Turbulence Intermittency - Contributing Mechanisms

Velocity-gradient (VG) intermittency is one of the most intriguing aspects of high Reynolds number turbulence. The Reynolds number scaling of various high-order VG moments have been extensively investigated in literature. However, the physical mechanisms that lead to intermittency have not been as widely studied. As energy cascades to smaller scales, the viscous dissipation must keep pace resulting is increasingly steeper velocity gradients. On an average, cascade rate balances viscous dissipation. At high Reynolds numbers, dissipation is not uniformly distributed in space, but is highly sporadic. In this work, we use direct numerical simulation data to examine if intermittency is merely due to the local imbalance between cascade and viscous dissipation, or caused by other phenomena. We focus specifically on non-local effects. We also demonstrate that the internal geometry of the VG tensor is nearly independent Reynolds number. Further, velocity gradient triple decomposition is used to exhibit that the largest values of dissipation occur in regions of pure shear, rather than solid body rotation. Thus, the enstrophy intermittency is due to shear, rather than solid-body rotation.