J . Fluid Mech. (1991), vol. 227, pp. 543-566 Printed in Great Britain 543 Horizontal jets and vortex dipoles in a stratified fluid By S. I. VOROPAYEV, YA. D. AFANASYEV AND I. A. FILIPPOV Institute of Oceanology, USSR Academy of Sciences, Krasikova 23, Moscow 117218, USSR (Received 23 August 1988 and in revised form 4 December 1990) When a horizontal force is applied locally to some volume of a viscous density- stratified fluid, flows with high concentration of vertically oriented vorticity (vortex dipoles) are generated. The processes of generation and evolution with time of these unsteady flows in a stratified fluid are studied. A convenient way to produce and study these flows in the laboratory is to use a submerged horizontal jet as a ‘point ’ source of momentum. The main governing parameter (the ‘force’) is easily controlled in this case. Two regimes were studied : starting jets with dipolar vortex fronts (the force acts continuously) and impulsive vortex dipoles (the force acts for a short period of time). A conductivity microprobe, aluminium powder, shadowgraph, thymol-blue and other techniques have been used to measure the velocity and density distributions in the flows. It is found that in both regimes the flows are self- similar : the lengthscale of the flows increases with time as ti for starting jets and as ti for vortex dipoles. Detailed information about the generation mechanism, kinematics and dynamics of the flows is obtained. On the basis of similarity principles a theoretical explanation of the experimental results is given. The theory is in good agreement with the results obtained. 1. Introduction and summary Axisymmetric vortex structures in a homogeneous fluid are a well-known phenomenon. They are easily formed in a viscous fluid when a force is applied locally to some volume of fluid. If a force acts for a short period of time, vortex rings are produced (see for example Batchelor 1967, photos 7.2.2 and 7.2.3). If the force acts continuously, a starting jet with a spherical vortex at its leading edge is generated (figure 1). Eventually this vortex travels away from the origin, forming a steady axisymmetric jet behind it. The steady axisymmetric jet in a homogeneous fluid was studied theoretically by Slezkin (1934), Landau (1944) and Squire (1951). The vortex dipole, a flat analogue of a vortex ring, is easily formed in a stratified fluid, where the vertical motion is suppressed by the force of gravity (figure 2). In general, it is not important what kind of physical mechanism suppresses the motion perpendicular to the direction of the applied force and makes the flow quasi-two- dimensional. It can be the force of gravity in a stratified fluid (Voropayev 1983,1987 ; van Heijst & Flor 1989), the Coriolis force in a rotating homogeneous fluid (Flierl, Stern & Whitehead 1983), the force of surface tension in a soap film (Couder & Basdevant 1986), or a magnetic field in a layer of mercury (Papaliou 1985; Nguyen Duc & Sommeria 1988). In all these cases vortex dipoles are formed easily. They are stable, have a long lifetime and are not destroyed in symmetric collisions (Couder & Basdevant 1986 ; Afanasyev, Voropayev & Filippov 1988 ; van Heijst 1989 ;