Drag mitigation by steady blowing and Coanda effect on a square back Ahmed body

Abstract

The impact of converging steady blowing on the global drag past a square back Ahmed bluff body is investigated experimentally. The dynamics of the recirculation bubble are studied for a width to height aspect ratio, w/h of 1.346, with a ground clearance of g/w=0.154, and for a nominal Reynolds number of Reh=2.86×105. When unforced, the bistable wake alternates between a left and right skewed states. Adaptable Coanda devices are used in combination with side air-knife blowers to deflect steady blowing jets into a converging (boat-tail) configuration. Two jet injection angles are investigated: 45° and 90°. Planar PIV and time-history force and pressure measurements indicate that inwards steady blowing reshapes the recirculation bubbles and induces significant variations of the pressure drag. On one hand, bilateral steady blowing leads to the interruption of bistability associated with a removal of the vertical pressure gradient for velocity ratios VR≈0.35−0.4. This has a favourable impact on the total drag with a reduction up to 3%. On the other hand, the boat-tailing effect shortens the recirculation length which tends to increase drag. This counter-productive effect limits and then outweighs drag reduction for higher velocity ratios. Another symmetry breaking is brought to light for intense highly deflected forcing.