CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM

what is the meaning of p_rgh?

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree4Likes
  • 2 Post By Phicau
  • 1 Post By Phicau
  • 1 Post By HPE

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 23, 2016, 11:36
Default what is the meaning of p_rgh?
  #1
New Member
 
Simone Colucci
Join Date: Mar 2016
Location: Pisa (Italy)
Posts: 23
Rep Power: 6
S.Colucci is on a distinguished road
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
S.Colucci is offline   Reply With Quote

Old   September 26, 2016, 13:45
Default
  #2
Member
 
Hasan Celik
Join Date: Sep 2016
Posts: 58
Rep Power: 6
PositronCascade is on a distinguished road
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?
PositronCascade is offline   Reply With Quote

Old   September 26, 2016, 23:15
Default
  #3
Senior Member
 
Pablo Higuera
Join Date: Jan 2011
Location: Auckland
Posts: 567
Rep Power: 14
Phicau is on a distinguished road
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.
Phicau is offline   Reply With Quote

Old   June 14, 2020, 05:07
Default
  #4
New Member
 
Himanshu
Join Date: Jan 2017
Posts: 25
Rep Power: 5
Himanshu_Shrivastava is on a distinguished road
Quote:
Originally Posted by Phicau View Post
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?
Himanshu_Shrivastava is offline   Reply With Quote

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

Old   June 14, 2020, 17:55
Default
  #6
HPE
Senior Member
 
HPE's Avatar
 
Herpes Free Engineer
Join Date: Sep 2019
Location: The Home Under The Ground with the Lost Boys
Posts: 670
Rep Power: 8
HPE is on a distinguished road
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.
HPE is offline   Reply With Quote

Old   June 15, 2020, 09:49
Default
  #7
New Member
 
Himanshu
Join Date: Jan 2017
Posts: 25
Rep Power: 5
Himanshu_Shrivastava is on a distinguished road
Thank You for the replies.
Himanshu_Shrivastava is offline   Reply With Quote

Old   June 15, 2020, 10:07
Default
  #8
Senior Member
 
Pablo Higuera
Join Date: Jan 2011
Location: Auckland
Posts: 567
Rep Power: 14
Phicau is on a distinguished road
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
Phicau is offline   Reply With Quote

Old   June 15, 2020, 12:18
Default
  #9
HPE
Senior Member
 
HPE's Avatar
 
Herpes Free Engineer
Join Date: Sep 2019
Location: The Home Under The Ground with the Lost Boys
Posts: 670
Rep Power: 8
HPE is on a distinguished road
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.
HPE is offline   Reply With Quote

Old   November 6, 2020, 22:21
Default
  #10
Member
 
Hasan Celik
Join Date: Sep 2016
Posts: 58
Rep Power: 6
PositronCascade is on a distinguished road
Quote:
Originally Posted by Phicau View Post
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.
PositronCascade is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Who can tell me what is the meaning of the macro in fluent UDF:C_PREMIXC_RATE(c,t) ChenZhan Fluent UDF and Scheme Programming 1 May 12, 2017 04:40
What is the meaning of pd in foam-extended 3.1 lxwd OpenFOAM Running, Solving & CFD 2 February 3, 2017 12:54
Meaning of Ychar and Ypmma aylalisa OpenFOAM Pre-Processing 2 October 20, 2013 06:49
What's meaning of UDF FUNCTION zhaoxinyu Fluent UDF and Scheme Programming 0 March 31, 2010 09:04
want to know meaning Sangamesh Siemens 0 May 15, 2007 06:15


All times are GMT -4. The time now is 10:12.