The Atwood number is a dimensionless number in fluid dynamics

Fluid dynamics

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 used in the study of hydrodynamic instabilities in density stratified flows.
It is a dimensionless density ratio defined as


where

= density of heavier fluid

= density of lighter fluid

Field of application

Atwood number is an important parameter in the study of Rayleigh–Taylor instability and Richtmyer–Meshkov instability. In Rayleigh–Taylor instability,
the penetration distance of heavy fluid bubbles into the light fluid is a function of acceleration time scale, where g is the gravitational acceleration and t is the time.