Investigation of Aerodynamic Performance due to the Interaction between the Rear Diffuser Angle and the Cooling Airflow Outlet

The purpose of this research is to understand the combined flow of underneath flow and engine-cooling exit flow, as well as the interaction between the rear diffuser angle and the cooling airflow outlet on aerodynamic performance. A 1/8 scale simplified notchback model with an unrestricted internal cooling airflow path was used in this study. And a series of experimental investigations were conducted into aerodynamic performance on different rear diffuser angles in the conditions of closing and opening the cooling airflow path, and on different combinations of the rear diffuser angle and the layout of cooling airflow outlet. The test results show that, after opening the cooling airflow path, the minimum drag can be obtained at a smaller rear diffuser angle; and the lift doesn't continuously decrease with the increase of rear diffuser angle, but decreases first and then increases at an angle. To gain a further understanding of the flow mechanism of the combined flow and the interaction, CFD was also employed to obtain the flow details and the distributions of static pressure on models surfaces. The simulation results indicate that, when the upper and the lower vortices on the wake region achieve a relative equilibrium, the drag reaches the minimum value. After opening the cooling airflow path, the separation region of the wake is increased, and the relative equilibrium of upper and lower vortices can be achieved at a smaller rear diffuser angle. In addition, the pressure distribution of the model under surface changes dramatically after opening the airflow path, particularly the variation tendency of pressure distribution on the surface of rear diffuser takes a marked change on different rear diffuser angles.