G. J. F. van Heijst, R. C. Kloosterziel, and C. W. M. Williams
University of Utrecht/Eindhoven University of Technology, The Netherlands

The photographs displayed above show the formation of a tripolar vortex from an unstable axisymmetric vortex in a solidly rotating homogeneous fluid. The tripole is a symmetric, linear arrangement of three patches of distributed vorticity of alternate signs, and the axis of the vortex configuration rotates steadily about the center of the core vortex. In the experiment the initial vortex is an isolated cyclonic vortex, consisting of a core of cyclonic relative vorticity surrounded by a ring of anticyclonic relative vorticity. Under certain conditions such a vorticity distribution is unstable to perturbations of azimuthal wave number m. The photographs show an experiment in which m = 2 is the most unstable mode, and the flow structure is observed to show a gradual transition into a stable tripolar vortex.

Figure 1(a)

Figure 1(b)

Figure 1(c)

Figure 1(d)

The tripole formation process is visualized by adding dye to the fluid. In the experiment shown here two different dyes were used in order to demonstrate that the tripole s satellite vortices result from a pure rolling-up of the outer ring of anticyclonic vorticity, whereas the vortex core (containing the cyclonic vorticity) is hardly affected by the instability, its shape only changing from circular to elliptical.

The (plan view) photographs were taken by a corotating camera at times t=3.9 T (a), 7.5