porous pressure drop. Bugs?

whiz · February 18, 2010, 03:32

Hi, all,
I am trying to simulate a case in which I include the porous media.

I meet a problem: the mass flowrate from CFD is always larger than the measured data.
So I did a very simple case:
1. air with variable properties flow through a pipe.
2. All the domain is porous, and I defined the permeability and inetial coefficient.

Inlet: pressure and temperature
outlet: pressure

Then I compare the mass flowrate from FLUENT with that from Darcy-Forchheimer equation. The FLUENT software always over predict the mass flowrate.

Can anybody help me explain this? overcome this problem?......
Regards,

hvem10 · February 18, 2010, 17:28

And you have of course used Presto og higher order for pressure and minimum second order for momentum and calculated as double precision.
Pressure drop calculations (and mass flow as in your case) is always very sensitive to how accurate you model the problem.
Have you tried to change the mesh, pressure drop is also sensitive to the mesh. Mesh independant studies can easy show a difference in pressure drop on 10-20 %

whiz · February 19, 2010, 20:25

Dear Susan,
Thanks for your reply.
But I still think there are problem with the porous model in FLUENT software.

You can try like this:
1. Define the permeability and inetial coefficient (as you want).
2. define the inlet as: velocity inlet, temperature
outlet: pressure
3. Compute the pressure drop in FLUENT when the case get converged
4. substitute the permeability, ineatial coefficient, viscosity, density and velocity into the "Darcy-Forchheimer" equation. You will find the pressure drop values are different.

PS: you can define the air properties as: ideal gas, use the sutherland model to compute the dynamic vistosity.

Regards,
Wu

whiz · February 19, 2010, 20:27

I am sure what I am talking about is independent on the mesh, convergence or something like that. It is because the porous model in FLUENT software has bug(s).

and, if you use CFX software, you can compare the two software. You will find the porous model in CFX is right.

cloudnqh · March 16, 2010, 13:02

I'm also thinking something is wrong with porous medium model in Fluent. My case is different, if you compare the porous medium model results to detailed model results, the porous medium model always has lower pressure drop or lower mass flow rate if boundary conditions are the same for both models.

Quote:

Originally Posted by whiz

I am sure what I am talking about is independent on the mesh, convergence or something like that. It is because the porous model in FLUENT software has bug(s).

and, if you use CFX software, you can compare the two software. You will find the porous model in CFX is right.

whiz · March 16, 2010, 20:18

Dear cloudnqh,
you are right!

I am sure there are bugs in FLUENT about the porous model.

February 18, 2010, 03:32	porous pressure drop. Bugs?	#1
whiz New Member wu Join Date: Jul 2009 Posts: 14 Rep Power: 17	Hi, all, I am trying to simulate a case in which I include the porous media. I meet a problem: the mass flowrate from CFD is always larger than the measured data. So I did a very simple case: 1. air with variable properties flow through a pipe. 2. All the domain is porous, and I defined the permeability and inetial coefficient. Inlet: pressure and temperature outlet: pressure Then I compare the mass flowrate from FLUENT with that from Darcy-Forchheimer equation. The FLUENT software always over predict the mass flowrate. Can anybody help me explain this? overcome this problem?...... Regards,

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February 18, 2010, 17:28		#2
hvem10 New Member Susan Join Date: Oct 2009 Posts: 26 Rep Power: 17	And you have of course used Presto og higher order for pressure and minimum second order for momentum and calculated as double precision. Pressure drop calculations (and mass flow as in your case) is always very sensitive to how accurate you model the problem. Have you tried to change the mesh, pressure drop is also sensitive to the mesh. Mesh independant studies can easy show a difference in pressure drop on 10-20 %

February 19, 2010, 20:25		#3
whiz New Member wu Join Date: Jul 2009 Posts: 14 Rep Power: 17	Dear Susan, Thanks for your reply. But I still think there are problem with the porous model in FLUENT software. You can try like this: 1. Define the permeability and inetial coefficient (as you want). 2. define the inlet as: velocity inlet, temperature outlet: pressure 3. Compute the pressure drop in FLUENT when the case get converged 4. substitute the permeability, ineatial coefficient, viscosity, density and velocity into the "Darcy-Forchheimer" equation. You will find the pressure drop values are different. PS: you can define the air properties as: ideal gas, use the sutherland model to compute the dynamic vistosity. Regards, Wu

February 19, 2010, 20:27		#4
whiz New Member wu Join Date: Jul 2009 Posts: 14 Rep Power: 17	I am sure what I am talking about is independent on the mesh, convergence or something like that. It is because the porous model in FLUENT software has bug(s). and, if you use CFX software, you can compare the two software. You will find the porous model in CFX is right.

March 16, 2010, 20:18		#6
whiz New Member wu Join Date: Jul 2009 Posts: 14 Rep Power: 17	Dear cloudnqh, you are right! I am sure there are bugs in FLUENT about the porous model.