a gravity-driven instability. A heavy fluid is placed above the light fluid and the gravitational field is pointing downward. The perturbed fluid interface forms rising bubbles and falling spikes.
Richtmyer-Meshkov instability:
a shock-driven instability. In this simulation, a shock with Mach number 1.5 propagates downward, hitting the fluid interface. Later the shock is reflected by the ground and bounced back. A mushroom is formed after the fluid interface is re-shocked.