Atwood number

It is a dimensionless number in use in fluid dynamics for the study of hydrodynamic instabilities in density stratified flows. It is defined as: While L and H refer to heavier and lighter fluid respectively. It is an important parameter in the study of Rayleigh–Taylor instability and Richtmyer–Meshkov instability.

Archimedes number

The Archimedes number is a dimensionless number used to determine the motion of fluids due to density differences. It is named after the ancient Greek scientist and mathematician Archimedes. Archimedes number is the ratio of gravitational forces to viscous forces and can be written as: