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September 7, 2004, 05:58 
about porous cells

#1 
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Hi, as you can imagine this is someone who is struggling to model flow through a porous medium. There are two things that are really annoying about Star CD and this type of flow. The first thing is the permeability coefficients; why is it that while in all the literature in fluid mechanics the permeability is given in [L^2], whereas in Star CD, you have to: a) define it in terms of a pressure drop, and then b) find a linear relationship with two coefficients given in alfa [M L^4] and beta [M L^3 T]!. The other thing is the porosity factor, I dont know if you have notice it but, I measured the concentration of the species [moles/L^3] in a void space and then in a porous medium and they are both the same, even though there suppose to be a solid fraction. I would be greatful if someone out there could tell me how to convert from normal permeability units in to 'alfa and beta' terms, without having to consider a pressure drop? and also why is the porosity not working properly? Kind regards CM


September 7, 2004, 09:28 
Re: about porous cells

#2 
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Yeah you are absolutly right. I have asked myself several times why CD uses this variant for porosity definition. In Fluent for example it is very simple. You can define your permeability in [L^2]and in addition to that you can define a pressure jump. Thats it. So I would recommend to use FLUENT if you have it.
And I agree for some alpha beta values you can't reach a converged solution!!! e.g for alpha = 3 and beta = 80 and a flowspeed of 2030 m/s convergence is impossible. I work with STARCD for six and FLUENT for 2 years. And from day to day I have to ascertain that FLUENT is superior in so much cases especially the new 6.1 version. It is an absolut mystery why CD make simple things so complicated. (not only the porosity!!) I hope some CD developers have a look at this thread so that they realize that there solution is not ideal. 

September 10, 2004, 13:13 
Re: about porous cells

#3 
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CMB, Wow. It seems you are trying to make a square peg fit in a round hole. Instead of worrying about the 'alfa and beta', have you tried specifing the porous diffusivity and schmidt numbers in the Additional Scalar Properties panel? (If you have very little pressure drop, then don't try to 'convert from normal permeability units'). Good luck, R


September 11, 2004, 00:47 
Re: about porous cells

#4 
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10Sep2004 Dear Sir
Sub: Technical Support Could you elaborate on how Starcd solves porous media flow using NavierStokes Equation for solving x,y,z momentum for a 3D case as Darcy's law is the governing equation for fluid flow in porous media. I guess the pressure gradient term in Darcy Law acts as a momentum sink term (negative momentum source term) in the NavierStokes equation. In a nutshell, connect with an example (Pressure Gradient) NavierStokes and (Pressure Gradient)Darcy Law Please explain for Case 1 : I have only porous media flow with an example Case 2 : I have a channel flow with porous media fitted at one end of the tube with an example Please Reply Thanks Bharath 

September 11, 2004, 00:48 
Re: about porous cells

#5 
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10Sep2004 Dear Sir
Sub: Technical Support Could you elaborate on how Starcd solves porous media flow using NavierStokes Equation for solving x,y,z momentum for a 3D case as Darcy's law is the governing equation for fluid flow in porous media. I guess the pressure gradient term in Darcy Law acts as a momentum sink term (negative momentum source term) in the NavierStokes equation. In a nutshell, connect with an example (Pressure Gradient) NavierStokes and (Pressure Gradient)Darcy Law Please explain for Case 1 : I have only porous media flow with an example Case 2 : I have a channel flow with porous media fitted at one end of the tube with an example Please Reply Thanks 

September 20, 2004, 09:55 
Re: about porous cells

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September 20, 2004, 10:06 
Re: about porous cells

#7 
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you are right the flow through the porous medium in Star is modelled with Darcy's law, just as it says in the manual it is Ki ui = grad p, where K is the permeability, u the velocity and p the pressure, this would be your reduced momentum equation in the porous medium. Then you can modify it for polar or cartesian coordinates or do whatever you want with it. The problem is that, it is not very straightforward the way you define the permeability in Star, it is still a mystery for most of us.


October 28, 2004, 22:12 
Re: about porous cells

#8 
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Can someone resolve the 'mystery' ?


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