Slip

The Slip condition assumes that there are no viscous effects at both sides of the slip wall and hence, no boundary layer develops. From a modeling point of view, this may be a reasonable approximation if the important effect of the wall is to prevent fluid from leaving the domain.

• Theory for the Slip Wall Boundary Condition in the COMSOL

Multiphysics User’s Guide

See Also

Moving Wall

If the wall moves, so must the fluid on both sides of the wall. Hence, this boundary condition prescribes u = uw. Enter the components of the Velocity of moving wall uw (SI unit: m/s).

Specifying this boundary condition does not automatically cause the associated wall to move. An additional Moving Mesh interface needs to

be added to physically track the wall movement in the spatial reference

Important

frame.

• The Moving Mesh Interface in the COMSOL Multiphysics User’s

Guide

See Also

Inlet

The Inlet node includes a set of boundary conditions describing the fluid flow condition at an inlet. The Velocity boundary condition is the default.

In most cases the inlet boundary conditions appear, some of them slightly modified, in the Outlet type as well. This means that there is nothing in

the mathematical formulations to prevent a fluid from leaving the domain

Tip

through boundaries where the Inlet type is specified.

• Velocity

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•Pressure, No Viscous Stress

•Normal Stress

•Laminar Inflow

•Mass Flow

B O U N D A R Y S E L E C T I O N

From the Selection list, choose the boundaries that represent inlets. See Table 4-4 and

Table 4-5 for available boundary conditions by physics interface.

TABLE 4-4: SINGLE-PHASE FLOW, LAMINAR INLET BOUNDARY CONDITIONS

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V E L O C I T Y

This section displays when Velocity is selected as the Boundary condition. The option is

available for the Inlet and Outlet boundary features.

•Select Normal inflow velocity (the default) to specify a normal inflow velocity magnitude u = nU0 where n is the boundary normal pointing out of the domain. Enter the velocity magnitude U0 (SI unit: m/s).

•If Velocity field is selected, it sets the velocity equal to a given velocity vector u0 when u = u0. Enter the velocity components u0 (SI unit: m/s) to set the velocity equal to a given velocity vector.

Also enter the settings as described in Additional Boundary Condition Settings for Turbulent Flow Interfaces.

P R E S S U R E , N O V I S C O U S S T R E S S

The Pressure, no viscous stress boundary condition specifies vanishing viscous stress along with a Dirichlet condition on the pressure. Enter the Pressure p0 (SI unit: Pa) at the boundary.

This section displays when Pressure, no viscous stress is selected as the Boundary condition. The option is available for the Inlet and Outlet boundary features. Depending on the pressure field in the rest of the

Note domain, an inlet boundary with this condition can become an outlet boundary.

Also enter the settings as described in Additional Boundary Condition Settings for Turbulent Flow Interfaces.

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N O R M A L S T R E S S

Enter the magnitude of Normal stress f0 (SI unit: N/m2). Implicitly specifies that p f0 .

• Theory for the Normal Stress Inlet Boundary Condition in the

COMSOL Multiphysics User’s Guide

See Also

Also enter the settings as described in Additional Boundary Condition Settings for Turbulent Flow Interfaces.

L A M I N A R I N F L O W

Select a flow quantity to specify for the inlet:

•If Average velocity is selected, enter an Average velocity Uav (SI unit: m/s).

•If Flow rate is selected, enter the Flow rate V0 (SI unit: m3/s).

•If Entrance pressure is selected, enter the Entrance pressure pentr (SI unit: Pa) at the entrance of the fictitious channel outside of the model.

Then specify these parameters:

•Enter the Entrance length Lentr (SI unit: m) to define the length of the inlet channel outside the model domain. This value must be large enough so that the flow can

reach a laminar profile. For a laminar flow, Lentr should be significantly greater than 0.06ReD, where Re is the Reynolds number and D is the inlet length scale (this

formula is exact if D is the diameter of a cylindrical pipe and approximate for other geometries).

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•Select the Constrain endpoints to zero check box to force the laminar profile to go to zero at the bounding points or edges of the inlet channel. Otherwise the velocity is defined by the boundary condition of the adjacent boundary in the model.

For example, if one end of a boundary with a Laminar inflow condition connects to a Slip boundary condition, then the laminar profile has a maximum at that end.

