# Weber number

The Weber number ($W$) is a dimensionless quantity that is useful when analyzing fluid flow when the system in question includes an interface between two fluids. In particular, it is the ratio of the fluid's inertia and the surface tension: $W = \frac{\rho v^{2} L}{\sigma}$. Here, $\rho$ is the density of the moving fluid, $v$ is its velocity, $L$ is a characteristic length scale (this is generally the length the fluid flows through; for example, for a cylindrical flow, it would be the diameter of the cylinder formed by the fluid), and $\sigma$ is the surface tension.