Vortex core line

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A vortex core line is a line-like trace that marks the center of a vortex in a fluid’s velocity field. Many methods exist to detect these lines. A 2004 study by Jiang, Machiraju and Thompson reviewed nine vortex-detection methods, five of which target vortex core lines. One common method, from Sujudi and Haimes (1995), says a point lies on a core line if the velocity vector there is an eigenvector of the velocity gradient tensor, with the other eigenvalues being complex. Another widely used approach is the Lambda2 method, which is Galilean invariant, so adding a uniform flow doesn’t change the result.