Aerodynamic drag reduction of an Ahmed body based on deflectors

The effects of deflectors on the aerodynamic drag and near wake of an Ahmed model with a 25 degrees slant angle were experimentally investigated. Experiments were conducted in a closed circuit low speed wind tunnel at a Reynolds number of 8.7 x 10(5), based on the free stream oncoming velocity (U-infinity) and model length (l). The deflectors were placed at the side edges and leading edge of the slant. The height or width of the deflectors was 1%, 2% and 3% of l. Pressure distribution on the rear slanted surface and the vertical base was measured using pressure scanners. The flow velocities and static pressure were measured by Cobra probe at 0.5 l and l downstream in the near wake. Oil film flow visualization technique was deployed to capture the flow structure on the slant. In the absence of control, shear layer separation from the side edges and leading edge of the slant results in remarkable local low pressure regions on the slant, which contribute significantly to the aerodynamic drag. There is a D-shape separation bubble on the slant and the near wake is dominated by one pair of counter rotating trailing vortices. When the height of the deflector placed at the side edges of the slant is 1% of l, its effects on the near wake are negligibly small and the corresponding drag is increased slightly, compared to the uncontrolled case. With the deflector height increased to 2% and 3% of l, the D-shape separation bubble and the trailing vortices were noticeably suppressed, with the drag reduction reaching 3.5% and 7.2%, respectively. The deflector at the leading edge of slant changes the near wake of the model similar to that behind an Ahmed model of larger slant angle (e.g. 30 degrees or 35 degrees), eliminating the D-shape separation bubble on the slant. The corresponding drag reduction reaches 93%, 10.7% and 10.9% for the deflector width of 1%, 2% and 3% of l, respectively. For the 25 degrees slant Ahmed model, the deflector placed at the leading edge of the slant is more efficient in reducing drag and suppressing the trailing vortices than those at the side edges of slant. (C) 2015 Elsevier Ltd. All rights reserved.