Practical considerations in the identification of vortices amidst vortex interactions

An isolated vortex is characterized by two features: the vorticity distribution and the concomitant strain distribution. Any vortex identification criterion determines the size of a vortex based on the relative variation of these two fields. In a flow composed of interacting vortices, the interaction modifies the distribution of vorticity and strain, and therefore affects the identification of these vortices using a vortex identification criterion. Considering flows resulting from superposition of vortices whose isolated fields are known \emph{a priori}, we study the influence of vortex interactions in the context of using local vortex identification parameters---$\lambda_{ci}$, $\lambda_{cr}/\lambda_{ci}$, $\Delta$, $Q$, and $\lambda_2$---to extract the individual vortices. Our results give useful guidelines for the application of local vortex identification criteria in complex flows.