Little question concerning pressure dimension

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 April 28, 2008, 10:15 Hi folks, I was wondering a #1 Member   Leonardo Nettis Join Date: Mar 2009 Posts: 72 Rep Power: 9 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

 April 28, 2008, 10:23 The momentum equation's divide #2 Senior Member   Sandeep Menon Join Date: Mar 2009 Location: Amherst, MA Posts: 387 Rep Power: 16 The momentum equation's divided by density in the first case, and hence the consistency for pressure dimensions. (You'll have kinematic viscosity here) __________________ Sandeep Menon University of Massachusetts Amherst https://github.com/smenon

 April 28, 2008, 16:55 Hi Sandeep, thanks for your #3 Member   Leonardo Nettis Join Date: Mar 2009 Posts: 72 Rep Power: 9 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

 March 24, 2009, 14:01 #4 New Member   Milos Stanic Join Date: Mar 2009 Location: Novi Sad, Serbia Posts: 29 Rep Power: 9 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?

 March 25, 2009, 03:23 #5 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,910 Rep Power: 27 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)? :-) __________________ 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 open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

 March 25, 2009, 14:14 #6 New Member   Oleksiy Kurenkov Join Date: Mar 2009 Location: Nueremberg Posts: 16 Rep Power: 9 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?

March 26, 2009, 03:34
#7
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Milos Stanic
Join Date: Mar 2009
Posts: 29
Rep Power: 9
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!
Attached Images
 p_p_01_U_11.jpg (17.6 KB, 23 views)

 March 26, 2009, 15:31 #8 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,910 Rep Power: 27 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, __________________ 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 open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

 March 26, 2009, 16:31 #9 New Member   Milos Stanic Join Date: Mar 2009 Location: Novi Sad, Serbia Posts: 29 Rep Power: 9 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!

 March 26, 2009, 16:41 #10 Senior Member   Alberto Passalacqua Join Date: Mar 2009 Location: Ames, Iowa, United States Posts: 1,910 Rep Power: 27 You can email me (albert.passalacqua@gmail.com) the case, if you want. Thanks, __________________ 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 open-source implementation of quadrature-based moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using.

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