what is the meaning of p_rgh?

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 September 23, 2016, 11:36 what is the meaning of p_rgh? #1 New Member   Simone Colucci Join Date: Mar 2016 Location: Pisa (Italy) Posts: 23 Rep Power: 6 Hi, I have a simple question. In many solvers (for example in reactingTwoPhaseEulerFoam, OpenFoam 4.0) the PEqn solves an equation for p_rgh, instead of p. p_rgh is defined as: Code: `p_rgh = p - rho*gh;` where : Code: `volScalarField gh("gh", (g & mesh.C()) - ghRef);` If I consider, for example, a vertical pipe with the z_bottom (vertical coordinate)=-10 and the z_top=0, rho*g*h has a physical meaning, but if the coordinates are z_bottom = 0 and z_top = 10, what does it mean? Thank you in advance Simone

 September 26, 2016, 13:45 #2 Member   Hasan Celik Join Date: Sep 2016 Posts: 58 Rep Power: 6 Can you clarify your point little bit more? I mean, I guess h is the difference in height in both cases you mentioned, so isn't it 10?

 September 26, 2016, 23:15 #3 Senior Member   Pablo Higuera Join Date: Jan 2011 Location: Auckland Posts: 567 Rep Power: 14 Hi Simone, the truth is that it has no physical meaning, it is just a convenient mathematical artifact. It is explained in my thesis (page 111): https://repositorio.unican.es/xmlui/...=1&isAllowed=y Best, Pablo Ivory and geth03 like this.

June 14, 2020, 05:07
#4
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Himanshu
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Quote:
 Originally Posted by Phicau Hi Simone, the truth is that it has no physical meaning, it is just a convenient mathematical artifact. It is explained in my thesis (page 111): https://repositorio.unican.es/xmlui/...=1&isAllowed=y Best, Pablo
Hi Pablo,
So, it mean no matter what the co-ordinate system our model is in. the result will be same as long as our BC remain same.
To elaborate my question, if I did simulation for one geometry and now I translate that geometry by 10 cm. Will the result be same if I perform simulation on this new translated geometry?

 June 14, 2020, 17:37 #5 Senior Member   Pablo Higuera Join Date: Jan 2011 Location: Auckland Posts: 567 Rep Power: 14 Hi, not necessarily. The main advantage is to avoid large round-off errors if numbers are large, as P can be. Best, Pablo Himanshu_Shrivastava likes this. __________________ Check out my new project: olaFlow --> The olaFlow Support Thread

June 14, 2020, 17:55
#6
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Herpes Free Engineer
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Quote:
 To elaborate my question, if I did simulation for one geometry and now I translate that geometry by 10 cm. Will the result be same if I perform simulation on this new translated geometry?
- I haven't read the other replies, so if misunderstood, my apologies.
- The governing equations of fluid dynamics are Galilean invariant, which means that making any transformation or rotation on the computational domain within an inertial frame of reference will not change the predictions if the boundary conditions are modified accordingly.
- Therefore, translating the domain by 10 cm in a given direction will not change the numerical predictions if the control volume keeps modelling the same volume.

 June 15, 2020, 09:49 #7 New Member   Himanshu Join Date: Jan 2017 Posts: 25 Rep Power: 5 Thank You for the replies.

 June 15, 2020, 10:07 #8 Senior Member   Pablo Higuera Join Date: Jan 2011 Location: Auckland Posts: 567 Rep Power: 14 Hi HPE, despite the equations being Galilean invariant, once you discretize them it is another story. The round off errors and the solution will be different. To what extent is a question of the processes and how long you run the simulation for, but "chaos" plays a role here and results may be noticeably different. For example, you can observe this in my simulations when waves break. Best, Pablo __________________ Check out my new project: olaFlow --> The olaFlow Support Thread

 June 15, 2020, 12:18 #9 Senior Member     Herpes Free Engineer Join Date: Sep 2019 Location: The Home Under The Ground with the Lost Boys Posts: 670 Rep Power: 8 Hi, There are circumstances where Galilean invariance breaks down for the finite volume method in Eulerian frame, true; but from an engineering perspective, the reason in the current context is the acute sensitivity of the governing equations of particular physics on initial/boundary conditions. What you have observed can also happen for a restarted simulation wherein the restart is not bitwise, which is very common in any simulation of any software. Personally, I would not be concerned with such ultra academic break downs of Galilean invariance in discretised domains since even the Navier-Stokes equations have only weak solutions, which means there might be some flow behaviour not obeying N-S in some part of universe at some point. But, who cares. Thank you for your kind remarks. __________________ The OpenFOAM community is the biggest contributor to OpenFOAM: User guide/Wiki-1/Wiki-2/Code guide/Code Wiki Nilsson/Guerrero/Holzinger/Holzmann/Nagy/Santos/Nozaki/Jasak-FSB OpenFOAM Governance and Technical Committees Report bugs/Request features: OpenFOAM (ESI-OpenCFD-Trademark) Report bugs/Request features: FOAM-Extend (Wikki-FSB) Report bugs: OpenFOAM (Foundation) How to create a MWE. OpenFOAM Journal

November 6, 2020, 22:21
#10
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Hasan Celik
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Quote:
 Originally Posted by Phicau Hi Simone, the truth is that it has no physical meaning, it is just a convenient mathematical artifact. It is explained in my thesis (page 111): https://repositorio.unican.es/xmlui/...=1&isAllowed=y Best, Pablo
I think this is not correct. It is pseudo-hydrostatic pressure as it is explained in prghPressureFvPatchScalarField.H. It is nothing related to dynamic or pseudo-dynamic pressure to my opinion.