On the identification of a vortex

A definition of a vortex in incompressible flow in terms of the eigenvalues of the symmetric tensor S² + Ω² was proposed. This definition captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers, unlike a pressure-minimum criterion. This definition was compared with prior schemes/definitions using exact and numerical solutions of the Euler and Navier-Stokes equations for a variety of laminar and turbulent flows. It accurately identifies the vortex core in flows where the vortex geometry is intuitively clear.