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7.19.6 User Inputs for Porous Media
When you are modeling a porous region, the only additional inputs for the problem setup are as follows. Optional inputs are indicated as such. 1. Define the porous zone.
2. Define the porous velocity formulation. (optional)
3. Identify the fluid material flowing through the porous medium. 4. Enable reactions for the porous zone, if appropriate, and select the reaction mechanism.
5. Enable the Relative Velocity Resistance Formulation. By default, this option is already enabled and takes the moving porous media into consideration (as described in Section 7.19.6). 6. Set the viscous resistance coefficients ( or
in Equation 7.19-1,
in
in Equation 7.19-2) and the inertial resistance coefficients (
Equation 7.19-1, or in Equation 7.19-2), and define the direction
vectors for which they apply. Alternatively, specify the coefficients for the power-law model.
7. Specify the porosity of the porous medium.
8. Select the material contained in the porous medium (required only for models that include heat transfer). Note that the specific heat capacity, , for the selected material in the porous zone can only be entered as a constant value.
9. Set the volumetric heat generation rate in the solid portion of the porous medium (or any other sources, such as mass or momentum). (optional) 10. Set any fixed values for solution variables in the fluid region (optional).
11. Suppress the turbulent viscosity in the porous region, if appropriate. 12. Specify the rotation axis and/or zone motion, if relevant.
Methods for determining the resistance coefficients and/or permeability are presented below. If you choose to use the power-law approximation of the porous-media momentum source term, you will enter the
coefficients and in Equation 7.19-3 instead of the resistance coefficients and flow direction.
You will set all parameters for the porous medium in
the Fluid panel (Figure 7.19.1), which is opened from the Boundary Conditions panel (as described in Section 7.1.4).
Figure 7.19.1: The Fluid Panel for a Porous Zone
Defining the Porous Zone
As mentioned in Section 7.1, a porous zone is modeled as a special type of fluid zone. To indicate that the fluid zone is a porous region, enable
the Porous Zoneoption in the Fluid panel. The panel will expand to show the porous media inputs (as shown in Figure 7.19.1).
Defining the Porous Velocity Formulation
The Solver panel contains a Porous Formulation region where you can instruct FLUENT to use either a superficial or physical velocity in the porous medium simulation. By default, the velocity is set to Superficial
Velocity. For details about using the Physical Velocity formulation, see Section 7.19.7.
Defining the Fluid Passing Through the Porous Medium
To define the fluid that passes through the porous medium, select the
appropriate fluid in the Material Name drop-down list in the Fluid panel. If you want to check or modify the properties of the selected material, you can click Edit... to open the Material panel; this panel contains just the properties of the selected material, not the full contents of the standard Materials panel.
If you are modeling species transport or multiphase flow,
the Material Name list will not appear in the Fluid panel. For species calculations, the mixture material for all fluid/porous
zones will be the material you specified in the Species Model panel. For multiphase flows, the materials are specified when you define the phases, as described in Section 23.10.3.
Enabling Reactions in a Porous Zone
If you are modeling species transport with reactions, you can enable reactions in a porous zone by turning on the Reaction option in the Fluid panel and selecting a mechanism in the Reaction Mechanism drop-down list.
If your mechanism contains wall surface reactions, you will also need to specify a value for the Surface-to-Volume Ratio. This value is the surface area of the pore walls per unit volume (
), and can be thought of as a
measure of catalyst loading. With this value, FLUENT can calculate the total surface area on which the reaction takes place in each cell by multiplying
by the volume of the cell. See Section 14.1.4 for details
about defining reaction mechanisms. See Section 14.2for details about wall surface reactions.
Including the Relative Velocity Resistance Formulation
Prior to FLUENT 6.3, cases with moving reference frames used the absolute velocities in the source calculations for inertial and viscous
resistance. This approach has been enhanced so that relative velocities are used for the porous source calculations (Section 7.19.2). Using the Relative Velocity Resistance Formulationoption (turned on by default) allows you to better predict the source terms for cases involving moving meshes or moving reference frames (MRF). This option works well in cases with non-moving and moving porous media. Note that FLUENT will use the appropriate velocities (relative or absolute), depending on your case setup.
Defining the Viscous and Inertial Resistance Coefficients
The viscous and inertial resistance coefficients are both defined in the same manner. The basic approach for defining the coefficients using a Cartesian coordinate system is to define one direction vector in 2D or two direction vectors in 3D, and then specify the viscous and/or inertial resistance coefficients in each direction. In 2D, the second direction, which is not explicitly defined, is normal to the plane defined by the specified direction vector and the direction vector. In 3D, the third direction is normal to the plane defined by the two specified direction vectors. For a 3D problem, the second direction must be normal to the first. If you fail to specify two normal directions, the solver will ensure that they are normal by ignoring any component of the second direction that is in the first direction. You should therefore be certain that the first direction is correctly specified. You can also define the viscous and/or inertial resistance coefficients in each direction using a user-defined function (UDF). The user-defined options become available in the corresponding drop-down list when the UDF has been created and loaded into FLUENT. Note that the coefficients defined in the UDF must utilize theDEFINE_PROFILE macro. For more