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siw October 29, 2010 02:50

Porous loss model coefficient query

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 ('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.


ghorrocks October 29, 2010 07:02

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.

enghamed August 23, 2011 00:58

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?


ghorrocks August 23, 2011 06:38

What are you trying to do?

enghamed August 24, 2011 00:34

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?

ghorrocks August 24, 2011 06:21

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.

enghamed August 24, 2011 06:51

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.

ghorrocks August 24, 2011 07:41

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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