9.4.3 Modeling Inputs for Compressible Flows

To set up a compressible flow in ANSYS FLUENT, you will need to follow the steps listed below. (Only those steps relevant specifically to the setup of compressible flows are listed here. You will need to set up the rest of the problem as usual.)

1.   Set the Operating Pressure in the Operating Conditions dialog box.

Boundary Conditions Operating Conditions...

(You can think of $p_{\rm op}$ as the absolute static pressure at a point in the flow where you will define the gauge pressure $p$ to be zero. See Section  8.14 for guidelines on setting the operating pressure. For time-dependent compressible flows, you may want to specify a floating operating pressure instead of a constant operating pressure. See Section  9.4.4 for details.)

2.   Activate solution of the energy equation in the Energy dialog box.

Models Energy Edit...

3.   (Pressure-based solver only) If you are modeling turbulent flow, activate the optional viscous dissipation terms in the energy equation by turning on Viscous Heating in the Viscous Model dialog box. Note that these terms can be important in high-speed flows.

Models Viscous Edit...

This step is not necessary if you are using one of the density-based solvers, because the density-based solvers always include the viscous dissipation terms in the energy equation.

4.   Set the following items in the Create/Edit Materials dialog box:

Materials Create/Edit...

(a)   Select ideal-gas in the drop-down list next to Density.

(b)   Define all relevant properties (specific heat, molecular weight, thermal conductivity, etc.).

5.   Set cell zone conditions and boundary conditions (using the Boundary Conditions task page and Cell Zone Conditions task page), being sure to choose a well-posed cell zone or boundary condition combination that is appropriate for the flow regime. See below for details. Recall that all inputs for pressure (either total pressure or static pressure) must be relative to the operating pressure, and the temperature inputs at inlets should be total (stagnation) temperatures, not static temperatures.

Cell Zone Conditions Boundary Conditions

These inputs should ensure a well-posed compressible flow problem. You will also want to consider special solution parameter settings, as noted in Section  9.4.5, before beginning the flow calculation.

Boundary Conditions for Compressible Flows

Well-posed inlet and exit boundary conditions for compressible flow are listed below:

• For flow inlets:
  • Pressure inlet: Inlet total temperature and total pressure and, for supersonic inlets, static pressure

  • Mass flow inlet: Inlet mass flow and total temperature

• For flow exits:
  • Pressure outlet: Exit static pressure (ignored if flow is supersonic at the exit. All the information travels downstream in a supersonic region, hence the pressure at the outlet can be computed by directly extrapolating from the adjacent cell center [ 32]. Therefore, it is not meaningful to use the exit static pressure prescribed in the boundary conditions task page, and the exit static pressure is ignored).

It is important to note that your boundary condition inputs for pressure (either total pressure or static pressure) must be in terms of gauge pressure--i.e., pressure relative to the operating pressure defined in the Operating Conditions dialog box, as described above.

All temperature inputs at inlets should be total (stagnation) temperatures, not static temperatures.