Effect of stroke on structure of vortex ring array in circular synthetic jets

Abstract

Array of vortex rings in circular synthetic jets was investigated experimentally by instantaneous velocity measurements using a hot-wire anemometer. The ensemble averaged velocities were derived by referencing the oscillatory flow in a circular orifice. The contours of the ensemble averaged velocity showed that the elliptical shaped high-velocity regions were generated per a cycle of the oscillatory flow. These high-velocity regions were caused by vortex rings, consequently their travelling and disappearance in the downstream indicated the convection and collapse of the vortex rings. The structures of vortex array were examined for seven dimensionless strokes of the oscillatory flow range of L/d = 0.712-7.10, where the stroke L was the length of the fluid ejected in an oscillatory cycle and d is the diameter of the orifice. It was found that vortex rings with nearly equal intervals along the jet axis travelled downstream, and collapsed in a further downstream location. In the process of vortex collapse, no direct interactions with the neighbouring vortices were observed. For the dimensionless strokes larger than 3.6, it was observed the vortex ring accompanied weak and small trailing vortex rings. It was also found that dimensionless convection velocities of vortex ring were independent of the stroke, but the spatial intervals of vortex rings increased as the stroke was increased. As the stroke was increased, the location of the vortex collapse moved downstream. The correlation between the location of the vortex collapse and the starting point of jet width growth was good. The effect of the stroke on the jet evolution was attributed to the change in the location of the vortex collapse.

Recent advancements toward the understanding and control of turbulent flows (Summary of ICJWSF2013)

“…To use synthetic jets effectively, the understanding of the fundamental features is important. The experimental studies on the synthetic jet actuator using bubbles produced by electric discharge , on the effect of strokes on the vortex ring structures (Koso, et al, 2013), and on the motion and deformation of vortex rings evolved in a synthetic jet ejected into a crossflow (Miyakoshi and Hasegawa, 2013) are presented. The flow characteristics, focusing on the streamwise mean velocity along the jet centerline, are clarified numerically by a discrete vortex method .…”

Flow characteristics of two-dimensional impinging synthetic jets

“…For instance, steady continuous jets have been used to control flow. Recently, synthetic jets have been widely applied to flow control and to the driving source of small fluid machinery, replacing continuous jets to downsize and reduce the weight of equipment [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Regarding the fundamental behavior of synthetic jets, Nishibe et al [8] compared synthetic and continuous jets, finding that the flow characteristics of two-dimensional synthetic jets under the free-jet condition in sufficient downstream are similar to those of continuous jets, including the unsteady characteristics.…”