Distortion and Evolution of a Localized Vortex in an Irrotational Flow

Distortion and evolution of a localized vortex in an irrotational flow. This paper examines the interaction of an axisymmetric vortex monopole, such as a Lamb vortex, with a background irrotational liow. At leading order, the monopole is advected with the background flow velocity at the center of vorticity. However, inhomogeneities of the llow will cause the monopole to distort. It is shown that a shear-diffusion mechanism, familiar from the study of mixing of passive scalars, plays an important role in the evolution of the vorticity distribution. Through this mechanism, nonaxisymmetric vorticity perturbations which do not shift the center of vorticity are homogenized along streamlines on a Ren3 time scale, much faster than the Re decay time scale of an axisymmetric monopole. This separation of time scales leads to the quasisteady evolution of a monopole in a slowly varying flow. The asymptotic theory is verified by numerically computing the linear response of a Lamb monopole to a time-periodic straining flow and it is shown that a large amplitude, @(Ren3), distortion results when the monopole is forced at its resonant frequency. 6 1995 American Institute of Physics.