The effect of inclined outlet geometry on vortex rings

In this study, pressure-driven pistons were used to generate finite impulses of flow that exited round cylinders into an ambient surrounding. Flow from cylinders with inclined trailing edges was compared with flow from a flat axisymmetric cylinder that produced a round vortex ring. The piston stroke-to-diameter ratio L/D was 1, the flow Reynolds number U₀D/(2v) was 23000, and the cylinder incline angles were 0° (axisymmetric case), 14°, and 27°. Flow visualization and instantaneous PIV fields revealed a complex flow structure exiting the inclined cylinders. Initially, asymmetric vortex tubes form with angles smaller than the cylinder incline angle. Entrainment of ambient fluid on the ‘short’ cylinder side is much stronger than that on the ‘long’ side resulting in a larger initial circulation about the short side. Upstream of the vortex ring, an impulsive sweep of entrained fluid flows from the short to the long side. Some of this fluid is wrapped into the vortex tube while the rest is ejected outward past the long cylinder edge. The vortex structure moves toward the short cylinder side at an angle to the cylinder axis. Propagation angle increases, propagation speed decreases, and penetration distance decreases as the cylinder exit angle is increased. The same qualitative flow structure and trends were observed at a smaller Reynolds number of 2800.