
[Sponsors] 
May 5, 2007, 23:36 
Hi,
I was running icoFoam w

#1 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
Hi,
I was running icoFoam with liddrive 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 2020, with 20x20 grid it is 4040, with 40x40 it is 8080. Shouldn't it converge to a certain range? Thanks in advance, Billy. 

May 6, 2007, 10:46 
Increasing even more, using a

#2 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
Increasing even more, using a 60x60 grid gives very high pressure range like 1E+41E+4.
Billy. 

May 6, 2007, 19:15 
I also tried a grid with one a

#3 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.


May 6, 2007, 19:15 
I also tried a grid with one a

#4 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
I also tried a grid with one and two layers, the pressure is increases in the same degree that the layers increase.


May 7, 2007, 11:16 
The figure shows pressure cont

#5 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
The figure shows pressure contours around the inlet. The outlet is located on other side.
Both domains have same thickness. 

May 7, 2007, 15:41 
Basically the pressure seems t

#6 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
Basically the pressure seems to increase at the same rate the volume of the cell decreases.


May 8, 2007, 17:45 
Does anyone know why this happ

#7 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
Does anyone know why this happens? Don't want this topic to die.


May 8, 2007, 17:51 
Please, post a case. I'll try

#8 
Senior Member
Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
Posts: 1,904
Rep Power: 27 
Please, post a case. I'll try to run it.
A.
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

May 8, 2007, 18:00 
I would think about two things

#9 
Member
rafal zietara
Join Date: Mar 2009
Location: Manchester, UK
Posts: 60
Rep Power: 9 
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 

May 8, 2007, 18:38 
Thanks. Two test cases are inc

#10 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 

May 8, 2007, 18:54 
I wasn't able to include secon

#11 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
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. 

May 8, 2007, 19:12 
You may have to set inlet velo

#12 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
You may have to set inlet velocity like
0.1, 0, 0. 

May 8, 2007, 19:15 
Patch 0 is inlet,
Patch 1 is

#13 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
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. 

May 8, 2007, 19:20 
I post two more figures with i

#14 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
I post two more figures with icoFoam.


May 8, 2007, 19:30 
I believe the reference pressu

#15 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
I believe the reference pressure is only important in closed cavities. This case the outlet is set to zero.


May 9, 2007, 17:42 
The gradient seems ok. The pro

#16 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
The gradient seems ok. The problem is the value. Anyone have suggestions why this happens? It almost looks like the pressure is per layer.


May 9, 2007, 18:06 
Hi Billy
I should state tha

#17 
New Member
Joakim Möller
Join Date: Mar 2009
Posts: 26
Rep Power: 9 
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 gridindependence? You have walls in your simulation, but I can't see that you have resolved any boundarylayers. When you change the meshdensity, how does the velocities change? Best regards /Joakim 

May 9, 2007, 19:44 
1) I am trying to simulate the

#18 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
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. 

May 9, 2007, 23:41 
Hi Billy
Isn't your problem

#19 
New Member
Joakim Möller
Join Date: Mar 2009
Posts: 26
Rep Power: 9 
Hi Billy
Isn't your problem a 3D problem? I would assume that you get a parabolic flow profile in the xyplane 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 zdirection? Regards /Joakim 

May 10, 2007, 10:35 
Actually I tried with 10 layer

#20 
Senior Member
Billy
Join Date: Mar 2009
Posts: 167
Rep Power: 9 
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. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Two fundamental questions about icoFoam while updating the velocities and pressure  dbxmcf  OpenFOAM Running, Solving & CFD  6  April 12, 2016 16:05 
Density in icoFoam Densidad en icoFoam  manuel  OpenFOAM Running, Solving & CFD  8  September 22, 2010 04:10 
UDF Interpretation  Suvash  FLUENT  0  November 1, 2005 00:10 
UDF interpretation trough TUI  David  FLUENT  2  March 18, 2005 15:26 
Result interpretation  Pilli  FLUENT  2  November 19, 2000 17:58 