The Free Surface feature assumes that the density and viscosity of the fluid outside the domain are negligible compared to the corresponding values inside the domain. On the free surface the following condition for the stress, τ, along the surface is applied:

Here pext is the pressure outside the free surface domain (SI unit: Pa) and fst denotes the surface tension forces (SI unit: N/m2). In the surface tension terms, ∇s is the surface gradient operator (∇s = (I − ni niT)∇ where I is the identity matrix) and σ is the surface tension coefficient (SI unit: N/m).