Exploring the link between intermittency in scalar dissipation ($\chi )$ and energy dissipation ($\varepsilon )$ rates

The occurrence of spatial and temporal intermittency in $\chi $, analogous to that seen in $\varepsilon $ for the velocity field, poses a formidable challenge in the formulation of subgrid scale models for $\chi $. As the scalar transport equation is known to be linear, intermittency in the scalar field must be inherited largely from non-linearity in the momentum equation. This occurrence may be explained physically as the coincidence of steepest gradients in the scalar field (which correspond to the largest magnitudes of $\chi )$ with those in the velocity field (largest magnitudes of $\varepsilon )$, caused by strong straining of material particles. To determine the extent of the inheritance, we attempt to establish a qualitative as well as quantitative correlation between intermittency in $\varepsilon $ and $\chi $. Any external role of the scalar forcing term in the intermittency of $\chi $ is also assessed by using two scalar forcing techniques in homogeneous isotropic turbulence, namely mean scalar gradient forcing and linear scalar forcing. A third, unforced configuration, the turbulent mixing layer is used as well, where scalar fluctuations are sustained naturally by a mean gradient present in the cross-stream direction. Appropriate conclusions are also drawn regarding the relevance of the Schmidt number to the extent of intermittency inheritance, in light of the spectral de-linking that happens at very high Schmidt numbers.