On identification of vortical structures in turbulent shear flow

In the present work, four criteria for vortex detection in turbulent flows are considered. Three of them involve the invariants of the velocity-gradient tensor and one involves the analysis of the Hessian of the pressure. The ability of the different methods in detecting vortical structures in wall-bounded flows - hairpin vortices in particular - is analyzed with the following procedure: i) the flow of a viscous incompressible fluid in a plane channel is simulated numerically with the use of a computational code for numerical integration of the Navier-Stokes equations, based on a mixed spectral-finite difference technique (the approach of the Direct Numerical Simulation is followed); ii) a turbulent-flow database, representing the turbulent statistically steady state of the velocity field through 10 viscous time units, is assembled; iii) all the vortex-identification techniques are applied to the database. It is shown that three of the methods considered give almost equivalent results in identifying hairpin-like vortical structures in the wall region of turbulent channel flow, while one of them gives slightly different results.