# pressure in incompressible solvers e.g. simpleFoam

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 August 23, 2010, 08:45 pressure in incompressible solvers e.g. simpleFoam #1 New Member   Christian Graurock Join Date: Jul 2010 Location: Berlin, Germany Posts: 6 Rep Power: 8 Dear All, I have a general question about the setup of the pressure in an incompressible solver like the simpleFoam-solver. As we all know in simpleFoam the "strange" pressure units are a result of the partition by the density. This is reasonable, because the density will not change in an incompressible flow. Hence the only property of the fluid which can be changed by the user is the kinematic viscosity. In my opinion in a pressure driven flow (pressure difference between inlet and outlet are known from an experiment) you have to normalise the fixed pressure values at inlet and outlet by the density. In common commercial flow solvers you have to define a reference density. That is not possible in simpleFoam and why I wrote down my thoughts in this forum. It would be gald if someone could help me in this open discussion. Regards, Christian

 August 24, 2010, 03:23 #2 New Member   Christian Graurock Join Date: Jul 2010 Location: Berlin, Germany Posts: 6 Rep Power: 8 Another reason for posting this massage is, that I have seen many users who don't care about the normalization of the pressure. I just want to make it clear (for me) in this forum. There are experimental data like: Inlet: 1,013,250 Pa Outlet: 101,325 Pa --> Delta p: 911,925 Pa With a density of air round about 1,2 kg/m³ you've got: Inlet: 844,375 m²/s² Outlet: 84,437.5 m²/s² --> Delta p: 759,937.5 m²/s² Which setup is the correct one? In my opinion it's the second one Thanks, Christian

 August 24, 2010, 05:41 #3 New Member   Björn Westendorf Join Date: Oct 2009 Location: Stuttgart, Germany Posts: 9 Rep Power: 9 Changing the scale and/or the unit of pressure should not change the physics of your problem.

 August 24, 2010, 06:02 #4 Member   Join Date: Mar 2009 Location: Norway Posts: 96 Rep Power: 9 Well, you could just have a look at the units atop in the p-file. From there it should be very clear if your normalization is correct.

 August 24, 2010, 06:07 #5 New Member   Christian Graurock Join Date: Jul 2010 Location: Berlin, Germany Posts: 6 Rep Power: 8 Thank you for your reply. I know, that the physics are the same, but the first setup only make sense in a compressible simulation, the second one in an incompressible simulation. If you set your pressure difference, which is known from an experiment without density-normalization to your simpleFoam-setup you will get a setup, which is different to the experiment (factor in this case 1.2). --> The setup does not mirror your experiment. Is this correct? From your reply I guess, that my thoughts are correct. Thanks a lot.

 August 24, 2010, 08:08 #6 Member   Sven Degner Join Date: Mar 2009 Location: Zürich Posts: 54 Rep Power: 9 Hi everyone, I'm with Cristian, was your experimental data's initialization with a pressure difference, you must dived the pressure through the density ("for a normalization"). After this the results should look like the experimental data's. A other opportunity is the work with a post processor. best regards Sven

August 24, 2010, 08:41
#7
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Björn Westendorf
Join Date: Oct 2009
Location: Stuttgart, Germany
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Christian, sorry that I misunderstood your Question.

Quote:
 If you set your pressure difference, which is known from an experiment without density-normalization to your simpleFoam-setup you will get a setup, which is different to the experiment (factor in this case 1.2). --> The setup does not mirror your experiment. Is this correct?
Yes, absolutely. You must divide the pressure (Unit Pa) by density first when using an incompressible solver.

August 24, 2010, 11:19
#8
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Christian Graurock
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Quote:
 Originally Posted by Bjw Yes, absolutely. You must divide the pressure (Unit Pa) by density first when using an incompressible solver.
Thanks, that's exactly what I was looking for.

April 8, 2016, 03:47
#9
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Prasanna
Join Date: Jan 2016
Location: Norway
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Quote:
 Originally Posted by chrizzl Dear All, I have a general question about the setup of the pressure in an incompressible solver like the simpleFoam-solver. As we all know in simpleFoam the "strange" pressure units are a result of the partition by the density. This is reasonable, because the density will not change in an incompressible flow. Hence the only property of the fluid which can be changed by the user is the kinematic viscosity. In my opinion in a pressure driven flow (pressure difference between inlet and outlet are known from an experiment) you have to normalise the fixed pressure values at inlet and outlet by the density. In common commercial flow solvers you have to define a reference density. That is not possible in simpleFoam and why I wrote down my thoughts in this forum. It would be gald if someone could help me in this open discussion. Regards, Christian

Hi,

I was reading this thread these days. And I found this will help to future readers. Like to add a comment.

In simplefoam continuity equation doesn't include the density because incompressible fluid. But in momentum equation density is a parameter(constant). Therefore pressure has introduced as pressure/density.

units [pressure/density] = m2/s3

cheers.

June 9, 2016, 11:01
#10
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Quote:
 Originally Posted by prasa units [pressure/density] = m2/s3
Hi Prasa

Thanks for wanting to clarify this but the unit should be m2/s2, right?

All the best,
Jan

June 9, 2016, 12:30
#11
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Prasanna
Join Date: Jan 2016
Location: Norway
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Quote:
 Originally Posted by Jan Brunner Hi Prasa Thanks for wanting to clarify this but the unit should be m2/s2, right? All the best, Jan
Hi Jan,

Yes it should be m2/s2.

Regards,
Prasa.

 Tags incompressible, pressure, reference density, simplefoam, solver

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