Determination of the threshold value of the quantity chosen for vortex representation in turbulent flow

In recent times much work has been devoted to the issue of vortex definition in turbulence. Several criteria for vortex identification have been devised and techniques for defining the correspondence between different criteria have been found, leading to the possibility of representing nearly-equally intense vortical structures using different identification methods. The issue remains open of calculating a threshold value of the quantity chosen for vortex eduction, being able to give a physically based representation of the vortical structure at hand. For this purpose we present a direct method based on some statistics of the velocity field, applied in particular to the issue of representing a hairpin vortex in the wall region of turbulent channel flow.