Free Surface
The Free Surface node defines the initial position of an external fluid interface - that is, an interface in which the viscosity in the fluid outside the domain can be neglected. By applying this feature the motion of the two-phase interface can be tracked. 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.
The Free Surface node can only be applied to outer boundaries of a Deforming Domain that do not have any boundary condition assigned under Moving Mesh.
To control the attachment angle between the free surface and adjacent walls, a Contact Angle subnode is by default added under the Free Surface node. A different contact angle subnode can be added from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.
model input
Enter a value for the Temperature T (SI unit: K). This option is only necessary when the surface tension coefficient comes from a library of coefficients, as explained below.
Free Surface
Enter an External pressure pext (SI unit: Pa).
Surface Tension
Select the Include surface tension force in momentum equation check box to include the surface tension force in the momentum equation.
The default Surface tension coefficient σ (SI unit: N/m) can be User defined, or select Library coefficient, liquid/gas interface (default) or Library coefficient, liquid/liquid interface.
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For Library coefficient, liquid/gas interface select an option from the list — Water/Air (default), Acetone/Air, Acetic acid/Air, Ethanol/Air, Ethylene glycol/Ethylene glycol vapor, Diethyl ether/Air, Glycerol/Air, Heptane/Nitrogen, Mercury/Mercury vapor, or Toluene/Air.
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For Library coefficient, liquid/liquid interface select an option from the list — Benzene/Water, 20°C, Corn oil/Water, 20°C, Ether/Water, 20°C, Hexane/Water, 20°C, Mercury/Water, 20°C, or Olive oil/Water, 20°C.
Mass Flux
The mass flux setting specifies the mass transfer across the boundary, due to processes such as boiling. The default Mass Flux Mf (SI unit: kg/(m2·s)) is User defined, with a value of 0.
 
The Free Surface feature imposes a mesh deformation in the normal direction. On adjacent boundaries, the mesh displacement parallel to those boundaries cannot be fixed if the contact point is moving. In this case a Mesh Slip, Symmetry/Roller, Prescribed Normal Mesh Displacement, or Prescribed Normal Mesh Velocity boundary condition under Definitions>Moving Mesh should be selected. A Mesh Slip condition is automatically used on boundaries that are adjacent to the Free Surface and dot not have a prescribed Moving Mesh boundary condition.
If a Free Surface feature is in contact with a solid boundary, the movement of the contact point is not compatible with the no slip condition. For laminar flow the boundary condition with the option should be selected.