Flow separation control over a rounded ramp with spanwise alternating wall actuation

An implicit large-eddy simulation is carried out to study turbulent boundary-layer separation from a backward-facing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying out-of-phase control and in-phase control to the wall-normal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shear-layers, which is responsible for the presence of large-scale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the large-scale vortices and the downstream recirculation zone and free shear-layer is studied by examining flow statistics. It is found that in comparison with the non-controlled case, the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow in-phase control strips. An in-depth analysis shows that the delay of the flow separation is attributed to the activation of the near-wall turbulence by the in-phase control strips and the improvement of the reattachment location is mainly due to the large-scale streamwise vortices, which enhance the momentum transport between the main flow and separated region.