On the time evolution of the local flow topology in a turbulent boundary layer

Abstract

The average rates of change of the invariants of the velocity gradient tensor (Q and R) have been determined experimentally in the outer layer of a turbulent boundary layer as a function of the invariants themselves. Subsequent integration yields trajectories in the QR plane describing the average evolution of the local flow topology following a fluid particle. The trajectories reveal inward spiralling orbits around and converging to the origin. The orbit’s period is nearly constant at 14.3δ/Ue, which may be regarded as the characteristic lifetime of the energy containing eddies in this part of the boundary layer. Furthermore, an empirical model for the average Q and R evolution is presented..