Fluid Properties

The Fluid Properties node adds the flow equations and the equations for the level set variable or the phase field variables (where applicable). For the level set and phase field physics interfaces, specify the fluid that each domain is initially filled with using the Initial Values node.

D O M A I N S E L E C T I O N

From the Selection list, choose the domains to use the fluid properties.

M O D E L I N P U T S

Specify the temperature to be used in the interface:

•Select User defined to enter a value or expression for the temperature (SI unit: K).

•In addition, select a temperature defined by a heat transfer interface present in the model (if any). For example, then select Temperature (ht/fluid1) to use the temperature defined by the fluid1 node in the heat transfer interface ht.

To define the Absolute Pressure, see the settings for the Heat Transfer in

Fluids node as described in the COMSOL Multiphysics User’s Guide.

Tip

F L U I D 1 A N D F L U I D 2 P R O P E R T I E S

Specify the Density and the Dynamic viscosity of Fluid 1 and Fluid 2 in the same way as in a single-phase flow interface (see Fluid Properties).

Care should be taken when using the Domain Material setting for the

materials properties for Fluid 1 and Fluid 2.

Caution

The material properties are obtained from the domain irrespective of the location of the interface. If two different materials are selected in domains 1 and 2, with the phase boundary initially coincident with the domain boundary, the model has convergence issues once the phase boundary moves away from the domain boundary. This is because a density and viscosity discontinuity occurs in the single phase regions at the boundary of the two domains. For this reason selecting the material directly is recommended when setting the material properties for Fluid 1 and Fluid 2.

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S U R F A C E T E N S I O N

This section is not available for the turbulent two-phase flow interfaces.

Note

Select the Neglect surface tension in momentum equation check box to neglect surface tension. Select a Surface tension coefficient (SI unit: N/m):

•To use a predefined expression, select Library coefficient, liquid/gas interface or

Library coefficient, liquid/liquid interface. Then select an option from the list that displays below (for example, Water/Air, Glycerol/Air and so forth).

•If User defined is selected, enter a value or expression for the surface tension coefficient (SI unit: N/m).

L E V E L S E T P A R A M E T E R S

This section is available with The Laminar Two-Phase Flow, Level Set

Interface and when turbulence is active.

Note

Specify the parameters for the level set method:

•Enter a value or expression for the Reintialization parameter (SI unit: m/s). The default value is 1. Use an approximate value of the maximum speed occurring in the

flow. Because the flow speed is not always known in advance, an initial computation might be needed to find a proper value for .

•Enter a value or expression for the Parameter controlling interface thickness ls

(SI unit: m). The default expression is tpf.hmax/2, which corresponds to half of the maximum mesh element size in the model. In general, the results are optimal if the default value is used for this parameter.

P H A S E F I E L D P A R A M E T E R S

This section is available with The Laminar Two-Phase Flow, Phase Field

Interface and when turbulence is active.

Note

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Define the parameters for the phase field method:

•Enter a value or expression for the Parameter controlling interface thickness pf

(SI unit: m). The default expression is tpf.hmax/2, which corresponds to half of the maximum mesh element size in the model. In general, simply use the default value for optimal accuracy.

•Enter a value or expression for the Mobility tuning parameter (SI unit: m·s/kg). The default is 1, which is a good starting point for most models. This parameter

determines the time scale of the Cahn-Hilliard diffusion and therefore also governs the diffusion-related time scale of the interface. Keep the parameter value large

enough to maintain a constant interface thickness but still low enough to not damp the convective motion. A too high mobility can also lead to excessive diffusion of droplets.

E X T E R N A L F R E E E N E R G Y

This section is available with The Laminar Two-Phase Flow, Phase Field

Interface and when turbulence is active.

Note

Add a source of external free energy as an external force term in the flow equation (the

Fext term in Equation 6-1). The external free energy fext is a user defined free energy. In most cases, the external free energy is zero.

M I X I N G L E N G T H L I M I T

This section is available for The Turbulent Flow, Two-Phase, Level Set

and Phase Field Interfaces.

Note

By default, the Mixing length limit lmix,lim (SI unit: m) is Automatic. Select Manual to enter a different value or expression.

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