic design and optimization

Computational Fluid Dynamics is nowadays largely employed as an effective optimization tool in the automotive industry, especially for what concerns aerodynamic design driven by critical factors such as the engine cooling system optimization and the reduction of drag forces, both limited by continuously changing stylistic constraints. The Ahmed reference model is a generic car-type bluff body with a slant back, which is frequently used as a benchmark test case by industrial as well as academic researchers, in order to investigate the performances of different turbulence modeling approaches. In spite of its relatively simple geometry, the Ahmed model possesses many of the typical aerodynamic features of a modern passenger car - a bluff body with separated boundary layers, recirculating flows and complex three-dimensional wake structures. Several experimental works have pointed out that the flow region which presents the major contribution to the overall aerodynamic drag is the wake flow behind the vehicle model: therefore, a more exact simulation of the wake and separation process seems to be essential for the accuracy of numerical drag predictions. As a consequence, a significant effort has been put in many computational studies carried out on the Ahmed model in the last two decades, in order to fix benefits and deficiencies of various turbulence modeling practices, from the steady-state RANS approach to the fully unsteady LES approach. Though now there are some generally accepted remarks, such as the difficulties encountered by some classical steady-state RANS models in giving accurate results for some critical flow regimes, it is authors' opinion that there are still some issues that need to be addressed, particularly for what concerns the differences and the possible improvements related to the passage from steady to unsteady approaches. In this paper a numerical investigation of turbulent flow around the Ahmed model, performed with the open-source CFD toolbox OpenFOAM®, is presented. Several URANS turbulence models, as well as different wall treatments, have been extensively tested on the notoriously critical 25° rear slant angle configuration of the Ahmed body. Simulations with the same models, but run in steady-state RANS mode, have been also provided in order to evaluate which kind of approach could be the best compromise as a sufficiently accurate and time-saving optimization tool for ground vehicle aerodynamic design. Drag predictions and other flow features, especially in terms of velocity profiles visualization in the rear region, have been critically compared with the experimental data available in the literature and with some prior numerical studies.