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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.

The drag reduction device of a bluff-body was developed by slanting the rear underbody as a hip-up shape and adding flaps to the rear-end. The experiments were conducted in an open-jet low turbulence wind tunnel while the bluff-body model was varied in both slant angles and rear flap configurations. Drag forces, surface pressures around the body, and the velocity distribution in wake were measured experimentally. The surface flow at the underbody was visualized by the oil-paint method. Force measurements showed that an underbody slant with rear flaps reduced the drag force. The most effective setting occurred when the underbody was slanted three degrees with the enclosure that had the upper, side and lower flaps. In spite of the negative pressure and the trailing vortex at the underbody, the underbody slant was useful to diminish the velocity defect in wake, which led to an increase in the rear-end pressure.

The characteristics of a notchback passenger vehicle wakes have been investigated. The experiment was mainly focused on the effect of rounding trunk-deck end on the steady and unsteady characteristics of the flow structure in the wake of the vehicle. The results showed that the formation of the trailing vortices were enhanced and the turbulence intensity in the wake was increased by rounding the trunk-deck end, although the frequency of the dominant fluid motion changed little. Simultaneous measurement of the velocity in the wake showed the dominant fluid motion over the rear of the vehicle and in its wake is highly correlated with the flow that comes through its underbody.