Atwood number
= Atwood number
{c}
{title2=$\mathcal A=\frac{\rho_h-\rho_l}{\rho_h+\rho_l}$}
The dimensionless density contrast $(\rho_h-\rho_l)/(\rho_h+\rho_l)$, conventionally using heavy and light densities. In the inviscid, zero-surface-tension heavy-over-light <Rayleigh-Taylor instability>, the <growth rate> is $s=\sqrt{\mathcal A gk}$. State the signed ordering if labels are not heavy/light.