Stretching response of Rortex and other vortex-identification schemes

By considering a uniaxial stretching coupled with an inevitable uniform radial contraction for incompressible flow, a straightforward comparison of the stretching response for several popular vortex-identification criteria and the recently proposed vortex vector (Rortex) is presented. In addition, the outcome of the triple-decomposition method in terms of the residual vorticity tensor is employed due to its planar coincidence with Rortex. The stretching sensitivity of the examined schemes significantly differs and, consequently, reopens the persisting vortex-identification problem that the requirement of orbital compactness of the motion inside a vortex contradicts with the allowance for an arbitrary axial strain.