Axial stretching and vortex definition

Four existing invariant criteria for defining a vortex are diagnosed analytically and demonstrated by the Burgers and Sullivan vortices. Two of the criteria are found disqualified as generally applicable, and one underestimates the core radius. The diagnosis leads to a general requirement for any possible criterion: it should be able to identify the vortex axis and allow for arbitrary axial strain. (C) 2005 American Institute of Physics.