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-   -   Interpretation of pressure icoFoam (http://www.cfd-online.com/Forums/openfoam-solving/59700-interpretation-pressure-icofoam.html)

billy May 5, 2007 23:36

Hi, I was running icoFoam w
 
Hi,

I was running icoFoam with lid-drive cvavity and I noticed that pressure changed in same degree, as I refined the mesh. Doubling the cells in xy direction, the pressure doubles also. Documentation says:

"In a closed incompressible system such as the cavity, pressure is relative: it is the pressure range that matters not the absolute values"

But why does the pressure range change? With 10x10 grid pressure range is -20-20, with 20x20 grid it is -40-40, with 40x40 it is -80-80. Shouldn't it converge to a certain range?

Thanks in advance,

Billy.

billy May 6, 2007 10:46

Increasing even more, using a
 
Increasing even more, using a 60x60 grid gives very high pressure range like -1E+4-1E+4.

Billy.

billy May 6, 2007 19:15

I also tried a grid with one a
 
I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.

http://www.cfd-online.com/OpenFOAM_D...your_image.gif

http://www.cfd-online.com/OpenFOAM_D...your_image.gif

billy May 6, 2007 19:15

I also tried a grid with one a
 
I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.

http://www.cfd-online.com/OpenFOAM_D...ges/1/4380.jpg

http://www.cfd-online.com/OpenFOAM_D...ges/1/4381.jpg

billy May 7, 2007 11:16

The figure shows pressure cont
 
The figure shows pressure contours around the inlet. The outlet is located on other side.
Both domains have same thickness.

billy May 7, 2007 15:41

Basically the pressure seems t
 
Basically the pressure seems to increase at the same rate the volume of the cell decreases.

billy May 8, 2007 17:45

Does anyone know why this happ
 
Does anyone know why this happens? Don't want this topic to die.

alberto May 8, 2007 17:51

Please, post a case. I'll try
 
Please, post a case. I'll try to run it.

A.

rafal May 8, 2007 18:00

I would think about two things
 
I would think about two things:
1. where is the reference pressure cell and
what is the value you are setting in your case.
2. Dont you think that it would be better to
have more dense mesh in the region of big
gradients(4 cells is not enough, remember you
calculate everything on discrete values).

hope this helps
rafal

billy May 8, 2007 18:38

Thanks. Two test cases are inc
 
Thanks. Two test cases are included: one and two layer hex meshes.

http://www.cfd-online.com/OpenFOAM_D...hment_icon.gif hexlayer1.tgz

billy May 8, 2007 18:54

I wasn't able to include secon
 
I wasn't able to include second case because it is bigger than 50k. But if you have Gmsh, the number of layer can be changed edting line 3:

l = 2.

Then running gmshToFoam.

billy May 8, 2007 19:12

You may have to set inlet velo
 
You may have to set inlet velocity like

0.1, 0, 0.

billy May 8, 2007 19:15

Patch 0 is inlet, Patch 1 is
 
Patch 0 is inlet,
Patch 1 is outlet,
Patch 2 is wall,
Patch 3 is wall.

Rafael,

I don't think it has to do with mesh density. The figures were done with interFoam and two fluids but this happens also with the icoFoam case I posted and the pressure variation is smoother.

billy May 8, 2007 19:20

I post two more figures with i
 
I post two more figures with icoFoam.

http://www.cfd-online.com/OpenFOAM_D...ges/1/4424.jpg

http://www.cfd-online.com/OpenFOAM_D...ges/1/4425.jpg

billy May 8, 2007 19:30

I believe the reference pressu
 
I believe the reference pressure is only important in closed cavities. This case the outlet is set to zero.

billy May 9, 2007 17:42

The gradient seems ok. The pro
 
The gradient seems ok. The problem is the value. Anyone have suggestions why this happens? It almost looks like the pressure is per layer.

joakim May 9, 2007 18:06

Hi Billy I should state tha
 
Hi Billy

I should state that I do not know the details of your simulation, but I must ask what your are trying to simulate:

1) From your pictures, it looks to me that your are trying to run a 2D simulation. Why use more than one layer?

2) Have you reached grid-independence? You have walls in your simulation, but I can't see that you have resolved any boundarylayers. When
you change the mesh-density, how does the velocities change?

Best regards

/Joakim

billy May 9, 2007 19:44

1) I am trying to simulate the
 
1) I am trying to simulate the flow inside a thin plate. I want to use several layers because I might have a temperature variation across thickness that I want to capture. This way I would solve equations for flow as well as temperature. However, I want to know also how much pressure is needed, so pressure shouldn't vary so much with the amount of layers I have.

2) This is for laminar flow not turbulent flow. I don't think this difference is due to grid indpendence, there is no cross thickness flow and I don't think pressure should be 1x, 2x, 3x higher as I refine the mesh.

With 3 layers, pressure is 3 times higher. I think this has to do with the pressure correction scheme.

http://www.cfd-online.com/OpenFOAM_D...ges/1/4437.jpg

joakim May 9, 2007 23:41

Hi Billy Isn't your problem
 
Hi Billy

Isn't your problem a 3D problem?

I would assume that you get a parabolic flow profile in the xy-plane between the plates. Your strange results is probobly due to your b.c. I assume that you are currently using empty as b.c. at bottom and top since you started with one layer. Shouldn't you use wall- or atleat slipwall conditions instead and something like 10 cells in the z-direction?

Regards

/Joakim

billy May 10, 2007 10:35

Actually I tried with 10 layer
 
Actually I tried with 10 layers and it blows up. Courant just increases to infinity. There is an inlet, outlet and all other surfaces are walls. I will try another geometry to see what happens.

Billy.


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