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Porous loss model coefficient query

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Old   October 29, 2010, 02:50
Default Porous loss model coefficient query
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siw
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Hi,

The two Momentum Source loss models (isotropic and directional) shown in the CFX Theory Guide (eqs. 1.146 and 1.149) show that the loss coefficient is multiplied by density / 2.

When comparing these equations with the Dupuit-Forchheimer equation (http://en.wikipedia.org/wiki/Darcy's...non-Darcy_flow) the loss coefficient there (Greek letter beta) is multiplied only by density.

I have dervied my loss coefficient from experimental data as like in the Dupuit-Forchheimer equation. So I just want to check if I should really be doubling this value in CFX-Pre so that CFX halves it as shown in the Theory Guide. I'm not sure why CFX uses the 1 / 2. But I've only found reference to the Dupuit-Forchheimer equation in wiki and no actual fluids textbooks.

Thanks.
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Old   October 29, 2010, 07:02
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I don't know the answer to your question, but I know what I would do if I needed to know.

I would set up a simple simulation with a block of porous material and simulate a known flow velocity across it to get a pressure drop. I would then work out the analytical pressure drop and make sure that the loss coefficient I put in was generating the pressure drop expected.

It is always better to work these things out for yourself anyway.
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Old   August 23, 2011, 00:58
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If I don't the value for linear ressistance coefficient, can I not input any value? just select thethe isotropic loss to linear and quadratic resistance coefficients but then don't tick the box for entering a value. Is this ok?

thanks
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Old   August 23, 2011, 06:38
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What are you trying to do?
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Old   August 24, 2011, 00:34
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I am modeling a sloshing tank with mesh motion. Now I want to put in cylindrical baffel inside the tank which want to be 50% porous. I set it as isotropic loss but I dont know the linear and quadratic resistance coefficients. Can I not tick the linear and quadratic resistance coefficients options so I dont have to enter the value for it?

Another problem that i have faced is that in the porous setting I cant define mesh motion, but i simulate sloshing by mesh motion so what should i do?
thx
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Old   August 24, 2011, 06:21
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I am not sure you can have a porous region with mesh motion. If this cannot be done then you should impose the porous region yourself as a momentum source term.
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Old   August 24, 2011, 06:51
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thx. can you tell me how should I impose the porous region as momentum source? how is this done? what do i need to do?

thx a lot.
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Old   August 24, 2011, 07:41
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Determine the equation of your porous region (ie pressure drop versus velocity) and impose that as a momentum source. This is discussed in the documentation. You will probably need to include the source term linearisation term - again, see the doco.
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