Experimental Investigation of Flow Separation Control Using an Array of Synthetic Jets

Abstract

An experimental study of flow control using an array of three synthetic jets has been undertaken in a separated laminar flow over a flap in water. PIV was employed to obtain the information about the extent of flow separation delay at different synthetic jet operating conditions. Laser induced florescence (LI F) flow visualisation technique was used to reveal the characteristics of the vortical structures produced by the synthetic jets which result in a delay of separation. It was obser ved that the flow separation delay is typically associated with the presence of two strea ks or three streaks of high streamwise velocity in the separating flow. Based on the resul ts from the present experiment, a parameter map indicating the flow patterns observed at different synthetic jet operating conditions is produced. In addition, a contour map showing the separation control effectiveness at the corresponding conditions is al so obtained. It was found that the twostreak flow pattern is mostly produced at a jet-to- freestream velocity ratio (VR) between 0.3 and 0.5 whereas the three-streak pattern occurs at 0.5<VR<1.5 at medium dimensionless frequencies. The LIF images reveal th at the two-streak flow pattern is caused by the legs of the hairpin vortices produced by synthetic jets whereas the threestreak pattern is caused by the longitudinal struct ures induced by the trailing legs of tilted vortex ring type structures. For the experimental c onditions tested in this study, operating the synthetic jets at a dimensionless stroke length around 2 and a VR around 0.5, which generate vortical structures with a streamwise spac ing not larger than the local boundary layer thickness before separation, would deliver th e best flow control effect with the least energy consumption.