Deteched Eddy Simulation Of Vehicle External Flow Field

With the development of economic, high grade highway construction, increasingly stringent emissions regulations and rising of oil price, the requests of vehicles on dynamic characteristics, fuel economy, driving stability and comfort become more and more notable. The research on automotive aerodynamics has played a vital role in automotive industry.Wind tunnel is the traditional method of automotive aerodynamic research, but there are some disadvantages such as long period of experiment, costly and blocking effect. Because of above aspects, numerical simulation for automotive aerodynamic research has a rapid development. Strong full three-dimensional effect, unsteady, massive separation and complex vortical structures are the typical features of vehicle external flow field. Using RANS method is difficult to predict the flow separation and capture the details of flow field. In this paper the model of Benz A160 with full scale rate is built in STAR-CCM+ software, and the numerical simulation by using detached eddy simulation (DES) method is carried out. The main work in this paper includes:(1) Detached eddy simulation over a reference Ahmed car model has been done, and some results are compared with RANS method. It indicates that there are few differences between RANS and DES in obtaining the time-average results, but DES method is particularly suited and clearly superior to RANS method for unsteady flows and capturing the structures which contain energy in the wake.(2) The simulation process of the Benz A160 is described in detail, including meshing, boundary conditions setting and the solver configuration. Then the detailed analysis of the external flow field such as pressure distribution, velocity and turbulence kinetic energy distribution in the wake are shown.(3) The ground effect on aerodynamic coefficients, pressure distribution and wake field effects are discussed. Based on the ground effect, the wheel rotating effect of external flow field is also analyzed, including the changes of aerodynamic coefficients, air flow field in the bottom of the car body and around the wheels.(4) Comparisons between polyhedral mesh and tetrahedral mesh show that polyhedral mesh with a small number of grid cells but the results have an equivalent accuracy and a better convergence performance.(5) Fixed the ground clearance at 130 millimeter the differences around the whole vehicle with the diffuser angle of 0°,5°,10°and 15°have been simulated. Then the model of diffuser angle at 10°is chosen and the wheels are removed, a further comparative analysis among the ground clearance of 50,100,150 and 200 millimeter is made. The results indicate that the best diffuser angle of the ground clearance at 130 millimeter is between 10°and 15°, and the best ground clearance with the diffuser angle of 10°is about 100 millimeter. All above conclusions provide some reference to improve the aerodynamic performance of the car.