Effects of Aspect Ratio on Vortex Dynamics in Starting Rectangular Jets

The vortex dynamics of starting rectangular jets with aspect ratios of 1, 4, 8, and 10 is investigated numerically at Reynolds number of 1000. The 45°-axis-switching for the square vortex ring and 90°-axis-switching for the rectangular vortex ring are found to be controlled by the secondary flow induced by the ring deformation. Vortex stretching appears near both the x–y and x–z centerline planes, which is weakened by the increasing aspect ratio. Vortex ring pinch-off on both centerline planes can be identified by the magnitude of the vorticity flux delivered into the vortex ring. The dimensionless time for pinch-off (the formation numbers) for the square jet is at about 2.0 on both diagonal plane and midplane. For the rectangular jets, the formation number is reduced from 2.3 to 1.5 on the major axis plane by increasing aspect ratio, while it approaches 1.7 on the minor axis plane, which is independent of the aspect ratio. Our results suggest that the vortex deformation accelerates the pinch-off process on both the major and minor axis planes. In particular, the appearance of secondary vortices on the major plane and the arc-shaped vortices on the minor plane signals the onset of pinch-off.