When a horizontal force is applied locally to some volume of a viscous densitystratified 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 t½ for starting jets and as t1/3 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.

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The results of an experimental investigation on the adjustment of vortex sand ripples under shoaling waves to changing of wave conditions are presented in this paper. A large wave tank was used to generate shoaling waves. Waves with small ͑S͒, moderate ͑M͒, and large ͑L͒ intensities ͑as specified by the wave paddle excursion͒ were used to model three basic cases of cyclic variation of wave forcing, namely, M-L-M, L-M-L, and L-S-L. Depending on the forcing transitions ͑L-M, M-L, or L-S͒, three main ripple adjustment processes were identified: ͑i͒ ripple splitting, ͑ii͒ ripple regrowth, and ͑iii͒ ripple flattening. Quantitative data on the time evolution of ripple characteristics were collected using the structured light technique. The results of the observations were explained by extending a simplified physical model proposed earlier for ripples under constant wave forcing to the case of changing wave forcing.

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