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Old   April 28, 2008, 10:15
Default Hi folks, I was wondering a
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Leonardo Nettis
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Hi folks,

I was wondering about the pressure dimension adopted in different solver I'm using (potentialFoam, SimpleFoam, rhoTurbFoam). Indeed I found these different lines in the 0/p files:

p: [0 2 -2 0 0 0 0] m^2/s^2

p: [1 -1 -2 0 0 0 0] Kg/(m*s^2)

Could someone please clarify this issue??
thanks

dino
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Old   April 28, 2008, 10:23
Default The momentum equation's divide
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Sandeep Menon
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The momentum equation's divided by density in the first case, and hence the consistency for pressure dimensions. (You'll have kinematic viscosity here)
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Old   April 28, 2008, 16:55
Default Hi Sandeep, thanks for your
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Leonardo Nettis
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Hi Sandeep,

thanks for your clarification. Anyway, do you mean that if we are in the first case the value to impose in the BC is p/rho (and not simply the pressure)?? Am I right?
thank you in advance.

dino
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Old   March 24, 2009, 14:01
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Milos Stanic
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Yes, you are right... It confused me too. Do you know by any chance what does it actually mean if I change my outlet pressure BC to any other number than 0?
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Old   March 25, 2009, 03:23
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Alberto Passalacqua
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Little exercise: try with simpleFoam to change the pressure at the outlet at different values, let's say p_outlet = 0, 50 and 1000. Then compare the velocity and pressure fields. What do you notice? Why? What kind of flow are you solving for (hint rho = const)? :-)
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Old   March 25, 2009, 14:14
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Oleksiy Kurenkov
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Hello together,

I use simpleFoam and I also found that the pressure used in OF is normalized llike p/rho. Question: what I do see in in Paraview as the pressure is p/rho or really pressure?
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Old   March 26, 2009, 03:34
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Milos Stanic
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As far as I noticed, what you see in ParaView by default (if boundary conditions are default) is p/rho in its relative form, where 1 is the pressure at the outlet (in boundary conditions set as 0). To switch to the relative pressure in [bar] unit you should divide the default number with 100.

To set things a bit clearer, I have attached a photo of my project where you can see the pressure distribution along the pipe elbow. If my interpretation is correct, the numbers on the scale say that if pressure on the outlet was 1bar, then the other pressures along the pipe would be 1+"number from the scale". For example the pressure on the outer side of the elbow would then be 1.407bar and on the inside the pressure would be 0.519.

P.S. - To Alberto: I'll give it a thought as soon as I catch some time. The thing that is bothering me is when I set my outlet pressure to 0, the simulation diverges and if I set it to 0.1, then it works perfectly!
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File Type: jpg p_p_01_U_11.jpg (17.6 KB, 23 views)
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Old   March 26, 2009, 15:31
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Alberto Passalacqua
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Hi Milos,

could you please post your diverging case (if it's not huge to run)? It seems very strange to me the divergence is caused by the pressure at the outlet.

Regards,
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Old   March 26, 2009, 16:31
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Sure, but the thing is .zip or .tar package exceed the allowed size and if I try uploading the files independently - it reports some sort of error. Got an idea how to do it? If you would like I can send it directly to your e-mail ( .tar.gz is around 3.3 MB)?

I did some roaming concerning that outlet pressure issue, found some literature and advices so after I do my reading and contemplating - I'll get back to you. Hopefully with a good understanding of how it all goes.

Cheers!
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Old   March 26, 2009, 16:41
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You can email me (albert.passalacqua@gmail.com) the case, if you want.

Thanks,
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