This section displays when Laminar inflow is selected as the Boundary condition for the Laminar Flow interface. However, it is not available

when the Use memory-efficient form check box is selected from Advanced

Note

Settings on the Laminar Flow node’s Settings window.

• Theory for the Laminar Inflow Condition

See Also

A D D I T I O N A L B O U N D A R Y C O N D I T I O N S E T T I N G S F O R T U R B U L E N T F L O W

I N T E R F A C E S

The Turbulent Flow, k- model and Turbulent Flow, low Reynolds number k- models, also requires you to specify k and via one of the following options:

•Select Specify turbulence length scale and intensity to enter values or expressions for the Turbulent intensity IT (unitless) and Turbulence length scale LT (SI unit: m).

The Turbulent intensity IT and Turbulence length scale LT values are related to the turbulence variables via

•If Specify turbulence variables is selected, enter values or expressions for the Turbulent kinetic energy k0 (SI unit: m2/s2) and Turbulent dissipation rate, 0 (SI unit: m2/s3).

The Turbulent Flow, k- model, also requires you to specify k and via one of the

following option:

•Select Specify turbulence length scale and intensity to enter values or expressions for the Turbulent intensity IT (unitless) and Turbulence length scale LT (SI unit: m).

The Turbulent intensity IT and Turbulence length scale LT values are related to the turbulence variables via

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•If Specify turbulence variables is selected, enter values or expressions for the Turbulent kinetic energy k0 (SI unit: m2/s2) and Turbulent dissipation rate, 0 (SI unit: 1/s).

For recommendations of physically sound values see Inlet Values for the

Turbulence Length Scale and Intensity.

Tip

For the Turbulent Flow, Spalart-Allmaras interface, also enter a value or expression for the Undamped turbulent kinematic viscosity 0 (SI unit: m2/s).

M A S S F L O W

The mass flow boundary condition is only available for the Inlet boundary type. Depending on the sign of the specified mass flow or flux an inlet boundary with this condition can become an outlet boundary.

Under Mass Flow Rate, select a Mass flow type—Pointwise mass flux, Mass flow rate,

Standard flow rate, or Standard flow rate (SCCM).

• Theory for the Mass Flow Inlet Condition

See Also

Pointwise Mass Flux

This option sets the mass flux parallel to the boundary normal. The flux perpendicular to the normal is set to zero. The mass flux is a model input, which means that COMSOL can take its value from another physics interface if applicable. If User defined is selected from the associated list, the Mass flux Mf (SI unit: kg/(m2·s)) is entered manually in the field. The default value is 0 kg/(m2·s).

Mass Flow Rate

This option sets the integrated mass flow across the entire boundary to a specific value. The mass flow is assumed to be parallel to the boundary normal, so the tangential velocity is set to zero.

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Enter the Normal mass flow rate m (SI unit: kg/s). The default value is 10 5 kg/s.

For 2D models, enter a Channel thickness dbc (SI unit: m). The default value is 1 m.

The Channel thickness is used only in defining the area across which the mass flow occurs—it is not a setting applied to the whole model. Line

2D integrals of mass flux over the boundary in post-processing or coupling variables still produces results per unit thickness and needs to be scaled appropriately for comparison with the specified mass flow.

Standard Flow Rate

This option sets a standard volumetric flow rate, according to the SEMI standard E12-0303. The flow rate is specified as the volumetric flow rate that would occur for the same number of moles per second, if the gas density was a standard value (the molar mass over a fixed standard volume). The flow occurs across the whole boundary in the direction of the boundary normal and is computed by a surface (3D) or line (2D) integral. The tangential flow velocity is set to zero.

The standard density can be defined directly, or by specifying a standard pressure and temperature, in which case the ideal gas law is assumed. Select an option from the

Standard flow rate defined by list—Standard density or Standard pressure and

temperature.

If Standard density is selected enter:

-The Standard flow rate Qsv (SI unit: m3/s) to specify the standard volumetric flow rate through the boundary. The default value is 10 6 m3/s.

-The Standard molar volume Vm (SI unit: m3/mol). The default is 0.0224136 m3/mol.

-The Mean molar mass Mn (SI unit: kg/mol). This can be selected as a model input from another physics interface or set to a user-defined value using the associated

list and field. The default is that User defined is selected from the list with a value of 0.032 kg/mol.

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