Turbulence with large density variations due to thermal and compositional fluctuations

Flow systems with large density variations are ubiquitous in technology and nature and span scales ranging from those encountered in astrophysical phenomena to the micro-scales in Inertial Confinement Fusion. Sources of variable density effects include compressibility aspects such as acoustic fluctuations and shock waves, temperature variations due to differential heating or chemical/nuclear reactions, mixing between fluids with different molar masses or phases, and phase transformations. At the extremes, only (sometimes indirect) observations of the flow physics are possible. Nevertheless, advances in theoretical analysis of the flow equations, diagnostic tools, and supercomputer power have led to significant recent progress in our understanding and modeling of turbulence coupled with large density variations. In particular, extreme scale computing increasingly allows controlled numerical experiments of ever more complex flows, where the coupling between large density gradients and other flow phenomena can be investigated in detail. In this talk, I will discuss shared inertial density effects over different turbulent flows, starting from incompressible homogeneous configurations to highly compressible inhomogeneous flows. These include homogeneous buoyancy-driven variable-density turbulence, shear driven mixing layers, Rayleigh-Taylor and Richtmyer-Meshkov instabilities, and shock-variable density turbulence interaction. Thus, I will use some of the largest direct numerical simulations to date to highlight mixing and turbulence asymmetries, and changes in spectral behavior and structure of mixing layers in the presence of large density variations. I will also discuss some of the challenges and open questions regarding variable density effects, and prospects for exa-scale computing to address these issues.