Multi-layer Rayleigh-Taylor instability in naturally occurring density-stratified mixing layers

In nature, gravitationally unstable stratified fluids are widespread, emerging from dynamic interactions where denser parcels of fluid rise over less dense ones in persistent and/or transient manners. These phenomena manifest in diverse environments on our planet, including atmospheres, coastal regions at intersection of seas, lakes, and rivers and radiatively heated cold waters beneath the temperature of maximum density. In these regions of unstable stratified fluids, intense gravitational convection and mixing take place, significantly influencing system functionality. The underlying physics governing this convective process has been explored through the classic Rayleigh-Taylor instability (RTI), which involves two horizontally arranged layers in an unstable configuration. Unstable stratified regions comprise multiple layers, each of which can exhibit either unstable or stable behavior relative to its neighboring layer. Despite their prevalence in fluid environments, there has been limited research focused on these multi-layer unstable fluids.

The present work presents an overview of multilayer RTI, focusing on its fundamental characteristics and underlying mechanisms. The distinct stages of multilayer RTI, encompassing initial growth of perturbations, formation of finger-like structures, and eventual transition to a fully turbulent regime are explored by invoking a compressible enstrophy transport equation.