A mixing layer is formed by bringing two streams of water, moving at different velocities, together in a lucite-walled channel. The Reynolds number, based on the velocity difference and the thickness of the shear layer, varies from about 45, where the shear layer originates, to about 850 at a distance of 50 cm. Dye is injected between the two streams just before they are brought together, marking the vorticity-carrying fluid. Unstable waves grow, and fluid is observed to roll up into discrete two-dimensional vortical structures. These turbulent vortices interact by rolling around each other, and a single vortical structure, with approximately twice the spacing of the former vortices, is formed. This pairing process is observed to occur repeatedly, controlling the growth of the mixing layer. A simple model of the mixing layer contains, as the important elements controlling growth, the degree of non-uniformity in the vortex train and the ‘lumpiness’ of the vorticity field.

A turbulent field is produced with an oscillating grid in a deep, rotating tank. Near the grid, the Rossby number is kept large, 0(3-33), and the turbulence is locally unaffected by rotation. Away from the grid, the scale of the turbulence increases, the r.m.s. turbulent velocity decreases, and rotation becomes increasingly important. The flow field changes dramatically at a local Rossby number of about 0.20, and thereafter remains independent of depth. The flow consists of concentrated vortices having axes approximately parallel to the rotation axis, and extending throughout the depth of the fluid above the turbulent Ekman layer. The number of vortices per unit area is a function of the grid Rossby number. The local vorticity within cores can be a factor of 50 larger than the tank vorticity 2Ω. The total relative circulation contained in the vortices remains, however, a small fraction of the tank circulation.The concentrated vortex cores support waves consisting of helical distortions, which travel along the axes of individual vortices. Isolated, travelling waves seem well-described by the vortex-soliton theory of Hasimoto (1972). The nonlinear waves transport mass, momentum and energy from the vigorously turbulent region near the grid to the rotation-dominated flow above. Interactions between waves, which are frequent occurrences, almost always result in a local breakdown of the vortex core, and small-scale turbulence production. Usually the portions of broken core reform within ½−1 rotation periods, but occasionally cores are destroyed and reformed on a much longer timescale.

FIGURE 1. Sketch of the experimental apparatus. FIGURE 2. Initial shear layer geometry: initial Richardson number, 0.05 < Ri < 0.2; initial Reynolds number, Re x 300; maximum Reynolds number, 300 < R+*, < 1900; hai/7ii %-1. difference Ap. The shear layer has an initial vertical length scale (maximum slope thickness) hi and the corresponding density scale is denoted as ri. The mean convection speed of the fluid in the channel is 8, the average density is pAV, and the gravity, g, acts vertically downward. Note that the centre of the shear region is displaced vertically by a small amount, 6, from the density interface. Consistent with the Boussinesq approximation, the density difference appears only in connexion with the gravity field as gAp/pAv. Including the kinematic viscosity, v, and the diffusion coefficient for salt D, the flowfield is uniquely determined by the six non-dimensional quantities Ri = gAph,/pAv(AU)a; Re = AUhi/v; SC = v / D ; AUlD; €/hi; qilh,. Ri and Re are the initial Richardson number and Reynolds number and measure respectively the importance of buoyancy and viscosity upon the developing instability. hi is the maximum slope thickness of the shear layer a t x = l cm. The value of