On the breakdown of vortex rings from inclined nozzles

Abstract

In a recent experimental investigation, Webster and Longmire [Phys. Fluids 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 sufficiently large. They attributed the breakdown to the presence of longitudinal vorticity, but did not elaborate on the mechanism involved. In this paper, we examined the above issue by focusing primarily on the large-scale structures of the inclined jet (i.e., the inclined vortex rings). To the author's knowledge, this area of research remains relatively unexplored. A study of it would certainly help to shed light on the mechanism involved in the breakdown of the inclined jets. Here we investigated 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°, 10°, 20°, and 45° were considered, and the Reynolds number of the flow ranged 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 diameter 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, circumferential flow was produced in the vortex core which increased with the nozzle angle and the Reynolds number. If sufficiently large, the circumferential flow was found to lead to core breakdown by initiating a wavy instability in the vortex filament which subsequently developed into a "bubble-type" breakdown and then a "double-helix"-type breakdown before 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 occur instead. A breakdown chart plotted using the Reynolds number and LID for different nozzle angles is presented. The chart enables one to determine the flow conditions under which the core breakdown would occur. © 1998 American Institute of Physics.