ON THE BREAKDOWN OF VORTEX RINGS FROM INCLINED NOZZLES

In a recent experimental investigation, Webster and Longmire [Phys. Fl uids 9, 655 (1997)] reported that the large-scale jet structures from inclined nozzles, which consisted of continuous inclined vortex rings, would undergo breakdown if the inclined angle of the nozzle was suffi ciently large. They attributed the breakdown to the presence of longit udinal vorticity, but did not elaborate on the mechanism involved. In this paper, we examined the above issue by focusing primarily on the l arge-scale structures of the inclined jet (i.e., the inclined vortex r ings). To the author's knowledge, this area of research remains relati vely unexplored. A study of it would certainly help to shed light on t he mechanism involved in the breakdown of the inclined jets. Here we i nvestigated the effects of the Reynolds number, the nozzle's inclined angle, and LID (see below) on the evolution of inclined vortex rings. Nozzles with the inclined angle of 5 degrees, 10 degrees, 20 degrees, and 35 degrees were considered, and the Reynolds number of the flow ra nged from 1447 to 4824. The LID was varied from 0.77 to 1.92, where L is the length of the slug of fluid ejected through the nozzle of diame ter D. The results showed that when inclined vortex rings were formed, they were subjected to a differential rate of vortex stretching, due in part to a nonaxisymmetric vortex roll-up. As a result, circumferent ial flow was produced in the vortex core which increased with the nozz le angle and the Reynolds number. If sufficiently large, the circumfer ential flow was found to lead to core breakdown by initiating a wavy i nstability in the vortex filament which subsequently developed into a ''bubble-type'' breakdown and then a ''double-helix''-type breakdown b efore the ring disintegrated into a chaotic motion. A simple physical model describing this transition process is proposed. However, if the circumferential flow was low, core bulging (or core swelling) might oc cur instead. A breakdown chart plotted using the Reynolds number and L ID for different nozzle angles is presented. The chart enables one to determine the flow conditions under which the core breakdown would occ ur. (C) 1998 American Institute of Physics.