7.2.1 Fluid Conditions

A fluid zone is a group of cells for which all active equations are solved. The only required input for a fluid zone is the type of fluid material. You must indicate which material the fluid zone contains so that the appropriate material properties will be used.

If you are modeling species transport and/or combustion, you will not select a material here; the mixture material is specified in the Species Model dialog box when you enable the model. Similarly, you will not specify the materials for a multiphase flow here; you will choose them when you define the phases, as described in Section  24.3.4.

Optional inputs allow you to set sources or fixed values of mass, momentum, heat (temperature), turbulence, species, and other scalar quantities. You can also define motion for the fluid zone. If there are rotationally periodic boundaries adjacent to the fluid zone, you will need to specify the rotation axis. If you are modeling turbulence using one of the $k$- $\epsilon$ models, the $k$- $\omega$ model, or the Spalart-Allmaras model, you can choose to define the fluid zone as a laminar flow region. If you are modeling radiation using the DO model, you can specify whether or not the fluid participates in radiation.

For information about porous zones, see Section  7.2.3.

Inputs for Fluid Zones

You will set all fluid conditions in the Fluid dialog box (Figure  7.2.1), which is accessed from the Cell Zone Conditions task page (as described in Section  7.1.4).

Figure 7.2.1: The Fluid Dialog Box

Defining the Fluid Material

To define the material contained in the fluid zone, select the appropriate item in the Material Name drop-down list. This list will contain all fluid materials that have been defined (or loaded from the materials database) in the Create/Edit Materials dialog box. If you want to check or modify the properties of the selected material, you can click Edit... to open the Edit Material dialog box; this dialog box contains just the properties of the selected material, not the full contents of the standard Create/Edit Materials dialog box.

If you are modeling species transport or multiphase flow, the Material Name list will not appear in the Fluid dialog box. For species calculations, the mixture material for all fluid zones will be the material you specified in the Species Model dialog box. For multiphase flows, the materials are specified when you define the phases, as described in Section  24.3.4.

Defining Sources

If you wish to define a source of heat, mass, momentum, turbulence, species, or other scalar quantity within the fluid zone, you can do so by enabling the Source Terms option. See Section  7.2.5 for details.

Defining Fixed Values

If you wish to fix the value of one or more variables in the fluid zone, rather than computing them during the calculation, you can do so by enabling the Fixed Values option. See Section  7.2.4 for details.

Specifying a Laminar Zone

When you are calculating a turbulent flow, it is possible to "turn off'' turbulence modeling in a specific fluid zone. To disable turbulence modeling, turn on the Laminar Zone option in the Fluid dialog box. This will disable the turbulence production, but transport the turbulence quantities. If, in addition, you want to set the turbulent viscosity to zero you can do that using the text command define/ boundary-conditions/fluid. You will be asked if you want to Set Turbulent Viscosity to zero within laminar zone?. If your response is yes, ANSYS FLUENT will set both the production term in the turbulence transport equation and $\mu_t$ to zero. This is useful if you know that the flow in a certain region is laminar. For example, if you know the location of the transition point on an airfoil, you can create a laminar/turbulent transition boundary where the laminar cell zone borders the turbulent cell zone. This feature allows you to model turbulent transition on the airfoil.

Disabling turbulence modeling in a fluid zone, can be applied to all the turbulence models except the Large Eddy Simulation (LES) model.

Specifying a Reaction Mechanism

If you are modeling species transport with reactions, you can enable a reaction mechanism in a fluid zone by turning on the Reaction option and selecting an available mechanism from the Reaction Mechanism drop-down list. See Section  15.1.3 more information about defining reaction mechanisms.

Specifying the Rotation Axis

If there are rotationally periodic boundaries adjacent to the fluid zone or if the zone is rotating, you must specify the rotation axis. To define the axis, set the Rotation-Axis Direction and Rotation-Axis Origin. This axis is independent of the axis of rotation used by any adjacent wall zones or any other cell zones. For 3D problems, the axis of rotation is the vector from the Rotation-Axis Origin in the direction of the vector given by your Rotation-Axis Direction inputs. For 2D non-axisymmetric problems, you will specify only the Rotation-Axis Origin; the axis of rotation is the $z$-direction vector passing through the specified point. (The $z$ direction is normal to the plane of your geometry so that rotation occurs in the plane.) For 2D axisymmetric problems, you will not define the axis: the rotation will always be about the $x$ axis, with the origin at (0,0).

Defining Zone Motion

To define zone motion for a rotating or translating reference frame, select Moving Reference Frame from the Motion Type drop-down list (visible if you scroll down using the scroll bar to the right of the Rotation-Axis Origin and Direction) and then set the appropriate parameters in the expanded portion of the dialog box.

To define zone motion for a sliding mesh, select Moving Mesh in the Motion Type drop-down list and then set the appropriate parameters in the expanded portion of the dialog box. See this section in the separate Theory Guide for details.

For problems that include linear, translational motion of the fluid zone, specify the Translational Velocity by setting the X, Y, and Z components. For problems that include rotational motion, specify the rotational Speed under Rotational Velocity. The rotation axis is defined as described above.

See Chapter  10 for details about modeling flows in moving reference frames.

Defining Radiation Parameters

If you are using the DO radiation model, you can specify whether or not the fluid zone participates in radiation using the Participates in Radiation option. See Section  13.3.6 for details.