Active Control Of Separated Flow Over 2d Back-Facing Ramp By An Array Of Finite-Span Slotted Synthetic Jets

A wind tunnel experimental investigation of the separated flow over a back-facing ramp at 25 degrees slant angle and equipped with an array of slotted synthetic jets (SJs) actuators for active flow control (AFC) has been conducted at height-based, h, Reynolds numbers between 9.41 x 10(3) <= Re-h <= 2.55 x 10(4). Both the baseline, i.e. without control, and controlled cases were inspected by means of surface pressure measurements and planar particle image velocimetry (PIV). This study represents a fundamental investigation of the basic mechanisms promoting the mitigation of the separation by means of the SJ technology. The AFC device consists of an array of twelve finite span slotted SJs having aspect ratio equal to 15, all fed by a common resonant cavity. For the baseline cases, it is found that the streamwise extension of the separated region decreases with increasing Re-h. This behavior is due to the nature of the boundary layer upstream of the ramp, which is transitional for the lowest Re-h and turbulent for the highest one, as is corroborated by velocity PIV measurements within the boundary layer. For Re-h = 1.06 x 10(4), under actuation with momentum coefficient c(mu) = 2.75 x 10(-3) and reduced frequency F+ = 0.34, the boundary layer undergoes an increment of the wall shear stress and acceleration at its edge. Phase-locked PIV measurements unveil that coherent vortical structures energize the boundary layer producing a 65% reduction of the extension of the separation bubble with respect to the corresponding baseline case. However, these benefits degrade by increasing the Reynolds number and for lower levels of c(mu) and F+. Phase-average flow fields reveal the existence of vortical structures over the ramp which determine characteristic wavy patterns of the streamlines. The scattering of the location of the instantaneous vortices taken at fixed phases proves the existence of concentrations of turbulent structures that participate in the buildup of the turbulence.