Nonlinear wind-up in a strained planar vortex

The response of a Gaussian vortex to a strong external strain field is examined using two complementary numerical schemes. When the strain is weak previous calculations have shown that a rebound phenomenon is operative: after enstrophy is transferred from the mean to the azimuthal component by the straining there is a reversal during which a significant fraction of the enstrophy moves back from the azimuthal component to the mean. Concomitantly the perturbation vorticity undergoes spiral wind-up and develops a short-scale radial structure which becomes ever finer with time. We show that the rebound behaviour is suppressed by strong strain and that the intricate radial structure is simultaneously inhibited. We also give some indication of the modifications that are introduced when a strongly strained vortex is allowed to relax after the forcing field is switched off.