Experimental analysis of the role of base blowing geometry on three-dimensional blunt body wakes

Abstract

This experimental study aims to investigate the effect of different base blowing configurations on the aerodynamics of a squareback Ahmed body of height h, width w and aspect ratio w/h>1. Four different slot configurations through which air is injected were studied. Each configuration had the same blowing area, equivalent to 10\% of the base area of the body, and was designated according to its geometric shape: square (S), vertical (V), cross (C) and horizontal (H). The same range of injected flow rates, Cq, was tested for each slot geometry at a Reynolds number Re = 65000, with corresponding wind tunnel measurements of aerodynamic forces, base pressure and wake characteristics using 2 Dimensions - 2 Components Particle Image Velocimetry (2D-2C PIV). Our experiments revealed the effect of blowing on the near wake and consequently on the base pressure and drag of the Ahmed body. In particular, the geometry of the slots was shown to be a crucial factor in influencing aerodynamics, especially at blowing flow rates close to Cq,opt, which is the blowing flow rate that provides the minimum drag coefficient. The centered square slot (S) is the configuration that achieves a greater drag reduction. This behavior is attributed to an elongation in the recirculation region behind the Ahmed body, a reduction in the backflow inside the recirculation bubble, and a decrease in the wake asymmetry associated with the Reflexional Symmetry Breaking (RSB) mode. Conversely, the vertically oriented slot geometries, such as the cross (C) and the vertical (V) configurations, showed limited drag reduction capability while maintaining or even intensifying the wake asymmetry. The horizontal (H) slot represented an intermediate case, mitigating the wake asymmetry to a large extent, but proving to be less effective in reducing the drag than the square case. The hierarchy of the blowing configurations was dictated by the modifications induced in the near wake behind the Ahmed body and the distance from the blowing injection to the shear layers enclosing the recirculation region, which influenced the asymmetry of the wake and the filling/emptying of the recirculation region, respectively.