Orbitally compact and loose vortex regions

The measure of orbital compactness of the motion of swirling material points inside a vortex is formulated without the assumption of incompressibility. Orbitally compact and loose vortex regions are distinguished. Their boundary is set very permissively and expressed through the introduced measure of orbital compactness. The present analysis is associated with the vortex-identification local trace criterion and explains its limited applicability. The comparison of compact and loose volumetric ratios for selected vortex-identification methods employs numerical data of four flow situations. The investigation shows that some vortex-identification criteria are too permissive, more or less ignoring the inherent vortex property of orbital compactness.