Active drag reduction of a high-drag Ahmed body based on steady blowing

Abstract

Active drag reduction of an Ahmed body with a slant angle of $25^{\circ }$, corresponding to the high-drag regime, has been experimentally investigated at Reynolds number $Re=1.7\times 10^{5}$, based on the square root of the model cross-sectional area. Four individual actuations, produced by steady blowing, are applied separately around the edges of the rear window and vertical base, producing a drag reduction of up to 6–14 %. However, the combination of the individual actuations results in a drag reduction 29 %, higher than any previous drag reductions achieved experimentally and very close to the target (30 %) set by automotive industries. Extensive flow measurements are performed, with and without control, using force balance, pressure scanner, hot-wire, flow visualization and particle image velocimetry techniques. A marked change in the flow structure is captured in the wake of the body under control, including the flow separation bubbles, over the rear window or behind the vertical base, and the pair of C-pillar vortices at the two side edges of the rear window. The change is linked to the pressure rise on the slanted surface and the base. The mechanisms behind the effective control are proposed. The control efficiency is also estimated.

Active Flow Control on a Square-Back Road Vehicle

“…The asymmetric forcing, obtained combining lower and lateral jets, leads to significantly better results with DR% of about 12.7% for V lat /V j = 1.4 and V low /V j = 1.2. The values of DR% found in the present investigation are in good agreement with those typical for continuously blowing jets [16,24].…”

“…In the latter case, it is fundamental to ensure that the boundary layer thickness growing on the floor is small compared to the clearance h. This is usually achieved by either mounting the model on a plate, where the boundary layer origin is fixed, or by providing a suction of the boundary layer. Stationary floor has been employed in several cases in the past (e.g., [8,16,25,30,31]). The mounting solutions can be differentiated as follows: a first solution where the model is connected to the floor through pins that very often correspond to the wheels and a second where a holding strut is located above the model.…”

“…The upper jet activation seems to be detrimental. A recent investigation by Zhang et al (2018) [16] showed that the blowing angle covers a fundamental role in defining the effectiveness of the flow control system and, more particularly of the top edge jet. In the present case, the choice of the blowing angle was set in a symmetric configuration and the optimization of this parameter is beyond the scope of the paper.…”

“…where Q indicates the total volumetric flow rate, A i indicates the cross-section area of the ith jet and V j,i the blowing speed of the ith jet. Equation (5) is obtained by extending the definition given in many previous investigations [16,24,46] to account also for the losses in the pneumatic line (∑ p losses · Q). We do this by estimating the losses in the pipings enclosed within the model using empirical relations [47].…”

“…Steady jet blowing located near the four edges of a square back model rear face was also studied by Zhang et al (2018) [16]. The model was an Ahmed body presenting a slant angle of 25 • on the top edge.…”