Setting Up Free-Stream Boundaries

For external compressible flows, a free-stream boundary allows you to model free-stream conditions at a far-field boundary. For atmospheric flows, you can let Simcenter STAR-CCM+ calculate the free-stream pressure and temperature from a specified altitude.

In the case of atmospheric flows, by default Simcenter STAR-CCM+ uses the US 1976 Standard Atmosphere model to compute the pressure and temperature at the specified altitude. Alternativelly, you can define the atmosphere using a table. An atmosphere table must provide one column for the altitude and two further columns for the pressure and temperature at that altitude, respectively.

For more information, see Theory Guide—Free-Stream.

To set up a free-stream boundary:
  1. Select the Regions > [region] > Boundaries > [boundary] node and set Type to Free Stream.
  2. Select the Physics Conditions > Flow Direction Specification node and set the flow direction of the fluid that enters or leaves through the boundary using one of the following Methods:
    • Boundary-Normal—this is the default. The flow direction is normal to each boundary face.
    • Components—set this option to specify the flow direction as individual angle components using the Physcs Values > Flow Direction node.

      This method is particularly useful for specifying the swirling flow from the inlet of an Axisymmetric Swirl simulation. In an axisymmetric space, all three velocity components are functions of only the axial and radial coordinates, z and r. With this method, you can define the swirling flow from the inlet by specifying the appropriate z and r components of the swirling inflow.

    • Angles—set this option to set the flow direction angles directly using the Physcs Values > Flow Angles node.
  3. Select the Physics Conditions > Free Stream Option node and, depending on the available flow data, do one of the following:
    Available Flow DataSteps
    Mach number, pressure, and temperature
    1. Set Free Stream Option to Mach Number + Presssure + Temperature.
    2. Expand the Physics Values node and set the following values:
      • Mach Number
      • Pressure—set the far-field working pressure.
      • Static Temperature—set the far-field static temperature.
    Mach number and altitude
    1. Set Free Stream Option to Mach Number + Altitude.
    2. Expand the Physics Values node and set the following values:
      • Mach Number
      • Altitude—set the height at which you want Simcenter STAR-CCM+ to extract the free-stream pressure and temperature from the atmosphere.
    Altitude, length scale, Reynolds number
    1. Set Free Stream Option to Altitude + Length Scale + Reynolds Number.
    2. Expand the Physics Values node and set the following values:
      • Altitude—as for Mach Number + Altitude.
      • Mach Number—set Length Scale to the characteristic length of the flow problem and Reynolds Number to the ratio of inertial over viscous forces.
  4. If you want Simcenter STAR-CCM+ to extract the free-stream pressure and temperature from your own atmosphere table:
    1. Import the atmosphere table under Tools > Tables.
    2. Select the [boundary] > Physics Conditions > Atmosphere Type Option node and set Option to User Table.
    3. Select the [boundary] > Physics Values > Atmosphere Table node and set the following values:
      • Table—select the imported atmosphere table.
      • Table: Altitude, Table: P, Table: T—set the table columns for altitude, pressure, and temperature, respectively.
      • Units: Altitude, Units: P, Units: T—set the units for altitude, pressure and temperature, respectively.
  5. If you use the Mach Number + Altitude or the Altitude + Length Scale + Reynolds Number option in Step 3, ensure that you set initial values for pressure, static temperature, and velocity that are consistent with the free-stream values. You can expose the free-stream values that Simcenter STAR-CCM+ calculates internally and set them as initial values as follows:
    1. Right-click the [boundary] > Physics Conditions > Free Stream Option node and selecting Print Average Pressure, Velocity, Temperature Values.
    2. Copy and paste the values from the Output window to the properties of each initial condition.
For more information, see Flow Boundaries Reference—Free Stream.