Stationary Free Surface
This postprocessing feature is used to estimate the deformation of a free surface from the pressure distribution on the boundary when using the Frozen Rotor or Frozen Rotor with Initialization study types. When computing the flow field, a slip condition together with a constant pressure level pext is applied on the boundary. The free surface deformation ηFS is evaluated from the linearized free surface condition in a separate study step, Wall Distance Initialization, using the pressure from the Frozen Rotor study step:
(4-1)
Here, σ is the surface tension coefficient, x = x0 represents the position of the undisturbed surface (the feature selection), and is its unit normal. The correct study sequence can be triggered by toggling the study window once a selection for the Stationary Free Surface feature has been made.
The Stationary Free Surface feature is supported by laminar and turbulent flow, and is applicable for small surface deformations (ηFS and its gradient should be small compared to the dimensions of the computational domain). If the surface deformation is large, a time-dependent study with a Deforming Domain and a Free Surface feature should be used instead.
Iterative solvers are necessary to reduce the cost of models with large number of degrees of freedom. The fluid flow interfaces use a Smoothed aggregation AMG solver per default. When the Stationary Free Surface feature is active and has a nonempty selection in a frozen rotor or stationary study step, a Geometric multigrid solver is used instead to ensure robustness of the model.
The Choice of Solver and Solver Settings in the CFD Module User’s Guide and Studies and Solvers in the COMSOL Multiphysics Reference Manual
Stationary Free Surface
Enter an External pressure pext corresponding to the average pressure level on the free surface.
Surface Tension
Select the Include surface tension force in momentum equation checkbox to include the surface tension force in the momentum equation.
The Surface tension coefficient σ (SI unit: N/m) can be specified from predefined libraries, by selecting Library coefficient, liquid/gas interface (the default) or Library coefficient, liquid/liquid interface, or be set to User defined.
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For Library coefficient, liquid/gas interface select an option from the list — Water/Air (the 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.
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For Library coefficient, liquid mixture/gas interface, select an option from the Predefined binary mixture list — Acetone and Water/Air, Acetic acid and Water/Air, Ethanol and Water/Air (default), Diethyl ether and Water/Air, Glycerol and Water/Air, or Toluene and Water/Air. If there are Transport of Concentrated Species (tcs) or Transport of Diluted Species (tds) interfaces in the component, you can alternatively select the appropriate interface option to input the surface tension values of each corresponding interacting species and the species mass fraction or concentration is directly input from the corresponding species transport interface.
Only one Stationary Free Surface feature can be applied in a Rotating Machinery, Fluid Flow interface, and it requires the study to be stationary (Frozen Rotor).