Impact of transverse strain on linear, transitional and self-similar turbulent mixing layers

Mean strain rates can arise in fluids due to geometric deformation, or from bulk compression/expansion as from implosions/explosions. For interfacial instabilities, such as the Richtmyer–Meshkov instability (RMI), and the resulting turbulent mixing layers, the effect of strain depends on the direction of application. To analyse the influence of transverse strain rates, which is in the direction orthogonal to the amplitude or mixing layer growth, simulations are conducted in a Cartesian geometry with a moving mesh to control the strain application. Two regimes are analysed under the application of transverse strain rates. In the linear regime, a linearised potential flow model and supporting simulations demonstrate that transverse compression amplifies the instability growth. In contrast, simulations of the RMI-induced turbulent mixing layer show a decrease in the mixing layer width under transverse compression. The turbulent flow deviates from the self-similar state that is observed in the absence of strain, due to shear production and a modified turbulent length scale. The change in turbulent length scale causes a change in the dissipation rate, altering the evolution of the mixing layer. Predictive models for the mixing layer width and the domain-integrated turbulent kinetic energy are presented, which require scaling the drag/dissipation terms by the inverse of the transverse expansion factor to align with simulation results.