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

抄録

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.