Coupled effects of variable density and isothermal background stratification on vorticity and mixing dynamics of compressible Rayleigh--Taylor instability*

Rayleigh-Taylor instability (RTI) occurs at the perturbed interface between fluids with different densities, where the acceleration and density gradient are in opposite directions. RTI is observed in natural flows, such as atmospheric and geological flows, and in engineering applications, such as the mixing stage of ramjets and scramjets and Inertial Confinement Fusion (ICF). We study the coupled effects of compressibility controlled by the isothermal background stratification strength and large molar mass differences between the mixing fluids through Direct Numerical Simulations (DNS) of the two-dimensional, single-mode RTI by solving fully compressible multi-species Navier-Stokes equations. From the low Atwood number cases, it is observed that when the molar mass ratio between the mixing fluids is small, the growth of the mixing layer ceases, and the flow becomes molecularly well-mixed as the background stratification increases. However, our high Atwood number simulations suggest that when the molar mass ratio between the mixing fluids is large, there is a more complex flow evolution where a stronger background stratification leads to a relatively faster mixing growth rate, but it also suppresses the vortical structures within the mixing layer. In addition to the overall flow evolution, we will compare each term of the vorticity transport equation for the low and high Atwood number cases under weak and strong background stratifications at different time instants.