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pressure in incompressible solvers e.g. simpleFoam

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Old   August 23, 2010, 08:45
Default pressure in incompressible solvers e.g. simpleFoam
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Christian Graurock
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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
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Old   August 24, 2010, 03:23
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Christian Graurock
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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/m you've got:
Inlet: 844,375 m/s
Outlet: 84,437.5 m/s
--> Delta p: 759,937.5 m/s

Which setup is the correct one? In my opinion it's the second one
Thanks,
Christian
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Old   August 24, 2010, 05:41
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Bjrn Westendorf
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Changing the scale and/or the unit of pressure should not change the physics of your problem.
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Old   August 24, 2010, 06:02
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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.
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Old   August 24, 2010, 06:07
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Christian Graurock
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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.
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Old   August 24, 2010, 08:08
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Sven Degner
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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
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Old   August 24, 2010, 08:41
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Bjrn Westendorf
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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.
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Old   August 24, 2010, 11:19
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Christian Graurock
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Quote:
Originally Posted by Bjw View Post
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.
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