Evolution of Local Structure of Turbulent Flow Along Pathlines

Abstract

The evolution of invariants of the velocity gradient tensor is examined to determine local topologies of flow within shear flow turbulence. In a temporal direct numerical simulation of a round jet, a large number of fluid pathlines were computed simultaneously, and values of invariants at locations along pathlines were stored. It turns out that trajectories in the invariant space, corresponding to fluid pathlines, are far more varied than those of the conditional mean field that has been determined before. Several trajectories have segments where the invariants have much larger values than that expected from their joint pdfs. Corresponding large changes are also observed in the space of the invariants of the strain rate tensor. Although less frequent, these large departures may have consequences for the evolution of turbulent flow fields.