heavy over light

LITTurn a glass of water upside down and the water falls out, even though air pressure could hold it up: the flat interface is unstable. That is the Rayleigh–Taylor instability. A heavy fluid above a light one under gravity lets small ripples grow exponentially at rate σ = √(A·g·k), with Atwood number A = (ρ₁ − ρ₂)/(ρ₁ + ρ₂) and wavenumber k. The same instability shapes supernova remnants and the Crab Nebula's filaments.

COMPUTEDLinear growth for several modes, from the slider densities, drawn as fingers that grow until they saturate (MODEL: a soft amplitude cap). Tap to flip the stack: light over heavy is stable (σ imaginary: the ripples just oscillate).

ρ top ρ bottom flip stack

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