CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Pressure correction equation [SIMPLE]

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

Like Tree2Likes
  • 1 Post By FMDenaro
  • 1 Post By AliE

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   January 22, 2018, 19:36
Default Pressure correction equation [SIMPLE]
  #1
Member
 
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 10
kh.aa is on a distinguished road
In developing a code...

After we solve the pressure correction equation PP, we need to correct the velocities u,v and the pressure P, such that:

P = P* + URF * (PP - PPref)
where PPref is a reference value, usually assigned to the cell (2,2)
URF is the under-relaxation factor

Why would we use a reference pressure in correcting the pressure after the pressure correction equation?

Thanks ...
kh.aa is offline   Reply With Quote

Old   January 25, 2018, 05:21
Default
  #2
Senior Member
 
Join Date: Dec 2017
Posts: 153
Rep Power: 8
AliE is on a distinguished road
Quote:
Originally Posted by kh.aa View Post
In developing a code...

Why would we use a reference pressure in correcting the pressure after the pressure correction equation?

Thanks ...
Hello,

The problem is well know in literature and in text books (see for example Fezinger and Peric). You have to specify a reference pressure every time you have defined Neumann BC (zero gradient) at all your boundaries (e.g. lid driven cavity). This is due to the fact that the pressure-correction equation is a Poisson-like equation and if you fix the pressure's derivative everywhere, then your solution is defined up to an arbitrary constant. In other words your matrix is singular and this may create huge problem when you are solving the linear system. This ambiguity is removed by defining the pressure value in one cell of your choice, since your solution depends on pressure gradient only and not on absolute pressure. If, for example, you have an outlet where usually the pressure is fixed, then you have not to define a reference (your equation has at least one dirichlet condition) and everithing is fine as is.

Hope this help,

Alie
AliE is offline   Reply With Quote

Old   January 25, 2018, 05:30
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,760
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
I would highlight that fixing a value is not a necessary condition. If the prescribed Neumann BC.s satisfies the compatibility condition in its discrete form the solution is ensured (up to an arbitrary function).
AliE likes this.
FMDenaro is offline   Reply With Quote

Old   January 25, 2018, 05:53
Default
  #4
Member
 
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 10
kh.aa is on a distinguished road
Quote:
Originally Posted by AliE View Post
Hello,

The problem is well know in literature and in text books (see for example Fezinger and Peric). You have to specify a reference pressure every time you have defined Neumann BC (zero gradient) at all your boundaries (e.g. lid driven cavity). This is due to the fact that the pressure-correction equation is a Poisson-like equation and if you fix the pressure's derivative everywhere, then your solution is defined up to an arbitrary constant. In other words your matrix is singular and this may create huge problem when you are solving the linear system. This ambiguity is removed by defining the pressure value in one cell of your choice, since your solution depends on pressure gradient only and not on absolute pressure. If, for example, you have an outlet where usually the pressure is fixed, then you have not to define a reference (your equation has at least one dirichlet condition) and everithing is fine as is.

Hope this help,

Alie
Thanks, that helped of course but I am not getting it quite well thought.

The problem here is with that "reference pressure", I've always like a dead value always there, e.g. in all of obstacles, I have to set u, v, ... and all the variables to zero. However, the pressure is always there with a great value "Pref"

According to that, I am getting unrealistic, I think, pressure contours.

To be specific, I am talking here about 2D incompressible flow, non orthogonal coordinates with uniform inlet, outlet, wall and axi-symmetric with some obstacles in the flow.

Thanks..
kh.aa is offline   Reply With Quote

Old   January 25, 2018, 05:57
Default
  #5
Member
 
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 10
kh.aa is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
I would highlight that fixing a value is not a necessary condition. If the prescribed Neumann BC.s satisfies the compatibility condition in its discrete form the solution is ensured (up to an arbitrary function).
Actually, I tried that to set it zero and drop it at all, I ended up with more iterations but the results are more realistic, somehow.
kh.aa is offline   Reply With Quote

Old   January 25, 2018, 06:04
Default
  #6
Senior Member
 
Join Date: Dec 2017
Posts: 153
Rep Power: 8
AliE is on a distinguished road
Ok , if you have an outlet then you have not to specify a pressure reference from some other cell, since the pressure is already fixed there.

Standard BC for pressure your case should be:

inlet: zero gradient
obstables/walls: zero gradient
outlet: fixed pressure (e.g. p=0)

Do not calculate the correction using something like "P-P(2,2)", your pressure reference here is the p at oulet, fixed using a dirichlet BC.
AliE is offline   Reply With Quote

Old   January 25, 2018, 06:10
Default
  #7
Member
 
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 10
kh.aa is on a distinguished road
Quote:
Originally Posted by AliE View Post
Ok , if you have an outlet then you have not to specify a pressure reference from some other cell, since the pressure is already fixed there.

Standard BC for pressure your case should be:

inlet: zero gradient
obstables/walls: zero gradient
outlet: fixed pressure (e.g. p=0)

Do not calculate the correction using something like "P-P(2,2)", your pressure reference here is the p at oulet, fixed using a dirichlet BC.
Ok, Got it, Thank you

So I just correct the pressure P = P* + URF * (PP )

And where is exactly can I find something in that issue, I went through Fezinger and Peric and I did not find anything about this, I think

If you could please mention to me something to get it explained

Thank you so much
kh.aa is offline   Reply With Quote

Old   January 25, 2018, 06:19
Default
  #8
Senior Member
 
Join Date: Dec 2017
Posts: 153
Rep Power: 8
AliE is on a distinguished road
Ok, and this is correct if you have assigned a p=0 bc at outlet that is a common practice. I remeber that in Ferzinger's book there are few lines about this problem somewhere. If you are not able to find out where, than look into Versteeg's book or simply search for this online and you wil find the answer.

Cheers,

Alie
kh.aa likes this.
AliE is offline   Reply With Quote

Reply

Tags
code, pressure-correction, solver

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
Calculation of the Governing Equations Mihail CFX 7 September 7, 2014 07:27
The correction on pressure equation of SIMPLE algorithm in MRFSimpleFOAM solver renyun0511 OpenFOAM Running, Solving & CFD 0 November 10, 2010 02:47
Pressure Correction Equation morteza OpenFOAM Running, Solving & CFD 2 September 4, 2007 07:16
BCs for Pressure Correction Equation (SIMPLE) Bharath Somayaji Main CFD Forum 1 March 1, 2006 07:12
what the result is negatif pressure at inlet chong chee nan FLUENT 0 December 29, 2001 06:13


All times are GMT -4. The time now is 10:05.