  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.
