Free surface

A free surface refers to a boundary surface between a gas and a liquid. The reason for the marking of the term "free" arises from the large density difference with respect to the gas and the liquid ( the density ratio of water to air is 1000). The low gas density means that the inertia force can generally be ignored in comparison with the inertia force of the liquid. It means the fluid motion is "independent" or " free" of the inertia force of the gas.

Free surface or in English " free surface" is a term used in computational fluid dynamics, or fluid dynamics.

There are numerous numerical calculation methods to calculate free surface. The oldest is the 1965 published Marker -and- Cell (MAC) method of Harlow and Welch. A newer method is published by Nichols and Hirt volume-of - fluid (VOF ) method. Still further, there is the Arbitrary Lagrangian Eulerian ( ALE) method.