The transitory behavior of a finite span synthetic jet, following the onset of a (pulse) input signal, was investigated and characterized using hot-wire anemometry and particle image velocimetry techniques. Measurements were performed in two planes: along the centerline of the synthetic jet slit (x-z plane) and across the jet slit (x-y plane) with varying stroke lengths and Reynolds numbers. The synthetic jet parameter matrix included actuation frequencies (fact) of 300 and 917Hz, stroke lengths from 16 to 50 times the slit width, and Reynolds numbers (based on the averaged orifice velocity) between 85 and 364. The transitory evolution of the synthetic jet consists of four stages: (1) in the x-y plane, the lead vortex pair advects downstream without spreading in the cross-stream direction while in the x-z plane, two vortices with opposite senses are formed (one on each end of the slit edge); (2) the vortex pair in the x-y plane moves in the cross-stream and streamwise directions, while the edge-vortices in the x-z plane move downstream and towards the center of the jet to form an array of vorticity concentrations of opposite sense; (3) accumulation of consecutive vortex pairs in the x-y plane, while in the x-z plane, consecutive pairs of the edge vortices propagate downstream and merge with the previous vortices, resulting in a three-dimensional vortex line; and (4) the combined leading vortex pair (in the x-y plane) detaches and moves downstream while the main jet penetrates through the leading vortex pair. During this time period the vortex lines (in the x-z plane) have a higher velocity in the center than the sides. Similar transitory behavior was observed for different stroke lengths, where as the stroke length increases the transient time decreases. Moreover, the spanwise vorticity concentrations lose their coherence as the stroke length increases.
Physics of Fluids, Volume 19, Issue 19, 2007.