This feature is used to define the two-phase interface separating the modeled fluid in a free surface domain from the outer fluid. By applying this boundary condition the motion of the two-phase interface can be tracked. The free surface follows the fluid motion in the normal direction. In the tangential direction the free surface motion is governed by the fluid motion in combination with the surface tension of the fluid-fluid interface. More information on the Free Surface feature can be found under Multiphase Flow Interface in the CFD Module User’s Guide.