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Old   April 30, 2014, 05:04
Default porous and filters
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Hello,

I have to model a filter. For now, I haven't the geometry and still don't really know what it will look like.

But to prepare myself, I would like to ask some general questions about porosity.

First : What is the true difference with permeability?

When running simulations with STARCCM+, one has to give the porosity, the inertial and viscous resistance coefficients.

From Darcy's Law we have -dp/l = mu*v / k, so Pv could be given by :
Pv = -dp/v/l if we admit that the flow is laminar and isotropic?


Problem is that I won't have a lot of data (no p=f(v) available).
How should I proceed? If I don't know the porosity, will STARCCM+ be ok only with Pv?

About Darcy-Forccheimer : in which situations do we use this kind of formula? in case of high Reynolds? are these situations called the "non-Darcy flows" ?

Thank you for any help.

Last edited by yorelchr; April 30, 2014 at 07:32.
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Old   May 1, 2014, 20:11
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Porosity (in Star-CCM) is the volume fraction of the porous medium, it is the ratio of open volume to total volume. It is needed only for two situations: 1) unsteady simulations by replacing the regular fluid density with the product of the fluid density and the porosity and 2) unsteady or steady simulations involving heat transfer / energy equation, the porosity is used to compute the effective thermal conductivity and specific heats of the fluid/solid mixture in the porous region.

Darcy's law is derived from Stokes flow (creeping flows) and can be applied to laminar or turbulent flows, with the added complication of modeling turbulence in porous media. The Inertial and viscous coefficients can be tensors, so the porous media does not need to be isotropic.

The Forchheimer term adds a quadratic term which helps Darcy's law account for non-linear dependence of pressure drop on velocity. It is more general than Darcy's Law and generally superior, but the absence of data limits its use. The Darcy-Forchheimer equation is better than the regular Darcy's law for high velocities within the porous media. In Star-CCM the Darcy-Forchheimer equation is used by default. The inertial resistance coefficient is the Forchheimer term. To use the regular Darcy's law, simply set this coefficient to be zero. Darcy's law and modifications of it is generally used as long as the flow within the porous media is a continuum because there are few better alternatives for predicting the flow through porous media.
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Old   May 7, 2014, 11:41
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ok thank you for these explanations.

I'd have another question, about interfaces: what is the difference between internal interface and porous baffle interface?
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Old   May 7, 2014, 12:20
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internal interfaces provide the numerical connection between two physical regions (solid-fluid, solid-solid, fluid-fluid type interfaces. Internal interfaces don't alter the physics of either region and the interface itself doesn't contain any physics.

A porous baffle interface simulates a porous or permeable membrane of infinitesimal thickness at the location of the interface. The porous baffle interface does have its own physics. There is a pressure drop across the interface depending on the velocity at the interface. The porous baffle interface uses a equivalent Darcy-Forchheimer equation as a porous region. The porous baffle interface is useful for modeling perforated plates, wire screens, porous regions, etc. without actually having to CAD and mesh those objects explicitly.
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Old   May 7, 2014, 13:00
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ok, so I have a first conduct, followed by a porous zone, then another conduct; since the porous zone is already "porous", do I need to add porosity on the interfaces (conduct1porous zone and porous zone/conduct2) or can I just let them as internal interfaces?
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Old   May 7, 2014, 13:44
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You should use an internal interface. Remember that the internal interface does not introduce new physics whereas a porous baffle interface does.

Using an internal interface would result in:
conduct1|int interface|porous zone & porous zone|int interface|conduct2

Using a porous baffle interface would result in:
conduct1|porous baffle|porous zone & porous zone|porous baffle|conduct2
using a porous baffle interface in this manner would cause additional redundant pressure drop at the inserted interfaces which do not exist physically.

