Atwood number

Etymology
Probably named after.

Noun

 * 1)  A dimensionless number used in the study of hydrodynamic instabilities in density stratified flows: $$ \mathrm{A} = \frac{\rho_1 - \rho_2} {\rho_1 + \rho_2} $$, where $$\rho_1$$ is the density of the heavier fluid and $$\rho_2$$ is the density of the lighter fluid.