7.3.10 Inputs at Pressure Far-Field Boundaries

Summary

You will enter the following information for a pressure far-field boundary:

• static pressure

• Mach number

• temperature

• flow direction

• turbulence parameters (for turbulent calculations)

• radiation parameters (for calculations using the P-1 model, the DTRM, the DO model, or the surface-to-surface model)

• chemical species mass or mole fractions (for species calculations)

• discrete phase boundary conditions (for discrete phase calculations)

All values are entered in the Pressure Far-Field dialog box (Figure  7.3.10), which is opened from the Boundary Conditions task page (as described in Section  7.1.4).

Figure 7.3.10: The Pressure Far-Field Dialog Box

Defining Static Pressure, Mach Number, and Static Temperature

To set the static pressure and temperature at the far-field boundary in the Pressure Far-Field dialog box, enter the appropriate values for Gauge Pressure and Mach Number in the Momentum tab. The Mach number can be subsonic, sonic, or supersonic. Set the Temperature in the Thermal tab.

Defining the Flow Direction

You can define the flow direction at a pressure far-field boundary by setting the components of the direction vector. If your geometry is 2D non-axisymmetric enter appropriate values for X and Y in the Pressure Far-Field dialog box (Figure  7.3.10). If your geometry is 2D axisymmetric, enter the appropriate values for Axial, Radial, and (if you are modeling axisymmetric swirl) Tangential-Component of Flow Direction.

If your geometry is 3D, you can choose a Coordinate System that is Cartesian, Cylindrical, or Local Cylindrical. In the Cartesian coordinate system, enter the appropriate values for X, Y, and Z-Component of Flow Direction. If the direction cosine data on the boundary is available, then use the cylindrical or local cylindrical coordinate system and specify the Axial, Radial, Tangential-Component of Flow Direction. For Cylindrical, axis parameters need to be specified on the adjacent cell zone of the boundary face. For Local Cylindrical Swirl, specify the Axis Origin and Axis Direction.

Defining Turbulence Parameters

For turbulent calculations, there are several ways in which you can define the turbulence parameters. Instructions for deciding which method to use and determining appropriate values for these inputs are provided in Section  7.3.2. Turbulence modeling is described in Chapter  12.

Defining Radiation Parameters

If you are using the P-1 radiation model, the DTRM, the DO model, or the surface-to-surface model, you will set the Internal Emissivity and (optionally) External Black Body Temperature Method. See Section  13.3.6 for details.

Defining Species Transport Parameters

If you are modeling species transport, you will set the species mass or mole fractions under Species Mass Fractions or Species Mole Fractions. See Section  15.1.5 for details.

Defining Discrete Phase Boundary Conditions

If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the pressure far-field boundary. See Section  23.4 for details.

Default Settings at Pressure Far-Field Boundaries

Default settings (in SI) for pressure far-field boundary conditions are as follows:

Gauge Pressure	0
Mach Number	0.6
Temperature	300
X-Component of Flow Direction	1
Y-Component of Flow Direction	0
Z-Component of Flow Direction	0
Turbulent Kinetic Energy	1
Turbulent Dissipation Rate	1

Calculation Procedure at Pressure Far-Field Boundaries

The pressure far-field boundary condition is a non-reflecting boundary condition based on the introduction of Riemann invariants (i.e., characteristic variables) for a one-dimensional flow normal to the boundary. For flow that is subsonic there are two Riemann invariants, corresponding to incoming and outgoing waves:

where $v_n$ is the velocity magnitude normal to the boundary, $c$ is the local speed of sound and $\gamma$ is the ratio of specific heats (ideal gas). The subscript $\infty$ refers to conditions being applied at infinity (the boundary conditions), and the subscript $i$ refers to conditions in the interior of the domain (i.e., in the cell adjacent to the boundary face). These two invariants can be added and subtracted to give the following two equations:

where $v_n$ and $c$ become the values of normal velocity and sound speed applied on the boundary. At a face through which flow exits, the tangential velocity components and entropy are extrapolated from the interior; at an inflow face, these are specified as having free-stream values. Using the values for $v_n$, $c$, tangential velocity components, and entropy the values of density, velocity, temperature, and pressure at the boundary face can be calculated.