Setting Initial Conditions

You specify the initial pressure and temperature conditions in the cylinder, ports, and plenums using constant values or tables that describe these quantities as functions of crank angle. For data in tabular form, Simcenter STAR-CCM+ In-cylinder evaluates the initial values at the specified start angle using linear interpolation between the data points. Additionally, for a cylinder sector model, you specify an initial swirl in terms of a rotational velocity about the central axis. For a RANS simulation, you specify constant values for the turbulence quantities.

For the initial pressure and temperature conditions in the cylinder, ports, and plenums, use relevant and verified engine data if available. Such data can be derived from experimental results or a 1-D simulation.

You require the following pressure data:

Cylinder: absolute total pressure
Intake port / intake plenum: absolute total pressure
Exhaust port / exhaust plenum: absolute static pressure

If no such data is available, initialize the engine components as follows:

Cylinder: Use the exhaust boundary conditions. This recommendation assumes that the simulation starts close to the end of the exhaust phase.
Intake port / intake plenum: Use the intake boundary conditions.
Exhaust port / exhaust plenum: Use the exhaust boundary conditions.

The supported file formats are .csv, .txt, .dat.

You can describe the initial pressure and temperature values in a single table or in separate tables.

Turbulence is generated around the valves during the intake process and hence the solution is not a strong function of initial turbulence values. The best way of initializing turbulence parameters is to utilise the results of a 3D simulation of the gas exchange process that runs over multiple cycles. By default, Simcenter STAR-CCM+ In-cylinder calculates the initial turbulence kinetic energy $k_{i n i t}$ and turbulence dissipation rate $ε_{i n i t}$ using the following strategy based on the rpm, stroke, and bore values:

Calculates the mean piston velocity from $v_{m e a n}^{p} = 2 \cdot s t r o k e \cdot r p m / 60$
Estimates the velocity fluctuation from $v' = 0.5 \cdot v_{m e a n}^{p}$ (it has been found that a factor of 0.5 gives results that are close to the 3D simulation).
Estimates the turbulence kinetic energy from $k_{i n i t} = \frac{3}{2} \cdot {(v')}^{2}$
Specifies the turbulence length scale as $l_{i n i t} = 0.025 \cdot b o r e$
Calculates the turbulence dissipation rate from $ε_{i n i t} = C_{μ}^{3 / 4} \cdot \frac{k_{i n i t}^{3 / 2}}{l_{i n i t}}$ , where $C_{μ} = 0.09$ .

For motored-test and charge-motion simulations, Simcenter STAR-CCM+ In-cylinder initializes the cylinder and the ports with air. For combustion simulations, Automatic Composition Initialization determines the initial gas composition, see In-cylinder Formulation—Gas Initialization and Boundary Conditions.

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

To set initial conditions:

Right-click the Engine > Cylinder / Cylinder Sector node and select Edit.
For a RANS simulation, within the Edit dialog, in the Initial Conditions tab, set the Turbulence Specification for the cylinder and the ports, such as K + Epsilon.

Set initial conditions for the cylinder / cylinder sector:

In the Cylinder group-box, set the initial pressure using one of the following methods:


Method	Procedure
Constant	From the Absolute Total Pressure drop-down menu, select Constant. Below the Absolute Total Pressure property, in the text-box, set the initial absolute total pressure in the cylinder.
Table	From the Absolute Total Pressure drop-down menu, select Table. Next to the File drop-down menu, click Import. In the Import Table dialog, navigate to the folder that contains the table. Select the file and click Open. In the Graphics window, a plot opens that displays the pressure in the cylinder as a function of crank angle. Set the following table properties: Crank Angle Column Absolute Total Pressure Column Crank Angle Units Pressure Units

Set the initial temperature using one of the following methods:


Method	Procedure
Constant	From the Temperature drop-down menu, select Constant. Below the Temperature property, in the text-box, set the initial temperature in the cylinder.
Table	Depending on whether the temperature data are described in the same or in a separate table as the pressure data: If the temperature data are described in the same table, from the File drop-down menu below the Temperature property, select the previously imported table. If the temperature data are described in a separate table, import the table as described for the pressure table using the Import button below the Temperature property. The plot in the Graphics window now displays the absolute pressure and temperature curves for the cylinder. Set the following table properties: Crank Angle Column Temperature Column Crank Angle Units Temperature Units

To determine the absolute total pressure and temperature values that Simcenter STAR-CCM+ In-cylinder uses for initialization, hover the mouse over the respective curve and stop at the specified crank angle.
The coordinates are displayed next to the pointer.
For a combustion simulation that uses the ECFM-3Z model, set the initial Fuel/Oxidizer Mixing State Specification to Unmixed or Premixed.
For a cylinder sector model, set the initial velocity in terms of an angular velocity Omega (in units of rpm) about the z-axis of the Cylinder Cylindrical Coordinate System.
For a RANS simulation, set the initial values for the turbulence quantities, such as Turbulent Dissipation Rate and Turbulent Kinetic Energy.

Set initial conditions for the ports:
1. Repeat Step 3 for the Intake Port [n] and the Exhaust Port [n] group boxes, where [n] specifies the number of the respective port.
  For exhaust and intake ports, you specify constant or tabular data for Absolute Static Pressure and Absolute Total Pressure, respectively.
2. Click Apply, then Close.
For more information, see Engine Reference—Cylinder Dialog: Initial Conditions.
Set initial conditions for the plenums:
1. Right-click the Engine > Plenum [n] node and select Edit.
2. For a RANS simulation, within the Edit dialog, in the Initial Conditions tab, set the Turbulence Specification for the plenum, such as K + Epsilon.
3. In the Fixed Plenum group box, set initial conditions for pressure, temperature, fuel/oxidizer mixing state specification, and turbulence quantities as described in Step 3.
4. Click Apply, then Close.
  For more information, see Engine Reference—Plenum Dialog: Initial Conditions.