Active Drag Reduction of a Simplified Car Model Using a Combination of Steady Actuations

Active drag reduction of an Ahmed model with a slant angle of 25o is experimentally investigated based on a combination of steady blowing over the rear window and behind the vertical base. The Reynolds number Re examined is 0.9 × 10 2.7 × 10. Steady blowing S1 was applied along the upper edge of the rear window, which has been demonstrated to be effective in suppressing the recirculation bubble on the slanted surface. This actuation led to a drag reduction up to 12%. Steady blowing S2 was deployed along two side edges of the rear window to break the well-known longitudinal C-pillar vortices, reducing drag by around 6%. Steady blowing S3 and S4 were applied along the upper and lower edges of the base to control the upper and lower recirculation bubbles behind the base to raise the base pressure, producing a drag reduction up to 12% and 15%, respectively. The combination of the four actuations achieved an impressive drag reduction of 25%, greatly higher than any previous drag reduction reported and in fact very close to the target set by automotive industries. Introduction The global warming and fast-climbing oil price in the past few years inspire resurgence in drag reduction research for vehicles. The method of steady blowing has been proven to be effective in delaying or even eliminating two and three-dimensional flow separation [2]. It is also relatively simple from a practical and implementation point of view. Naturally, this method has been widely deployed in studies on the active drag reduction of the Ahmed model. The wake of an Ahmed model with a slanted surface of 25o, corresponding to the high drag regime, comprises a separation bubble on the rear window, a pair of longitudinal C-pillar vortices at the side edges of the slanted surface and a recirculation torus behind the vertical base [1]. Controlling the interactions between the three types of coherent structures is the key of drag reduction techniques [5]. Wassen & Thiele [9] numerically deployed streamwise steady blowing along the upper and two side edges of the rear window, and the lower and two side edges of the base, producing a drag reduction by 6.4%. Under such actuation, the upper and lower recirculation bubbles behind the base were considerably enlarged longitudinally, and meanwhile the saddle point behind the bubbles was pushed farther downstream, resulting in a rise in the base pressure by 14%. In their numerical simulation, Bruneau et al. [3] used steady blowing through slots on the two side edges of the rear window to impair the C-pillar vortices, achieving 11% drag reduction. Aubrun et al. [2] deployed an array of steady microjets along the upper edge of the rear window, blowing normal to the slanted surface, achieving experimentally a drag reduction of 14%. This actuation was shown to be effective to reduce or suppress the recirculation bubble on the rear window, resulting in an increased pressure on the slanted surface. Previous investigations on active drag reduction have greatly enriched our knowledge in the control of the Ahmed model wake ( = 25) but also raised a number of issues that have yet to be clarified. Firstly, these efforts have achieved a rather limited success; the maximum drag reduction obtained for this model is only about 14% (Aubrun et al. [2]), substantially below the target (30%) set by automotive industries. Most of the previous studies focused on controlling one of the three types of coherent structures in the wake, neglecting the other two and their interactions. In their experimental and numerical investigations on the active drag reduction of an Ahmed model, Brunn et al. [4] found that, at  = 35, the synthetic jet placed at mid of the upper edge of the rear window reduced the separated flow region but at the same time triggered the development of C-pillar vortices; at  = 25, constant blowing near the two upper corners of the rear window weakened C-pillar vortices but increased the flow separation region. As a result, no significant drag reduction was achieved. Apparently, an effective and efficient active drag reduction technique requires a combination of different actuations schemes, i.e., producing actuations at different locations and orientations, which could not only weaken C-pillar vortices but also increase the pressure over the rear window and the vertical base. Yet, the optimum combination of different actuations for achieving a larger drag reduction remains elusive. As a matter of fact, there have been few studies that deployed a combination of different actuations for the control of an Ahmed body. This work aims to develop an effective active drag reduction technique for this model. Four different actuations, all based on steady blowing, are investigated. Steady blowing (S1) is arranged along the upper edge of the rear window to suppress the recirculation bubble on the slanted surface. Steady blowing (S2) is deployed along the two side edges of the rear window to break the C-pillar vortices. Steady actuations (S3 and S4) are applied along the upper and lower edges of the base to control the upper and lower recirculation bubbles behind the base to raise the base pressure. The dependence of each individual actuation on the momentum coefficient is examined, along with different combinations of S1, S2, S3 and S4 to maximize drag reduction. The net aerodynamic power saving is also estimated for each actuation scheme. Experimental Setup Experiments were conducted in a closed-circuit wind tunnel at Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology. The test section of the tunnel is 1.0 m high, 0.8 m wide and 5.6 m long. The flow speed in the test section is uniform to 0.1% and the longitudinal turbulence intensity is less than 0.4%. A flat plate (length × width × thickness = 2.6 m × 0.78 m × 0.015 m) was placed horizontally, 0.1 m from the floor of the test section, with its leading edge 1.5 m downstream of the exit plane of the tunnel contraction. The leading edge of the plate follows a clipper-built curve to minimize flow separation. A 1⁄2-scaled Ahmed model ( = 25) was placed on the plate, with its front end 0.3m downstream of the plate leading edge. The blockage ratio of the frontal surface of the model to the rectangular test section above the raised floor was around 3.9%. The right-handed Cartesian coordinate system (x, y, z) is defined in figure 1, with the origin O at the midpoint of the lower edge of the model vertical base. The model is 0.522 m in length (L), 0.1945 m in width (W) and 0.144 m in height (H), supported by four cylindrical struts of 15 mm in diameter. The ground clearance between the model underside and the surface of the raised floor was 25 mm. 101 235 z x z y 194.5