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In the past decade, the study of vortices and vortex motions "the sinews and muscles of fluid motions" (Küchemann 1965) has received impetus from problems arising in physics, engineering, and mathematics. The theory of line vortices and vortex rings is a part of modern macroscopic of liquid helium II (Donnelly & Roberts 1974) and has stimulated much work on assemblies of line vortices and the properties of vortex rings, particularly in bounded geometries. The persistent trailing vortices behind jumbo jets constitute a hazard to other aircraft, which led to a study of vortex formation by the roll-up of vortex sheets and the decay and interaction of vortex pairs. The attempt to understand turbulence in two dimensions has led to studies (following Onsager 1949) of the statistical mechanics of random arrays of point vortices.
Saffman et al. (Mon,) studied this question.