Porous media is not absorbing heat by radiation and density remain constant of overall control volume.
We are trying to formulate flow through "solar chimney with porous media as energy storage ".
In fluent we used :
1) k epsilon model
2) DO model for radiation
3) Pressure based model
4) Porous media
For elaborate details of problem refer document attached below .
Use either Darcy or non-Darcy model depending on the flow rate through the porous medium.
Limitations and Assumptions of the Porous Media Model
The porous media model incorporates an empirically determined flow resistance in a region of your model defined as "porous''. In essence, the porous media model is nothing more than an added momentum sink in the governing momentum equations. As such, the following modeling assumptions and limitations should be readily recognized:
- Since the volume blockage that is physically present is not represented in the model, by default ANSYS FLUENT uses and reports a superficial velocity inside the porous medium, based on the volumetric flow rate, to ensure continuity of the velocity vectors across the porous medium interface. As a more accurate alternative, you can instruct ANSYS FLUENT to use the true (physical) velocity inside the porous medium.
When applying the porous media model in a moving reference frame, ANSYS FLUENT will either apply the relative reference frame or the absolute reference frame when you enable the Relative Velocity Resistance Formulation. This allows for the correct prediction of the source terms.
ANSYS FLUENT will, by default, solve the standard conservation equations for turbulence quantities in the porous medium. In this default approach, turbulence in the medium is treated as though the solid medium has no effect on the turbulence generation or dissipation rates. This assumption may be reasonable if the medium's permeability is quite large and the geometric scale of the medium does not interact with the scale of the turbulent eddies. In other instances, however, you may want to suppress the effect of turbulence in the medium.
If you are using one of the turbulence models (with the exception of the Large Eddy Simulation (LES) model), you can suppress the effect of turbulence in a porous region by setting the turbulent contribution to viscosity, 📷, equal to zero. When you choose this option, ANSYS FLUENT will transport the inlet turbulence quantities through the medium, but their effect on the fluid mixing and momentum will be ignored. In addition, the generation of turbulence will be set to zero in the medium. This modeling strategy is enabled by turning on the Laminar Zone option in the Fluid dialog box. Enabling this option implies that 📷 is zero and that generation of turbulence will be zero in this porous zone. Disabling the option (the default) implies that turbulence will be computed in the porous region just as in the bulk fluid flow.
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 in the Cell Zone Conditions task page. (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
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 ANSYS FLUENT. Note that the coefficients defined in the UDF must utilize the DEFINE_PROFILE macro. For more information on creating and using user-defined function, see the separate UDF Manual.
If you are modeling axisymmetric swirling flows, you can specify an additional direction component for the viscous and/or inertial resistance coefficients. This direction component is always tangential to the other two specified directions. This option is available for both density-based and pressure-based solvers
The details of the above discussion in this link
https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node233.htm
Turbulence Modeling for Flows in Wall Bounded Porous Media:
An Analysis Based on Direct Numerical Simulations
Various models are available for simulating turbulent flows in porous media. Models based on the eddy viscosity assumption are often adopted to close the Reynolds stress term. In order to validate the assumptions behind such turbulence models, we studied the dynamics of macroscopic momentum and turbulence kinetic energy in porous media flows by utilizing
Direct Numerical Simulation (DNS). The Generic Porous Matrix (GPM) is composed of regularly arranged spheres. The resulting periodic porous medium is bounded by two walls. The DNS analyses with a Lattice Boltzmann Method were performed for various values of the applied pressure gradient, pore size to channel width ratio, and porosity. The DNS results were averaged over time and volume to obtain macroscopic results. The results show that the
macroscopic shear Reynolds stress in all Representative Elementary Volumes (REVs), independent of their location, is negligibly small, although the mean velocity gradient takes nonzero values near the wall. The turbulence kinetic energy production rate is generally balanced by the dissipation rate in each REV. The DNS results support a zero-equation turbulence model that accounts for the fact that turbulent structures are restricted in size by the pore scale. The DNS results also suggest that the Brinkman term, which expresses the diffusion of momentum, has an important effect near the wall where the gradient of the shear stress is large. Therefore, the Brinkman term should be taken into account in the macroscopic
momentum equation as a component of the total drag. A preliminary acroscopic model for calculating turbulent porous media flows has been proposed and compared with our DNS results.
Article Turbulence modeling for flows in wall bounded porous media: ...
Porous media & k-epsilon
https://www.cfd-online.com/Forums/openfoam-solving/196928-do-you-think-realistic-porous-media-k-epsilon.html
Modeling of Compressible flow through a porous medium: choking at pore scale and its implications
User Inputs for Porous Media in Fluent
http://jullio.pe.kr/fluent6.1/help/html/ug/node236.htm
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. Set the viscous resistance coefficients
define the direction vectors for which they apply. Alternatively, specify the coefficients for the power-law model.
6. Specify the porosity of the porous medium.
7. Select the material contained in the porous medium (required only for models that include heat transfer).
8. Set the volumetric heat generation rate in the solid portion of the porous medium (or any other sources, such as mass or momentum). (optional)
9. Set any fixed values for solution variables in the fluid region (optional).
10. Suppress the turbulent viscosity in the porous region, if appropriate.
11. Specify the rotation axis and/or zone motion, if relevant.
Respected Gamal B. Abdelaziz ,
My gratitude to your answers, I have followed your answers in RG portal, Appreciable clarity in your answers. Thank you
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