Effect of the vortex dynamics on the drag coefficient of a square back Ahmed body: Application to the flow control

A vortex generated behind a simplified vehicle induces a pressure force at the back wall that contributes to a significant part of the drag coefficient. This pressure force depends on two parameters: the distance of the vortex to the wall and its amplitude or its circulation. Therefore there are two ways to reduce the drag coefficient: pushing the vortices away from the wall and changing their amplitude or their dynamics. Both analytical studies and numerical simulations show that these two actions decrease the pressure force and consequently reduce the drag coefficient. The first action is achieved by an active control procedure using pulsed jets and the second action is achieved by a passive control procedure using porous layers that change the vortex shedding. The best drag coefficient reduction is obtained by coupling the two procedures.