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limit for porous alpha and beta coefficients

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Old   December 19, 2008, 20:15
Default limit for porous alpha and beta coefficients
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Hi all, Does anyone know if there is a maximum threshold (and what they are) for the alpha and beta coefficients used in the porous baffle assumption. I am using star-ccm+ but I guess the same limits would apply to star-cd; so if anyone knows that would be great.
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Old   December 19, 2008, 20:58
Default Re: limit for porous alpha and beta coefficients
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There is no upper and lower limit, what you need to be careful of is having too large a difference between the lowest and highest xyz components of alpha and beta. Star-CCM+ seems to actually be more picky about this... I cannot get away with more than about 3 maybe 4 orders of magnitude difference between the components.
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Old   December 21, 2008, 09:25
Default Re: limit for porous alpha and beta coefficients--
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Hi asdf,

Thanks for your kind advice. Just to clarify, do you mean the height of the porous interface? Or do you mean the relative magnitude of the alpha and beta coefficients?

Also in your calculations do you change the porosity coefficient to represent the actual (physical) porosity of your baffle? In my simulations I have assumed that this coefficient is needed when pressure drop data has been obtained from actual velocity instead of superficial velocity, and I have therefore considered the porosity coefficient equals unity; given the fact that cells are 100% open in the CFD sim. Is this justified in your opinion?

Thanks again for your help
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Old   December 21, 2008, 11:59
Default Re: limit for porous alpha and beta coefficients--
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The StarCD manuals (methodology/user) explain converting pressure drop to alpha and beta, actual versus superficial velocity, and porosity. It gives guidance regarding relative magnitude & recommends one coefficient (beta?) not be set to zero.

Sorry but it's been a while since I use porosity & don't remember it all off the top of my head.

I don't use CCM+. So I can't comment on what is done in that code.
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