Numerical investigation of compressible three-layer Rayleigh-Taylor instability

In this study, we investigate the unsteady mixing of three fluids, three-layer Rayleigh–Taylor instability (RTI) using two-dimensional direct numerical simulations (DNS). The investigated three-layer configurations with two interfaces consist of at least one acceleration-driven RT unstable interface where the direction of the acceleration points from the heavier to the lighter fluids to initiate the chaotic mixing. DNS are initialized with isopycnic background stratification, where the species densities of the three fluids are initially constant over the domain. We investigate three cases where stratified layers from top to bottom have heavy-intermediate-light, heavy-light-intermediate, and heavy-light-heavy densities. It is found that the globally unstable case with heavy-intermediate-light densities, in which both interfaces are unstable, exhibits symmetric mixing during the flow evolution and eventually behaves similarly to the classic two-layer RTI flow. For the remaining two cases, the lower interface of the three-layer RTI is stably stratified. For the heavy-light-intermediate case, whose second layer is weakly stable, pure fluids penetration is larger compared to the heavy-light-heavy case, whose second layer is initially strongly stable.