The Laminar Two-Phase Flow, Level Set Interface

The Laminar Two-Phase Flow, Level Set interface (), found under the Multiphase Flow>Two-Phase Flow, Level Set branch () in the Model Wizard, has the equations, boundary conditions, and volume sources for modeling laminar two-phase flow of two different immiscible fluids, separated by a moving interface, solving for the velocity field, the pressure, and the level set variable. The 0.5 contour of the level set variable defines the fluid-fluid interface.

•Filling of a Capillary Channel—Level Set: Model Library path

CFD_Module>Multiphase_Flow>capillary_filling_ls

Model

The main node is the Fluid Properties feature, which adds the Navier-Stokes equations and the level set equation and provides an interface for defining the properties of the fluids and the surface tension. For each of the fluids, enter the density and viscosity (these are typically taken from two different materials). Also specify temperature-dependent interfacial tension coefficients. Specify the fluid that each domain is initially filled with using the Initial Values node.

The following default nodes are added when using this interface—Fluid Properties, Wall

(with a default No slip boundary condition), Initial Interface, and Initial Values.

Right-click the Laminar Two-Phase Flow, Level Set node to add other features that implement, for example, boundary conditions and volume forces.

Except where noted below, the Laminar Two-Phase Flow, Level Set has the

same sections and settings as the Laminar Flow interface.

Note

I N T E R F A C E I D E N T I F I E R

The interface identifier is a text string that can be used to reference the respective physics interface if appropriate. Such situations could occur when coupling this interface to another physics interface, or when trying to identify and use variables defined by this physics interface, which is used to reach the fields and variables in expressions, for example. It can be changed to any unique string in the Identifier field.

The default identifier (for the first interface in the model) is tpf.

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

The default setting is to include All domains in the model. To choose specific domains,

select Manual from the Selection list.

P H Y S I C A L M O D E L

The Laminar Two-Phase Flow, Level-Set interface uses a level set method to track the

fluid-fluid interface.

This interface changes to a Laminar Two-Phase Flow, Phase Field interface if the Multiphase flow model selected is Two-phase flow, phase field. See The

Tip Laminar Two-Phase Flow, Phase Field Interface for details.

The Compressibility defaults to Incompressible flow to use the incompressible (constant density) formulation. Select Compressible flow (Ma<0.3) to use this formulation of the Navier-Stokes equations.

Select the Neglect inertial term (Stokes flow) check box to model flow at very low Reynolds numbers where the inertial term in the Navier-Stokes equations can be neglected. The interface then uses the linear Stokes equations instead. The Stokes equations are valid for creeping flow, which can occur in microfluidics and MEMS devices, where the flow speed or length scales are very small.

By default the Turbulence model type selected is None. If RANS, k- is selected, the interface changes to The Turbulent Flow, Two-Phase Flow,

Tip Level Set Interface.

D E P E N D E N T V A R I A B L E S

The dependent variables (field variables) are:

•Velocity field (SI unit: m/s)

•Pressure (SI unit: Pa)

•Level set variable

•Reciprocal initial interface distance (SI unit: 1/m)

The names can be changed but the names of fields and dependent variables must be unique within a model.

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A D V A N C E D S E T T I N G S

To display this section, click the Show button () and select Advanced Physics Interface Options. Normally these settings do not need to be changed. Select a

Convective term—Non-conservative or Conservative.

• Convective Term Formulation described for the Transport of Diluted

Species interface in the COMSOL Multiphysics User’s Guide

See Also

D I S C R E T I Z A T I O N

To display this section, click the Show button () and select Discretization. It controls the discretization (the element types used in the finite element formulation).

From the Discretization of fluids list select the element order for the velocity components and the pressure: P1+P1, P2+P1 (the default discretization), or P3+P2.

•P1+P1 means linear elements for both the velocity components and the pressure field. This can be computationally efficient but requires numerical stabilization of the Navier-Stokes equations.

•P2+P1 means second-order elements for the velocity components and linear elements for the pressure field. This are the default element orders for the Level Set and Phase Field interfaces.

•P3+P2 means third-order elements for the velocity components and second-order elements for the pressure field. This can add additional accuracy but it also adds additional degrees of freedom compared to P2+P1 elements.

C O N S I S T E N T A N D I N C O N S I S T E N T S T A B I L I Z A T I O N

To display this section, click the Show button () and select Stabilization. Any settings unique to this interface are listed below.

•The consistent stabilization methods are applicable to the Navier-Stokes equations— Streamline diffusion and Crosswind diffusion. These check boxes are not selected by

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