Extreme events and small-scale structure in computational turbulence

Detailed analyses have been made of data from a direct numerical simulation of turbulence on a periodic domain with $8192^3$ grid points designed to improve our understanding of small-scale structure and intermittency. At the Reynolds number of this simulation (1300 based on the Taylor scale) extreme events of dissipation and enstrophy as large as $10^5$ times the mean value are observed. These events are shown to possess a form that is different from similar events at low Reynolds numbers. Extreme vorticity appears to be ``chunky'' in character, in contrast to elongated vortex tubes at moderately large amplitudes commonly reported in the literature. We track the temporal evolution of these extreme events and find that they are generally short-lived, which suggests frequent sampling on-the-fly is useful. Extreme magnitudes of energy dissipation rate and enstrophy are essentially coincident in space and remain so during their evolution. Numerical tests show sensitivity to small-scale resolution and sampling but not machine precision. The connections expected between indicators of fine-scale intermittency such as acceleration statistics and the anomalous scaling of high-order velocity structure functions are also investigated.