Porous domain problem
 

Dear CFX Users,

I would like to calculate a steady, compressible flow with porous domains. The parameters of porous domains were not known therefore we completed a measurement to calculate this.
The porous domain (a special oil filter) was filled in a large tube and we measured the pressure drop before/after the oil filter. This value shows the pressure drop of oil filter only, the pressure drop of measurement system is about 50 Pa.
We evaluated our measurement; the points show a parabolic function. To calculate the parameters of porous domain, we used the next equation:
A*v^2+B*v=pressure drop
where:
- A = quadratic coefficient in porous domain
- B = linear coefficient in porous domain
(The velocity (v) was calculated in the cross section before the porous domain directly.)

We used these A, B coefficientsand true velocity option for the porous domain in the CFX model. Finally we set the value of porosity during the simulation to reach the measured points. With this method, we get all of measured points with simulation.
Now, we would like to apply this porous domain in a different geometry where the oil filter is same. We measured the pressure drop on this geometry too and we completed the simulation but the results don’t give back the measured pressure drops. I don’t understand, what is the reason….

Please help me. Perhaps is wrong our calculation method for the parameters of porous domain?
Best regards;
Roland

Do you have changed your filter's mesh ? I remember that porous media is really mesh dependent.

also take note that the true equation is :

dP = A x v^2 x L + B x v x L

L is the length of your filter (how big is the distance of porous media the flow have to pass by)

Thanks Thomas!

And what do you think, what is the "v" exactly?
I know, it is the superficial velocity. But to tell the truth, this definition is not clear for me fully...
Superficial velocity: the averaged velocity in the porous media? Or the velocity before the porous media?
If I have ribs before the porous media, which velocity is the superficial one? The velocity between the ribs before the porous domain? or the averaged velocity in porous material?

Thanks a lot
Roland