Coherent features in the sensitivity field of a planar mixing layer

Coherency in the topology of the instantaneous sensitivity fields of the planar mixing layer was captured using the sensitivity equation method (SEM). In the SEM approach, the partial differential equations governing the evolution of the sensitivity coefficients are derived, discretized, and solved directly, in the present work, using an unsteady finite-volume-based fractional-step algorithm. This allows the investigation of parameter-dependence without performing parametric studies. The present results, from numerical simulations run at Re δ 0 = 200 and Pr = 0.71 , provide a means to examine how and to what extent perturbations in the Reynolds/Prandtl number locally alter the structure of the flow. Specifically, a two-blade pattern appears as a dominant feature in the sensitivity solution and highlights the physical mechanism leading to vortex thickness growth and enhanced molecular mixing with increasing Re δ 0 and Pr. An expression describing the sensitivity of vortex thickness to changes in Re δ 0 is also derived and validated using the concept of "nearby" flows.