Surface Tension Effects

The surface tension force is a tensile force tangential to the interface separating two fluids, and it tries to keep the fluid molecules at the free surface in contact with the rest of the fluid. Its magnitude depends mainly on the nature of the fluid pair and on temperature. For a curved interface, the surface tension force can be resolved into two components:

(15)

where

,

(16)

and is the surface tension coefficient, n the unit vector normal to the free surface and directed from liquid to gas, t the unit vector in the tangential direction to the free surface, and K the mean curvature of the free surface. Note that in case of a constant surface tension coefficient , the tangential force is zero and the surface tension results in a force normal to the interface ().

In the VOF multiphase model case, the surface tension force is calculated according to the continuum surface force (CSF) model, i.e. it uses the smooth field of the phase volume fraction to calculate a vector normal to the interface:

(17)

The curvature of the interface may therefore be expressed in terms of the divergence of the unit normal vector n, as follows:

(18)

Now the surface tension force can be expressed as:

(19)

Eqn. 19 is applicable only when the surface tension coefficient is constant.