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CFX: what is "A true volume-porous media model?"

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Old   December 27, 2008, 10:49
Default CFX: what is "A true volume-porous media model?"
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whiz
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Hi, all, I see "A true volume-porous media model" from the website: http://www.ansys.com/assets/brochures/cfx-11.pdf

Can anybody explain it for me? and I will be grateful if anybody can give me some materials about this model.

Regards, whiz

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Old   December 30, 2008, 23:58
Default Re: CFX: what is "A true volume-porous media model
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R. Janny
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Hello whiz,

Before CFX 11, to model a porous media (like a screen plate or honeycomb), one would use a momentum-losses approach to account for pressure drop and deflection effects over a fluid sub-domain. This was achieved by defining a extra source term in the momentum equation, with help of loss coefficients (like the pressure loss coefficient, among others) within a given fluid sub-domain.

Now in CFX 11, there is a extra domain (other than fluid and solid) called "porous" which should be considered the preferred way to do the same thing as described above.

The details of this approach/model are described on "CFX-Solver theory manual". Basically, a generalized form of Navier-Stokes equation together with Darcy's law are solved over a given domain. Simple said, the Navier-Stokes equation are solved in a form which accounts for the volume porosity of the media and the pressure source is expressed in terms of the Darcy's law.

--Roberto
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Old   December 31, 2008, 06:59
Default Re: CFX: what is "A true volume-porous media model
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whiz
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Dear Janny, Thank you very much for your reply. Do you mean the "A true volume-porous media model" is the porous-domain model which is parallel to fluid-domain and solid domain? I know the moment equation in porous media, and moment equation of clear fluid as well. But I modeled one simple case with both fluid-domain and porous-domain, I noticed the velocity is not continous at the interface of "fluid-porous"? I want to know why the velocity is not continous there and I also want to know the theory of "fluid-porous" interface. Thank you very much! Whiz
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Old   December 31, 2008, 09:33
Default Re: CFX: what is "A true volume-porous media model
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R. Janny
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By "porous domain" I meant to say that CFX offers a formalism for fluid flow though porous media, including interface (zero thickness) boundary treatment.

This "discontinuity", or rather sharp variation, originates when considering interface treatment in the model. The previous models were not able to capture sharp gradients by sole using momentum sources.

The "true volume-porous" model uses a "double-node" approach at the porous interface, so to capture sharp pressure and velocity discontinuities that may occur. As far as I know, this "double-node" are really two nodes of the adjacent continuum elements that represent the potentials in the pore system on each side of the interface.

And as far as I understand, this "double-node" can account for both longitudinal and transversal pressure losses, if one need to model both (please, someone correct me if I'm wrong).

Additionally, the discretization of the pressure source in the "true volume-porous" model, on the interface, is spatially third-order accurate based on a "pressure redistribution term". The pressure redistribution term can produce spurious velocity fields, when using default Rhie Chow discretization. These wiggles may be greatly reduced by redistributing the body force with a discretization with reference on the porous drag source term.

Well, I really don't know about the specifics of CFX modeling for porous domain, more than what I've said.

--Roberto
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Old   December 31, 2008, 10:02
Default Re: CFX: what is "A true volume-porous media model
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whiz
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Dear Roberto, I have one paper "Analysis of fluid flow and heat transfer interfacial conditions between a porous medium and a fluid layer", by B. Alazmi, K. Vafai. In this paper, the author sumed up there are 5 methods to treat the fluid-porous interface. And as I know, there is "channel effect" between porous media and a wall, many scientist attribute this to the variable porosity near the wall. And the velocity profile from the CFX simulation seems reasonable, but I can't get any theory behind the result. whiz
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Old   January 2, 2009, 13:02
Default Re: CFX: what is "A true volume-porous media model
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bharath
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Dear friends,

can anybody explain me how to include the porous pressure drop, when the same porous domain treated as a heat source to?
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Old   January 3, 2009, 04:22
Default Re: CFX: what is "A true volume-porous media model
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whiz
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would you please explain your problem in more detail? whiz
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Old   January 3, 2009, 09:07
Default Re: CFX: what is "A true volume-porous media model
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bharath
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thanks for ur response whiz,

am doing simulation with tube bundles, where heat is removed and pressure drop occurs at the same bundles.so i model tube bundles as porous media and calculating quadratic resistance coefficient for a fixed pressure drop with out considering the heat transfer through bundles first.after obtaining the pressure drop through porous media,the heat source is supplied.now what is the problem is due to the heat transfer through porous,the pressure drop first i fixed is changed to a new value.but i don't want the drop to change.how can i get this by calculating the quadratic resistance with heat transfer effects. thanks in advance
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Old   January 5, 2009, 10:46
Default Re: CFX: what is "A true volume-porous media model
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whiz
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"the pressure drop first i fixed is changed to a new value"

as I know, for porous media, you just add source term to moment equation to take account of the pressure drop. In CFX, you just input a few coefficient, from the coefficient you provide, CFX then calculate the pressure drop. Do you input pressure drop directly? and where?

By the way, I guess the pressure drop is caused by the change of material properties such as density and viscousity. Regards, Whiz
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Old   January 5, 2009, 22:12
Default Re: CFX: what is "A true volume-porous media model
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bharath
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thanks whiz,

am not give the pressure drop as input directly.by giving the coefficient am obtaining the drop.when the source is not given as input the drop is matching however when source given as input drop is changed.
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Old   January 6, 2009, 03:55
Default Re: CFX: what is "A true volume-porous media model
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whiz
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because of the fluid properties change with temperatrue.
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Old   January 19, 2009, 00:02
Default Re: CFX: what is "A true volume-porous media model
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HekLeR
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The velocity is not continuous because the volume change also implies an area change at the interface. So, for equal mass flow rates in the open domain and the porous domain, the velocity has to go up in the porous media.

As explained by another poster, the interface is treated specially in CFX to allow this to happen discontinuously. Total pressure on side 1 and side 2 are assumed to be equivalent, accounting for the velocity change due to the flow area change.
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Old   January 19, 2009, 04:31
Default Re: CFX: what is "A true volume-porous media model
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whiz
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Dear HekLeR: in the CFX-pre, I see "conservative interface flux" about mass and momentum and turbulence option for the "fluid-porous" interface boundary. Do you have any insight theory about this technique? Kindly regards, whiz
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Old   June 3, 2013, 15:55
Default Porous
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Does anyone has idea regarding the Natural convection in Porous media in ansy14? If you have links and pdf than please share it.
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Old   June 5, 2013, 05:54
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What specifically are you looking for? There are lots of examples of porous flow and lots of natural convection, but not many of both I suspect.
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Old   June 5, 2013, 09:33
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Thank you for the reply Ghorrocks. Actually, I am looking for the Natural convection process that takes place in a porous media. If u have link or pdf just in porous flow beside natural convection than here is my ad- amarcheetri@gmail.com
Thanks
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Old   June 5, 2013, 23:29
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Why don't you just try running the case you are interested in? There should be enough info in the manuals to get you going.
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Old   June 6, 2013, 00:19
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thank you chris...I am going through the manual I have but it is straight forward...if you have proper pdf file regarding porous (in ansys) would you please send for me...thank you
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