The Atwood number (A) is a dimensionless number in fluid dynamics used in the study of hydrodynamic instabilities in density stratified flows. It is a dimensionless density ratio defined as
where is the density of the heavier fluid and is the density of the lighter fluid.
Atwood number is an important parameter in the study of RayleighâÂÂTaylor instability and RichtmyerâÂÂMeshkov instability. In RayleighâÂÂTaylor instability, the penetration distance of heavy fluid bubbles into the light fluid is a function of acceleration time scale where is the gravitational acceleration and is the time.