Analysis of scalar dissipation in terms of vorticity geometry in isotropic turbulence

Abstract

The mechanisms promoting scalar dissipation through scalar gradient production are scrutinised in terms of vorticity alignment with respect to strain principal axes. For that purpose, a stochastic Lagrangian model for the velocity gradient tensor and the scalar gradient vector is used. The model results show that the major part of scalar dissipation occurs for stretched vorticity, namely when the vorticity vector aligns with the extensional and intermediate strain eigenvectors. More specifically, it appears that the mean scalar dissipation is well represented by the sample defined by alignment with the extensional strain, while the most intense scalar dissipation is promoted by the set of events for which vorticity aligns with the intermediate strain. This difference is explained by rather subtle mechanisms involving the statistics of both the strain intensities and the scalar gradient alignment resulting from these special alignments of vorticity. The analysis, allowing for the local flow structure, confirms the latter scenario for both the strain- and rotation-dominated events. However, despite the prevailing role of strain in promoting scalar dissipation, the difference in the level of scalar dissipation when vorticity aligns with either the extensional or the intermediate strain mostly arises from rotation-dominated events.