Setting Reference Values

Flow simulations require the specification of a reference pressure and a reference pressure location. Additionally, for buoyant flows, you must specify a gravity vector and a reference altitude.

The reference pressure is simply a device that is used to reduce the numerical roundoff error in the numerical calculations involving pressure. This reduction is necessary since the differences in pressure are important, and these differences can be small relative to the absolute value of the pressure, as for example in fully incompressible or very low-Mach number flows. By subtracting a suitable constant reference pressure, a working pressure is obtained that is less prone to roundoff errors.

The default value of reference pressure is 101325 Pa. For simulations that use the Constant Density model or the Polynomial Density model, the actual value of the reference pressure has no relevance to the calculations. However, when using the Ideal Gas model, the reference pressure is used. In all cases, the reference pressure is used in the definition of the Absolute Pressure field function according to [eqn_link].

When simulating steady flow, or the flow of incompressible fluids, the equation for pressure is singular. If the boundary conditions do not define the pressure level (that is, there are no pressure boundaries), you can anchor the pressure level to a datum value in the solution domain. If the working pressure is not anchored to a known value somewhere in the domain, it can float arbitrarily high and cause numerical problems.

To set the reference pressure value:
  1. Select the Continua > [physics continuum] > Models > Reference Values > Reference Pressure node and specify a reference pressure value.
    For simulations using the Ideal Gas model, the following guidelines are suggested for setting the reference pressure:
    • If the Mach numbers are low (say 0.3 or less), set the reference pressure to some ambient value such that the working pressures are small.
    • If the Mach numbers are substantial, you can do one of the following:
      • Set the reference pressure to an ambient value such that the working pressures are small.
      • Set the reference pressure to zero so that working pressures are identical to the absolute pressures.
To set the reference pressure location:
  1. Depending on the Flow model, select one of the following nodes:
    • For the Segregated Flow model, select the Solvers > Segregated Flow > Pressure node.
    • For the Coupled Flow model, select the Solvers > [Coupled Implicit/Coupled Explicit] node.
  2. Set the pressure reference location using one of the following methods:
    MethodSteps
    Automatic

    Choose this method if absolute pressure levels are not important.

    Set Pressure Reference Location to Automatic Selection.

    Simcenter STAR-CCM+ identifies the cell such that the cell centroid has the min(x), min(y), min(z) in the domain. This condition can only occur in cells that are next to boundaries.

    To determine the appropriate cell, Simcenter STAR-CCM+ does the following:
    1. Checks the centroid of every cell that is next to a boundary, and selects all of the cells that have the smallest X component. Simcenter STAR-CCM+ checks all of the boundaries—wall boundaries as well as flow boundaries.
    2. From the cells that are selected in the previous step, Simcenter STAR-CCM+ selects the cells that have the smallest Y component.
    3. From the cells that are selected in the previous step, Simcenter STAR-CCM+ selects the cell that has the smallest Z component.

    Simcenter STAR-CCM+ uses the selected cell as the pressure reference location

    NoteThere is a known limitation with this method. Consider a rectangular prism as a flow domain: in the ideal case Simcenter STAR-CCM+ would select the cell at the lower left front edge of the prism. However, small numerical rounding errors in the centroid calculation and meshing can cause Simcenter STAR-CCM+ to select a different cell nearer the middle of the left side of the prism.
    Manual

    Choose this method if you want to determine absolute pressure levels accurately.

    For example, when you want to evaluate the pressure force on wall boundaries. Choose a location where the pressure is known, and where a fixed value is appropriate for the simulation.
    1. Set Pressure Reference Location to User Specified.

      A Pressure Reference Points node is added as a sub-node.

    2. Right-click this node and select New.
    3. Select the newly added Pressure Reference Points > PressureReferencePoint [n] node and set the following properties:
      • Point Coordinates— set the coordinates of the pressure reference point.
      • Reference System—set the coordinate system for the point coordinates.
      • Enabled—set to On in order to use the pressure reference point.

      When you select the PressureReferencePoint [n] node, the point is displayed in an active scene.
    The working pressure at the pressure reference location is set using the initial pressure value; the absolute pressure is the sum of the reference pressure plus the initial pressure. If the initial pressure value is changed during a simulation (by using a field function, for example), the pressure at the pressure reference location changes accordingly.

Simulations that use the Gravity model or the Boussinesq model require the definition of a gravity vector and an operating altitude. The operating altitude is a position vector defining the location at which the static and piezometric pressures are equal.

To set the gravity vector and the operating altitude:

  1. Select the Reference Values > Gravity node and set Value to the direction and magnitude of the gravity vector.
  2. Select the Reference Values > Reference Altitude node and set Value to the direction and value of the altitude vector.