Explanation

Rayleigh-Taylor instability is an instability caused by gravitational convection flow when heavy fluid is placed on light fluid. When two fluids with different density are accelerated, the situation is the same. Similar instabiility (Richtmyer-Meshkov Instability) also takes place in accelerating plasma and corresponds to the expanding shells of supernova explosion.

[Initial Condition] Fluids with different density is placed horizontaly and gravity is assumed toward the bottom (y<0). Initially both fulids are in gravitational and pressure equilibrium. Upper lime green region is high density region and lower aqua region is low density and their density ratio is 4. Perturbation is given in 10% of growth mode. In the figure, color shows density and arrows show velocity.

[Growth Stage] Discontinuity surface starts to surge sinusoidally, then heavy fluid sinks down to light fluid and the surface becomes in mashroom shape.