Dipolar vortices in a strain flow

The evolution characteristics of dipolar vortices in a strain flow were investigated both experimentally and theoretically. The laboratory experiments were performed in a stratified fluid, the strain flow being generated by four rotating horizontal discs, whereas the dipolar vortex was created by a pulsed injection of a small amount of fluid. Dye-visualization studies and particle-tracking techniques were used to obtain qualitative and quantitative information about the horizontal flow field. Depending on the initial orientation of the dipole, either a head–tail structure or a pair of elliptic-like monopolar vortices was formed. In the former case, the distance between the vortex centers was observed to remain nearly constant due to the opposing effects of strain and lateral diffusion, while in the latter case, the vortex centers were passively advected by the ambient flow. The head–tail formation could be explained kinematically by a simple point-vortex model. Full-numerical simulations based on the quasi-two-dimensional vorticity equation revealed a very good agreement with the laboratory observations.