Turbulent Flow Separation Control over a Two-Dimensional Ramp Using Synthetic Jets

Abstract

An experiment study of flow control using an array of three circular synthetic jets has been carried out in a separated turbulent flow over a two-dimensional ramp in a boundary-layer tunnel. The freestream velocity is 6:5 m=s and the Reynolds number based on the momentum thickness at a reference location upstream of the synthetic jet array is Re 1150. The synthetic array is operated at mean jet-to-freestream velocity ratios ranging from 0.2 to 0.5 and a reduced frequency based on the ramp height of F 0:2 and 0.6. Two-dimensional particle image velocimetry is employed to evaluate the control effect of synthetic jets at different actuator operating conditions in both timeand phase-averaged senses. Stereo particle image velocimetry is used to reveal the vortical structures thatwere responsible for theflow separation delay. It is found that at both reduced frequencies an increase in velocity ratio results in a further reduction in the height of the reverse-flow region, and somedifferences exist in the way inwhich the reverseflowresponds to jet actuation of the twodifferent reduced frequencies. It is also found that at the conditions tested in the present study, the interaction of synthetic jets with the turbulent boundary layer results in formation of hairpin vortices, which induce a further pair of streamwise vortices of the opposite sign of rotation directly underneath. It is believed that it is the streamwise vortices that produce a downwash bringing highmomentum fluid toward the wall, hence reducing the flow separation.