Effect of phase difference on the interaction of hairpin vortices induced by in-line twin synthetic jets

An experimental study is carried out to investigate the effect of operational phase difference of in-line twin synthetic jet actuators (SJAs) on the interaction of synthetic jet-induced hairpin vortices. The resulting vortex structures at four phase differences, i.e., Δϕ = 0°, 90°, 180°, and 270°, are presented and compared using both stereo dye visualization and particle image velocimetry (PIV) measurements. Three types of vortex structures are observed: one combined vortex at Δϕ = 90°, two completely separated hairpin vortices at Δϕ = 270°, and partially interacting vortex structures at Δϕ = 0° and 180°. The combined vortex is the strongest and most penetrates into the boundary layer. The completely separated hairpin vortices are the closest to the wall and hence are able to exert the most influence in the near-wall region. As for the partially interacting vortex structures, the head of one hairpin vortex interacts with legs of the other, producing complex vortex structures. Through this study it is also found that hairpin vortices issued from the upstream SJA are able to maintain their coherence more easily than their counterparts issued from the downstream SJA, regardless the phase difference. The secondary vortices captured by the PIV measurements are also compared.