The strain rate in evolutions of (elliptical) vortices in inviscid two-dimensional flows

In this paper the strain rate in evolutions of elliptical vortices in inviscid two-dimensional flows is considered. Previous work [Vosbeek et al., Phys. Fluids 9, 3315 (1997)] has revealed that there exists a relationship between the (in)stability of an elliptical vortex of uniform vorticity (Kirchhoff vortex) and the spatial distribution of the strain rate. It is examined here how the strain distribution evolves in time for unstable Kirchhoff vortices. Furthermore, it is shown that there also exists a relationship between strain and (in)stability of a vortex consisting of nested elliptical patches representing a more general vorticity distribution.