Rayleigh–Taylor instability

A stable arrangement of two fluids, with the denser fluid below the lighter one in a gravitational field, tends to resist small vertical displacements. The arrangement is unstable when acceleration acts from the denser fluid toward the lighter fluid: a small perturbation can then grow instead of being restored. The same principle applies in an accelerating frame or in flows where pressure and acceleration impose an adverse density arrangement. The instability concerns the evolution of an interface, not simply the fact that two fluids have different densities.

At early times, small interface disturbances grow; their subsequent evolution depends on the fluids, acceleration, perturbation scale, and effects such as viscosity and surface tension. In the nonlinear phase, protrusions form: lighter fluid can push into denser fluid while denser material descends into lighter fluid. These structures stretch and fold the interface, increasing mixing. Real experiments and numerical models can also be influenced by initial disturbances and three-dimensional flow, so the detailed morphology is not universal.

The instability is studied in fluid mechanics and is relevant to flows subject to adverse acceleration, including some combustion and inertial-confinement-fusion conditions. In astrophysics, deceleration of supernova ejecta can drive Rayleigh–Taylor growth at the boundary between the ejecta and surrounding shocked material. Simulations show finger-like structures and mixing of stellar material; these calculations help investigate the evolution and morphology of supernova remnants.

The Richtmyer–Meshkov instability also develops at a density interface, but is initiated by shock acceleration rather than sustained adverse acceleration. The mechanisms can be related in complex flows, but the names describe different drivers. Rayleigh–Taylor growth is also distinct from the Kelvin–Helmholtz instability, which is associated primarily with velocity shear across an interface.