Treatment of pressure

Naruto · June 7, 2014, 10:18

Hello,

Actually I am confused about the treatment of pressure in OpenFOAM. I have studied the source code having SIMPLE algorithm. I have found out that, at the very beginning of the simulation the solver divides the pressure with fluid density. In case of incompressible flow, most of the times a value of 1 (air) has been used.

But sometimes we need to use fluid which may not have a density of 1. I have changed the density value in system/forceCoeff file in OpenFOAM and found varied result.

Now my question is if I want to calculate pressure co-eff. Do I need to divide the pressure field provided by OpenFOAM with density? or OpenFOAM is by default providing dynamic pressure.

Thank you

shipman · June 7, 2014, 11:10

Hi Naruto,

Actually, as far as i know that relative pressure (P/rho) is used in the incompressible solvers of OF. So, the provided pressure field is already calculated dividing by rho.Therefore, you dont need to divide again. For further info about pressure treatment in OF you can look at my previous thread: http://www.cfd-online.com/Forums/ope...oam-error.html at post#18.

Hope this will help.

Baris

Naruto · June 8, 2014, 13:51

Dear shipman,
Thanks for your reply.
I think I am getting the philosophy a little. Yesterday I conducted some experiments by myself using pimpleFOAM. Actually if you want to calculate some quantities like force you could direct OpenFOAM by attaching a force function in OpenFOAM. I think you are aware of it. In the file you are required to input the fluid density. If you do not input the free stream fluid density, the solver would use the default value of 1 for density. But if you do, then it would use your entered value. That's what I found out.

So I think if I want to find out pressure co-efficient at a specific location, I would need to divide the quantity by density.

hcs129 · March 16, 2015, 21:16

Hi Shipman

You mentioned that the pressure input is not required to be divided by density. Is it true?
Several posts in this forum implies that the p-file setup should be corrected by density normalization.
post1
post2

Chenshu

shipman · March 17, 2015, 02:38

Hi Chenshu,

Please read the posts again carefully. As i said before, openfoam is using reference pressure (P/rho) not absolute pressure in the case of incompressible solvers. For the advantages about this you can see at your post 1 of you pasted. So, if you run a pressure driven case, which means that you may have experimental inlet and outlet pressures which are absolute pressure for you. If you wanna apply these conditions in to solver OF COURSE you must normalize these value by dividing density. On the other hand, if you wanna calculate the Cp or another force coefficient you dont need to divide as it is well explained at post 1.

Hope this helps you.

Baris

hcs129 · March 17, 2015, 03:25

Hi Shipman
Thank you for your promot reply. You are right. I misunderstanded you before.

Regards

June 7, 2014, 10:18	Treatment of pressure	#1
Naruto Member Join Date: Nov 2012 Posts: 62 Rep Power: 13	Hello, Actually I am confused about the treatment of pressure in OpenFOAM. I have studied the source code having SIMPLE algorithm. I have found out that, at the very beginning of the simulation the solver divides the pressure with fluid density. In case of incompressible flow, most of the times a value of 1 (air) has been used. But sometimes we need to use fluid which may not have a density of 1. I have changed the density value in system/forceCoeff file in OpenFOAM and found varied result. Now my question is if I want to calculate pressure co-eff. Do I need to divide the pressure field provided by OpenFOAM with density? or OpenFOAM is by default providing dynamic pressure. Thank you __________________ Happy Foaming

June 8, 2014, 13:51		#3
Naruto Member Join Date: Nov 2012 Posts: 62 Rep Power: 13	Dear shipman, Thanks for your reply. I think I am getting the philosophy a little. Yesterday I conducted some experiments by myself using pimpleFOAM. Actually if you want to calculate some quantities like force you could direct OpenFOAM by attaching a force function in OpenFOAM. I think you are aware of it. In the file you are required to input the fluid density. If you do not input the free stream fluid density, the solver would use the default value of 1 for density. But if you do, then it would use your entered value. That's what I found out. So I think if I want to find out pressure co-efficient at a specific location, I would need to divide the quantity by density. __________________ Happy Foaming

March 16, 2015, 21:16		#4
hcs129 New Member Hu Join Date: Jan 2014 Posts: 22 Rep Power: 12	Hi Shipman You mentioned that the pressure input is not required to be divided by density. Is it true? Several posts in this forum implies that the p-file setup should be corrected by density normalization. post1 post2 Chenshu Last edited by hcs129; March 16, 2015 at 21:21. Reason: adding detail

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June 7, 2014, 11:10		#2
shipman Senior Member Baris (Heewa) Join Date: Jan 2013 Location: Japan Posts: 130 Rep Power: 13	Hi Naruto, Actually, as far as i know that relative pressure (P/rho) is used in the incompressible solvers of OF. So, the provided pressure field is already calculated dividing by rho.Therefore, you dont need to divide again. For further info about pressure treatment in OF you can look at my previous thread: http://www.cfd-online.com/Forums/ope...oam-error.html at post#18. Hope this will help. Baris

March 17, 2015, 02:38		#5
shipman Senior Member Baris (Heewa) Join Date: Jan 2013 Location: Japan Posts: 130 Rep Power: 13	Hi Chenshu, Please read the posts again carefully. As i said before, openfoam is using reference pressure (P/rho) not absolute pressure in the case of incompressible solvers. For the advantages about this you can see at your post 1 of you pasted. So, if you run a pressure driven case, which means that you may have experimental inlet and outlet pressures which are absolute pressure for you. If you wanna apply these conditions in to solver OF COURSE you must normalize these value by dividing density. On the other hand, if you wanna calculate the Cp or another force coefficient you dont need to divide as it is well explained at post 1. Hope this helps you. Baris

March 17, 2015, 03:25		#6
hcs129 New Member Hu Join Date: Jan 2014 Posts: 22 Rep Power: 12	Hi Shipman Thank you for your promot reply. You are right. I misunderstanded you before. Regards