Alternatively you can eliminate the porous region and choose to model it as a single porous baffle in place of the porous region
conduct1|porous baffle|conduct2
using a porous baffle in this fashion accounts for pressure drop across the porous zone without actually needing to grid the region. The porous baffle is inserted as a substitute for the porous region.
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Old   May 8, 2014, 12:35
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Thank you very much for these explanations. I kept the porous zone since I already did the geometry.
But I have "discovered" that with porosity, we can't use multiphase models. I need to have air with particles. So I changed and I put "phase permeability" boundaries. But I don't understand how I can "tell" starccm that some particles do not cross the boundary. It is a problem of filters. I have to work in unsteady I guess? Should I use a field function that would decrease permeability for particles with time...but will some "stay" on the first side?
Thank you for any clue.
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Old   May 8, 2014, 12:58
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Do you even need to model a porous zone anymore? Sounds like you took a big detour that could have been avoided if you had been clearer about exactly what you wanted to model.

Permeable wall with phase permeability should do what you want. You can set different phase permeabilities in the phase conditions. Set the permeability to 0-1, 1 allows everything to pass through, 0 blocks. The phase conditions which show up until you setup the physics and define all the phases.

Be careful because this is a wall boundary condition and not an interface. You will also want to set the wall shear to 100% slip or 0 shear.

Not sure what you're trying to model, so don't know if unsteady is needed or not. It would be needing if you're interested unsteady behavior like particle tracking.
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Old   May 8, 2014, 16:58
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I'm trying to model the efficiency of a filter : air should cross it but the filter should retain some of the particles, unfortunately not all of them, else it would be maybe easier
I have some questions if you have some more time.
.
First question(even if of no use here) : can I use porous baffle even if the filter has a thickness of few millimeters or even centimeters, instead of representing the entire porous zone ?

Second question : about geometry construction and interfaces. When I've built my "conduct1 | filter | conduct2", I obtained 2 faces at each contact zone (for example one at the end of conduct 1 followed by one at the beginning ot the filter); then when transforming parts to regions, I got interfaces created by STAR. Is it better to build just one object and cut it so there is only one face?

Third question: if then I don't geometrically represent the filter and use permeability walls (since I have a multiphase case, porosity is bad for me), I would then have "conduct1 | permeable wall | conduct2 ", I go back to question 1, ie, is it ok if thickness is centimeters? will the drop pressure be ok?

Fourth question, (that could change everything) : air has to cross the filter but some particles should pass and others should be trapped into the filter. For that, should I go back to geometry representation of the filter in this way : "conduct1 | permeable wall | filter | permeable wall |conduct2", so I could put different permeability coefficients for the particle phase at the inlet and outlet of the filter (for example a permeability coefficient for phase particle smaller at the outlet of the filter), else I don't know how to tell STAR to keep the particles. And how can I control this: some particles cross the filter, others no. Do you think that with a field function I could do something?

Fifth question :about particle tracking; I've never used that before, do you think I should explore this way? if so, I will try to understand in what it consists exactly.

I have tried to find some examples on the net, but I did not find anything that could help. Maybe it isn't possible?

Again, thank you very much for your explanations!!!
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Old   May 15, 2014, 11:39
Default porosity and permeability coefficients
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Hi,

Back to the question about the difference between permeability and porosity.
When using the porous model, using Darcy's law, I put Pi=0 and Pv=dp/v/l and it was ok. I hadthe pressure drop I expected.
Since I don't want to use anymore the porous model, I tried first, using Porous baffle. The porous viscous resistence should be expressed in m/s, so how do I calculate it? is there any relation with the coefficient I've used with the porous model? As I couldn't find, I adjusted the coefficient by running several times the calculations; but isn't there a better way to solve that?
Now, I'm working with eulerian phase and permeability condition for the phases, and again I have to change the coefficient. This time I have to put a coefficient of permeability (with no dimension). How should I "choose" it?
Is there a relation between all these coefficients?
Thank you very much for any help.
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Old   May 20, 2014, 13:22
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How to define interface as phase permeable?
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Old   February 7, 2017, 04:49
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Good day,

I was wondering if you were able to solve this problem?

I am doing something similar and would appreciate some assistance.

Thank you
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