If Standard pressure and temperature 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 pressure Pst (SI unit: Pa). The default value is 1 atm (101,325 Pa).

-The Standard temperature Tst (SI unit: K). The default value is 273.15 K (0 degrees Celsius).

-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.

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.

Remember to enter the settings as described in Additional Boundary

Condition Settings for Turbulent Flow Interfaces.

Important

C O N S T R A I N T S E T T I N G S

To display this section, click the Show button () and select Advanced Physics Interface Options. Select a Constraint type—Bidirectional, symmetric or Unidirectional. Select the Use weak constraints check box as required.

Outlet

The Outlet feature includes a set of boundary conditions describing fluid flow conditions at an outlet. The Pressure, no viscous stress boundary condition is the default.

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Selecting appropriate outlet conditions for the Navier-Stokes equations is not a trivial task. Generally, if there is something interesting happening at an outflow boundary, extend the computational domain to include this phenomenon.

All of the formulations for the Outlet type are also available, possibly slightly modified, in other boundary types as well. This means that there is nothing in the mathematical formulations to prevent a fluid from

Tip entering the domain through boundaries where the Outlet boundary type is specified.

•Velocity

•Pressure, No Viscous Stress

•Normal Stress

•Vacuum Pump

•Laminar Outflow

•Pressure

•No Viscous Stress

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 outlets.

B O U N D A R Y C O N D I T I O N

Select a Boundary condition for the outlet. Pressure, no viscous stress is the default. See Table 4-6 and Table 4-7 for available boundary conditions by interface.

See Pressure, No Viscous Stress, Velocity and Normal Stress boundary

conditions as described for the Inlet feature.

See Also

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TABLE 4-6: SINGLE-PHASE FLOW, LAMINAR OUTLET BOUNDARY CONDITIONS

P R E S S U R E

This boundary condition prescribes only a Dirichlet condition for the pressure p = p0.

Enter the Pressure p0 (SI unit: Pa) at the boundary. While this boundary condition is flexible and seldom produces artifacts on the boundary (compared to Pressure, No Viscous Stress), it can be numerically unstable. Theoretically, the stability is guaranteed

by using streamline diffusion for a flow with a cell Reynolds number Rec u h 2 1 (h is the local mesh element size). It does however work well in

most other situations as well.

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

This section displays when Laminar Outflow is selected as the Boundary condition for a Laminar Flow (or Creeping Flow) interface. However, it is

not available when the Use memory-efficient form check box is selected

Note

from Advanced Settings on the Laminar Flow Settings window.

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 Exit pressure is selected, enter the Exit pressure pexit (SI unit: Pa) at the end of the fictitious channel following the outlet.

Then specify these parameters:

•Enter the Exit length Lexit (SI unit: m) to define the length of the fictitious channel after the model domain. This value must be large enough so that the flow can reach

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

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

•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.

• Theory for the Laminar Outflow Condition

See Also

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

The No Viscous Stress condition specifies vanishing viscous stress on the outlet. This condition does not provide sufficient information to fully specify the flow at the outlet and must be combined with pressure constraints on adjacent points.

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