- = 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.
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