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7.3.20 Porous Jump Boundary Conditions

Porous jump conditions are used to model a thin "membrane'' that has known velocity (pressure-drop) characteristics. It is essentially a 1D simplification of the porous media model available for cell zones. Examples of uses for the porous jump condition include modeling pressure drops through screens and filters, and modeling radiators when you are not concerned with heat transfer. This simpler model should be used whenever possible (instead of the full porous media model) because it is more robust and yields better convergence.

The thin porous medium has a finite thickness over which the pressure change is defined as a combination of Darcy's Law and an additional inertial loss term:

 \Delta p = - \left(\frac{\mu}{\alpha} v + C_2 \frac{1}{2} \rho v^2 \right) \Delta m (7.3-69)

where $\mu$ is the laminar fluid viscosity, $\alpha$ is the permeability of the medium, $C_2$ is the pressure-jump coefficient, $v$ is the velocity normal to the porous face, and $\Delta m$ is the thickness of the medium. Appropriate values for $\alpha$ and $C_2$ can be calculated using the techniques described in Section  7.2.3.

User Inputs for the Porous Jump Model

Once the porous jump zone has been identified (in the Boundary Conditions task page), you will set all modeling inputs for the porous jump in the Porous Jump dialog box (Figure  7.3.37), which is opened from the Boundary Conditions task page (as described in Section  7.1.4).

Figure 7.3.37: The Porous Jump Dialog Box

The inputs required for the porous jump model are as follows:

1.   Identify the porous-jump zone.

2.   Set the Face Permeability of the medium ( $\alpha$ in Equation  7.3-69).

3.   Set the Porous Medium Thickness ( $\Delta m$).

4.   Set the Pressure-Jump Coefficient ( $C_2$).

5.   Define the discrete phase boundary condition for the porous jump (for discrete phase calculations).

Identifying the Porous Jump Zone

Since the porous jump model is a 1D simplification of the porous media model, the porous-jump zone must be modeled as the interface between cells, rather than a cell zone. Thus the porous-jump zone is a type of internal face zone (where the faces are line segments in 2D or triangles/quadrilaterals in 3D). If the porous-jump zone is not identified as such by default when you read in the mesh (i.e., if it is identified as another type of internal face zone), you can use the Boundary Conditions task page to change the appropriate face zone to a porous-jump zone.

Define $\rightarrow$ Boundary Conditions...

The procedure for changing a zone's type is described in Section  7.1.3. Once the zone has been changed to a porous jump, you can open the Porous Jump task page (as described in Section  7.1.4) and specify the porous jump parameters listed above.

Defining Discrete Phase Boundary Conditions for the Porous Jump

If you are modeling a discrete phase of particles, you can set the fate of particle trajectories at the porous jump. See Section  23.4 for details.

Postprocessing for the Porous Jump

Postprocessing suggestions for a problem that includes a porous jump are the same as for porous media problems. See Section  7.2.3.

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