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.
The Free Surface feature is supported by laminar flow and by turbulence models using wall functions or automatic wall treatment but not low-Reynolds number wall treatment. It also requires the study to be time dependent. The node is inactive for Frozen Rotor studies.
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.
The predefined correlations are based on the data in Ref. 5, Ref. 6, and Ref. 7.
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 Wall boundary condition with the Navier Slipoption should be selected. For turbulence models with wall functions or automatic wall treatment the Wall feature with the No Slip option can be selected.