J. Fluid Mech. (2004), vol. 499, pp. 327–343. c  2004 Cambridge University Press DOI: 10.1017/S0022112003006980 Printed in the United Kingdom 327 The interaction of the piston vortex with a piston-generated vortex ring By JOHN E. CATER 1 , JULIO SORIA 1 AND T. T. LIM 2 1 Laboratory for Turbulence Research in Aerospace & Combustion, Department of Mechanical Engineering, Monash University, P.O. Box 31, Melbourne, Victoria 3800, Australia 2 Mechanical Engineering Department, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore (Received 10 April 2001 and in revised form 25 July 2003) This paper presents the results of an experimental investigation of the effects of a piston vortex on the vorticity evolution of a vortex ring. The rings are produced by the roll-up of a shear layer at a circular orifice in a plane wall and have a Reynolds number of 2000 based on the ejection velocity and orifice diameter. The generation mechanism is a piston moving inside a cylinder with a stroke length of two piston diameters. The experimental apparatus is similar to that used by Glezer & Coles (1990) where the piston finishes flush with the orifice, with the result that a piston vortex produced by the apparatus interacts with the vortex ring. Instantaneous velocity field measurements using cross-correlation digital particle image velocimetry reveal that the piston vortex not only increases the circulation of the ring but also creates an asymmetric vorticity distribution of the vortex core. It is found that ‘imperfect’ merging of the piston vortex with the primary vortex ring promotes the growth of an instability which leads to early transition to turbulence of initially laminar vortex rings. 1. Introduction The term vortex ring refers to a bounded region of vorticity in a fluid in which vortex lines form closed loops. The bounded region is usually referred to as the core of the ring. The vortex ring has fascinated researchers for a long time, partly due to its intrinsic properties, and partly due to its possible technological applications. The compact nature of a vortex ring also makes it ideal as a simple building block in modelling more complex flows such as the round jets described in Verzicco et al. (1997). A great deal of research has been conducted in this area over the past couple of decades, and a review of this work can be found in Shariff & Leonard (1992), and Lim & Nickels (1995). In most laboratory studies, a vortex ring is produced by moving a piston through a cylindrical tube with an open end protruding into a body of fluid. The resulting cylindrical vortex sheet, which forms at the tube exit, immediately rolls up into a vortex ring (henceforth referred to as the primary ring). However, as soon as the piston stops at the end of the generation process, a secondary vortex ring of opposite circulation is formed on the external surface of the tube exit. The secondary ring is found to affect not only the size, but also the stability of the primary ring. In an attempt to eliminate the secondary flow, Glezer & Coles (1990) designed a generator based on the setup shown in figure 1. It operates in such a way that at the beginning