Drag Force Reduction of a Bluff-Body with an Underbody Slant and Rear Flaps

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. Since the upward flow from the slanted underbody was blocked by the rear flaps, the vortex shedding into wake was repressed. In conclusion, in order to reduce the drag force of a bluff-body with the underbody slant, it is important to suppress the trailing vortex formation at the slanted underbody and the vortex shedding into wake as well as to raise the lowered pressure.