Statistics and growth rates of high-amplitude vorticity events in turbulence
Seminar Room 1, Newton Institute
Fluid turbulence is often characterized as a tangle of many intermittent vortices embedded in regions of straining motion. Although there have been many experimental and numerical studies on the evolution of isolated intense vortices, pairs of them or on the kinematics of ensembles of randomly distributed vortices, not much is known about their dynamic evolution in a fully turbulent flow. We present a high-resolution numerical simulation that monitors the formation and time evolution of high-amplitude vorticity regions. In order to identify and follow these events, we track the turbulence fields in local Cartesian frames of reference which move with Lagrangian tracers through the fluid. The local enstrophy -a measure of vorticity- shows temporal growth compatible on average with a classical prediction by Howarth and von Karman (1938). It remains well below a recently predicted rigorous upper bound for the enstrophy growth (Lu and Doering, 2008). However locally, enstrophy growth rates are detected that go beyond the mean trend and approach the predicted global bound.
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