Setting Boundary Conditions

Boundary conditions define the physical conditions on the surface of the cylinder, the valves, and the plenums.

By default, the gas enters through the inlet of the intake port / intake plenum and exits through the outlet of the exhaust port / exhaust plenum, where both inlet and outlet boundaries are modeled as pressure outlets. This setting results in the specification of an absolute total pressure inflow condition and an absolute static pressure outflow condition.

The inlet and outlet boundary conditions can have significant effect on the flow predictions. Therefore, if available, use relevant and verified engine data in tabular form to set both pressure and temperature as functions of crank angle. The supported file formats are .csv, .txt, .dat.

You can describe the pressure and temperature values in a single table or in separate tables. If no engine data are available for temperature, you can set a constant value instead.

Engine data for turbulence boundary values are usually not available, but reasonable settings for a RANS simulation are 10% for the turbulence intensity and 10% of the hydraulic diameter for the length scale.

For motored-test and charge-motion simulations, only air enters the engine through the inlets and the outlets (in case of backflow). For combustion simulations, Automatic Composition Initialization determines the gas composition, see In-cylinder Formulation—Gas Initialization and Boundary Conditions.

注	To modify the gas composition for combustion simulations, you must de-activate Automatic Composition Initialization, see Setting the Operating Conditions.

Wall boundaries are modeled as smooth walls with a constant temperature of 500 K, by default. However, Simcenter STAR-CCM+ In-cylinder also allows you to model rough walls. For the thermal specification, you can also set tables that describe the temperature on the wall as a function of crank angle or of location. Alternatively, you can apply adiabatic conditions.

A table that describes the wall temperature as a function of location must provide coordinates and temperature data for the engine with all valves closed and the piston at BDC. To make sure that the temperature data for the ports are not mapped onto the closed valves and vice versa, prepare separate tables for the valves.

To set the boundary conditions for flow boundaries:

For an inlet boundary, edit the Engine > [engine part] > Inlet [n] node, where [engine part] can be Cylinder or Plenum [n].
For a RANS simulation, within the Edit dialog, in the Boundary group box, set the Turbulence Specification for the boundary, such as Intensity + Length Scale.
Set the absolute total pressure using tabular data:
1. In the Pressure group-box, next to the File drop-down menu, click Import.
2. In the Import Table dialog, navigate to the folder that contains the table.
3. Select the file and click Open.
  In the Graphics window, a plot opens that displays the pressure as a function of crank angle.
4. Set the following table properties:
  - Crank Angle Column
  - Absolute Total Pressure Column
  - Crank Angle Units
  - Absolute Total Pressure Units

Set the temperature depending on whether engine data for the temperature are available or not:

Temperature Engine Data	Procedure
Available	Depending on whether the temperature data are described in the same or in a separate table as the absolute pressure data: If the temperature data are described in the same table, in the Static Temperature group-box, select the previously imported table from the File drop-down menu. If the temperature data are described in a separate table, import the table as described for the pressure table using the Import button in the Static Temperature group-box. The plot in the Graphics window now displays the pressure and temperature curves. Set the following table properties: Crank Angle Column Temperature Column Crank Angle Units Temperature Units
Not available	In the Static Temperature group-box, from the top drop-down menu, select Constant. The default constant temperature is 500 K. To set a specific temperature value, delete the $DefaultAmbientTemperature entry and set another value.

Temperature Engine Data

Procedure

Available

Depending on whether the temperature data are described in the same or in a separate table as the absolute pressure data:
- If the temperature data are described in the same table, in the Static Temperature group-box, select the previously imported table from the File drop-down menu.
- If the temperature data are described in a separate table, import the table as described for the pressure table using the Import button in the Static Temperature group-box.
The plot in the Graphics window now displays the pressure and temperature curves.
Set the following table properties:
- Crank Angle Column
- Temperature Column
- Crank Angle Units
- Temperature Units

Not available

In the Static Temperature group-box, from the top drop-down menu, select Constant.
The default constant temperature is 500 K.
To set a specific temperature value, delete the $DefaultAmbientTemperature entry and set another value.

For a combustion simulation that uses the ECFM-3Z or the ECFM-CLEH model, set the Fuel/Oxidizer Mixing State Specification at the boundary to Unmixed or Premixed.
For a RANS simulation, set the boundary values for the turbulence quantities, such as Turbulence Intensity and Turbulent Length Scale.
Click Apply, then Close.
To set the respective boundary conditions at the outlets, repeat Steps 1-5 using the Engine > [engine part] > Outlet [n] nodes, where [engine part] can be Cylinder or Plenum [n].
For outlet boundaries, you set constant or tabular data for Absolute Static Pressure, respectively.

To set the boundary conditions at a wall:

If you want to set different boundary conditions on specific faces of the Engine Part Surfaces Cylinder > Default : Gases, Intake/Exhaust Valve [n] > Default : Solids, or Plenum > Default : Gases 2, separate those faces into their own Engine Part Surface first.
See Splitting an Engine Part Surface.
Edit the Engine > [engine part] > [wall] node, where [engine part] is the respective Engine Part, such as Cylinder, and [wall] is the Engine Part Surface for which you want to set the wall boundary conditions, such as Liner.
Specify how the wall surface acts on the gas passing along it:
1. In the Edit dialog, set Shear Stress Specification to one of the following:
  - No-Slip—with this method, the fluid sticks to the wall and moves with the same velocity as the wall.
  - Slip—with this method, the wall represents an impenetrable but traction-free surface.
To apply a wall roughness:
1. Make sure that Shear Stress Specification is set to No-Slip.
2. Set Wall Surface Specification to Rough.
3. In the Roughness Height group-box, set the equivalent sand-grain roughness height.
  You obtain the value that is appropriate for your model either from the literature or empirically.
See Engine Part Surface Reference.
Specify the thermal conditions at the wall:
1. Set Thermal Specification to one of the following:
  - Temperature—set this option to specify the static temperature at the wall.
  - Adiabatic—set this option to not permit heat transfer across the boundary.
2. For the Temperature option, set the static temperature at the wall depending on whether engine data for temperature are available or not as described in Step 4.
  If the engine data for temperature vary spatially across the boundary, set the static temperature as follows:
  1. In the Static Temperature group-box, from the top drop-down menu, select TABLE XYZ.
  2. Next to the File drop-down menu, click Import.
  3. In the Import Table dialog, navigate to the folder that contains the table, select the file, and click Open.
  4. Set Temperature Column to the header of the column that contains the temperature values.
  If you set the table after the volume mesh is generated and the mesh is up-to-date, Simcenter STAR-CCM+ In-cylinder automatically maps the data onto the boundary and displays the temperature values in the Graphics window.