Application of Detached-Eddy Simulation for Automotive Aerodynamics Development

This paper presents a complete methodology for performing finite–volume–based detached–eddy simulation for the prediction of aerodynamic forces and detailed flow structures of passenger vehicles developed using the open–source CFD toolbox OpenFOAM R . The main components of the methodology consist of an automatic mesh generator, a setup and initialisation utility, a DES flow solver and analysis and post–processing routines. Validation of the predictions is done on the basis of detailed comparisons to experimental wind–tunnel data. Results for lift and drag are found to compare favourably to the experiments, with some moderate discrepancies in predicted rear lift. Point surface– pressure measurements, oil–streak images and maps of total pressure in the flow field demonstrate the approach’s capabilities to predict the fine detail of complex flow regimes found in automotive aerodynamics. Standard DES methods can cost an order of magnitude more than traditional methods, but optimisation and automation of mesh generation, setup and solution algorithms ensure quick turn–around times. Due to the fully parallel nature of these components, the entire process can be executed in a distributed fashion. Efficient solution algorithms provide exceptional accuracy when compared to Reynolds–averaged approaches without sacrificing stability, even when the flow exhibits high Courant numbers. The proposed methodology is highly customisable, which allows for targeted developments to suit the individual needs of aerodynamics CFD. On the basis of the results presented here, the methodology is found to be appropriate and suitable for use in the industrial development process. INTRODUCTION The use of numerical simulations to predict the aerodynamic characteristics of road vehicles is now a standard practice in automotive development [1]. The current state–of–the–art makes use of either finite–volume solvers based on the Reynolds– averaged Navier–Stokes (RANS) equations or lattice– Boltzmann solvers, both in the context of commercial, proprietary computational fluid dynamics (CFD) software packages. Recent developments in the field of automotive aerodynamics have generated the need for accelerated development of the application of CFD therein. First, the steady increase in the number of different vehicle models brought to market has not been matched by a commensurate increase in wind– tunnel capacity, necessitating the use of alternatives to experimental testing, preferrably CFD. Second, the continual improvement of vehicle aerodynamics, both in terms of drag coefficient as well as lift, requires the tools of aerodynamics development to perform at ever–increasing levels of accuracy; this applies to both wind–tunnel technology and CFD. Third, CFD methods must be able to keep pace with continually shortening development cycles, placing continually increasing demands on computational efficiency and robustness. Fourth, these increasing demands require the CFD software to be specifically tailored to the needs of the application, and therefore to contain considerable know–how derived from the application, as well as to be usable flexibly on a large scale. In the present paper, we present a complete methodology for carrying out automotive aerodynamics CFD that addresses precisely these issues. The methodology is based on the open–source CFD toolbox OpenFOAM R © [2] and is applied in an industrial context to a wide range of vehicles from the brands of the Volkswagen Group Audi, Volkswagen and SEAT.