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

TL;DR

This study investigates how vorticity alignment with strain axes influences scalar dissipation in isotropic turbulence, revealing that different alignments promote varying dissipation intensities through subtle flow mechanisms.

Contribution

It introduces a stochastic Lagrangian model to analyze the relationship between vorticity alignment and scalar dissipation, highlighting the distinct roles of strain and rotation in this process.

Findings

Scalar dissipation peaks when vorticity aligns with the intermediate strain eigenvector.

Mean scalar dissipation is best represented by vorticity aligned with the extensional strain.

Rotation-dominated events significantly influence the differences in scalar dissipation levels.

Abstract

The mechanisms promoting scalar dissipation through scalar gradient production are scrutinized 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.