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guy November 23, 2008 09:41

porosity value in porous baffle star-ccm+
Hi there,

Does anybody know why setting the porosity of a porous baffle affects results when no heat transfer is included. In my work, I have already considered the baffle porosity when determining the coeff.s in the pressure drop equation, so it doesn't make sense to do it again in STAR-CCM. This is why I have set porosity to 1 and carried on. Is this correct?

Guy November 26, 2008 17:22

Re: porosity value in porous baffle star-ccm+
It seems that the issue of having to specify a porosity as well as alpha and beta with star has cropped up in this forum with no real answer.

In my CFD models I want to get the air velocity behind two types of windbreaks, namely a porous fence (made of yorkshire boarding) and permeable screens.

The two materials have the same porosities yet have different alpha's and beta's. So I assume correctly setting their porosity coeff's in the software would be innacurate. That is unless porosity is basically a factor to "tune" CFD results, in which case I'm out of luck cos I have no experimental data on this. Am I right by letting this equal to 1 and what are the max values for the alpha/beta coeffs accepted by star-ccm+.

Hopefully someone out there has the answer

Pauli November 26, 2008 17:30

Re: porosity value in porous baffle star-ccm+
If I remember correctly, alpha and beta are for the pressure drop side while porosity has to do with heat transfer. It's in the manual.

Guy November 26, 2008 17:55

Re: porosity value in porous baffle star-ccm+
I know, i have read the manual. What it says is "The viscous shear stress is computed as a linear combination of the shear stress in the fluid and the shear stress at the wall. The porosity provides the linear weighting."

In fact the porosity specification has a huge effect on the velocity predictions.

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