Methods for Vortex Identification

Abstract: Initially, this paper presents an overview of the existing methods used for the identification of vortices in fluids, attempting to point out their different strengths and weaknesses. In addition, a new method is proposed to identify vortices directly from velocity field information. It defines a point inside a vortex as a point around which there is rotation. By looking at the velocity directions in the neighbourhood of a point we decide if there is rotation. A measure of the strength of rotation at the point is also introduced based on the magnitude of the velocity components in the neighbourhood. The method is tested on simulated data and performs well. Finally a Lagrangian identification method (Direct Lyapunov Exponents, DLE) is used to attempt to calibrate the most widely used Eulerian method in order to match an objectively defined edge. The results here indicate that it might not be possible to find a threshold for this method to match the edges found using the DLE method.