26.17.3 Checking Boundary and Cell Zone Conditions

Figure 26.17.5: The Boundaries and Cell Zones Tab in the Case Check Dialog Box

The following recommendations appear under the Boundaries and Cell Zones tab (Figure  26.17.5):

• Apply an axis boundary on the centerline (x-axis).

  For geometry that is axisymmetric or axisymmetric swirl (as set in the General task page), the centerline (x-axis) boundary type should be set to axis. See Section  7.3.17.

  Boundary Conditions

• Change inlet boundary conditions. Velocity inlet boundary conditions are not compatible with compressible flow.

  This boundary condition is intended for incompressible flows, and its use in compressible flows will lead to a nonphysical result because it allows stagnation conditions to float to any level (see Section  7.3.4). If you decide to select a different boundary type, go to the Boundary Conditions task page.

  Boundary Conditions

• Change outlet boundary conditions. A combination of pressure and outflow boundaries is not compatible.

  Outflow boundary conditions in ANSYS FLUENT are used to model flow exits where the details of the flow velocity and pressure are not known prior to solution of the flow problem. One of the limitations when using outflow boundary conditions is that outflow boundary conditions are not compatible with pressure inlets. Therefore, it is recommended that you use velocity or mass flow inlets instead of pressure inlets when used in combination with outflow boundaries. See Section  7.3.11 for a list of limitations that exist with outflow boundaries.

  Boundary Conditions

• Change outlet boundary conditions. Outflow boundary conditions are not compatible with the ideal gas law for density.

  Outflow boundaries cannot be used if you are modeling unsteady flows with varying density, even if the flow is incompressible. See Section  7.3.11 for more limitations that exist with outflow boundaries.

  Boundary Conditions

• Non-zero operating pressure set. This will be added to gauge pressure inputs.

  For cases that have density specified as the ideal gas law, and the operating pressure is greater than zero, the operating pressure will be added to the gauge pressure to yield the absolute pressure. For more information, see Sections  8.3.6 and 8.14.2.

  Boundary Conditions Operating Conditions...

• Apply positive non-zero pressure boundary conditions when using the ideal gas law for density.

  In compressible flows , isentropic relations for an ideal gas are applied to relate total pressure, static pressure, and velocity at a pressure inlet boundary. Your input of total pressure, $p'_0$, at the inlet and the static pressure, $p'_s$, in the adjacent fluid cell are related, as described in Equations  7.3-22 and 7.3-23 of Section  7.3.3. It is recommended that pressure boundary conditions are not set to zero for compressible flows that use the ideal gas law.

  Boundary Conditions

• Review turbulence specifications at flow boundaries. Default values detected.

  If your case setup has any of the turbulence models enabled, be sure to review the default parameters for the K and Epsilon Turbulence Specification Method in the outlet and inlet boundary conditions. ANSYS FLUENT's default parameters for the Backflow Turbulent Kinetic Energy and Backflow Turbulent Dissipation Rate are 1. You can either adjust the values, or select a different Turbulence Specification Method. For general information turbulence parameters, see Section  7.3.2.

  Boundary Conditions

• Assign a non-zero wall thickness for wall boundaries with shell conduction.

  When the Shell Conduction option is enabled in the Wall boundary condition dialog box, ANSYS FLUENT will compute heat conduction within the wall, in addition to conduction across the wall. therefore, you must specify a non-zero Wall Thickness in the Wall dialog box, because the shell conduction model is relevant only for walls with non-zero thickness. See Section  7.3.14 for information on shell conduction in thin walls.

  Boundary Conditions

• Assign a value of 0 or 1 for VOF at the inlet or outlet boundary conditions.

  When enabling the VOF model, the Volume Fraction in the inlet and outlet boundary conditions for each phase should be set either to 0 or 1. No intermediate values are permitted. For general information on boundary condition setup, see Section  24.2.9.

  Boundary Conditions

• Change the outlet boundary condition. Outflow boundary condition is not compatible with current multiphase settings.

  You cannot assign an outflow boundary condition when using the mixture and Eulerian multiphase models. Note the limitations of this boundary condition in Section  7.3.11. ANSYS FLUENT can model the effects of open channel flow using the VOF formulation. In such a case, outflow boundary conditions can be used at the outlet of open channel flows, to model flow exits where the details of the flow velocity and pressure are not known prior to solving the flow problem. See this section in the separate Theory Guide , under the heading Outflow Boundary, for more information.

  Boundary Conditions

• Review wall motion. Stationary wall motion relative to adjacent cell zone detected.

  In cases where the fluid zone motion type is specified as Moving Mesh or Moving Reference Frame, all wall zones should be set to Moving Wall in the Momentum tab in the Wall boundary conditions dialog box. The wall motion should be defined Relative to Adjacent Cell Zone. The exception to this is if the walls are stationary in the absolute frame. To define wall motion, see Section  7.3.14.

  Boundary Conditions

• Assign non-zero velocities when specifying a moving fluid zone.

  If selecting either Moving Mesh or Moving Reference Frame in the Fluid dialog box, be sure to set non-zero values for the rotational and translational velocities. Refer to Section  7.2.1 for user inputs.

  Cell Zone Conditions...

• Review flow specifications at inlet boundaries. Default values detected.

  For mass-flow-inlet and velocity-inlet boundary conditions, the default values in ANSYS FLUENT are $1 kg/s$ and $0 m/s$, respectively. Review the settings and adjust accordingly. See Sections  7.3.4 and 7.3.5 for default parameters of velocity inlets and mass flow inlets, respectively.

  Boundary Conditions

• Define the porous zone when using the heat exchanger model.

  Heat exchanger models always require the definition of the porous media zone on the primary side for the macro model and for both primary and auxiliary sides for the dual cell model. See this section in the separate Theory Guide for more information.

  Cell Zone Conditions