Atwood number

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

${\displaystyle \mathrm {A} ={\frac {\rho _{1}-\rho _{2}}{\rho _{1}+\rho _{2}}}}$

where ${\displaystyle \rho _{1}}$ is the density of the heavier fluid and ${\displaystyle \rho _{2}}$ is the density of the lighter fluid.