Vortex formation of a finite-span synthetic jet: High Reynolds numbers

The formation and evolution of flow structures of a high-speed, finite-span synthetic jet issued into a quiescent flow were investigated experimentally using Stereoscopic Particle Image Velocimetry for jet peak velocities up to 150 m/s. The effect of high jet Reynolds number (Re-o = 1150, 3450, and 5750, with corresponding Strouhal numbers of St = 0.215, 0.046, and 0.028, respectively) on a finite-span synthetic jet with aspect ratios of 18 and 24 were explored. It was found that the velocity and vorticity fields were greatly affected by the Reynolds number, with the lowest Reynolds number case producing the highest normalized peak velocities and vorticity. Moreover, for all three Reynolds numbers, axis switching was observed and its streamwise location increased as the Reynolds number increased. The Q-criterion was utilized for decoupling vortices from the vorticity concentrations in the flow field. This enabled the identification of vortices in the flow field and the reconstruction of the 3D vortex ring that was formed near the orifice, and to track its evolution and advection. At this range of high Reynolds numbers, secondary flow structures were observed, where these structures were formed either immediately downstream of the orifice or farther downstream, depending on the Reynolds number and the aspect ratio. It is apparent from this work that the effect of the Reynolds number has a pronounced influence on the evolution of the flow structures and, as a result, may alter the effectiveness of flow control utilizing these jets. Furthermore, investigation of the behavior of synthetic jets with low aspect ratios (AR < 25), at high Reynolds numbers (Re-o > 1000), with peak velocities exceeding 100 m/s, addresses a critical gap in the understanding the finite-span synthetic jet flow field for practical applications. (C) 2014 AIP Publishing LLC.