
[Sponsors] 
January 22, 2018, 18:36 
Pressure correction equation [SIMPLE]

#1 
Member
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 9 
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 underrelaxation factor Why would we use a reference pressure in correcting the pressure after the pressure correction equation? Thanks ... 

January 25, 2018, 04:21 

#2  
Senior Member
Join Date: Dec 2017
Posts: 153
Rep Power: 7 
Quote:
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 pressurecorrection equation is a Poissonlike 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 

January 25, 2018, 04:30 

#3 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,388
Rep Power: 67 
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).


January 25, 2018, 04:53 

#4  
Member
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 9 
Quote:
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 axisymmetric with some obstacles in the flow. Thanks.. 

January 25, 2018, 04:57 

#5 
Member
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 9 
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.


January 25, 2018, 05:04 

#6 
Senior Member
Join Date: Dec 2017
Posts: 153
Rep Power: 7 
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 "PP(2,2)", your pressure reference here is the p at oulet, fixed using a dirichlet BC. 

January 25, 2018, 05:10 

#7  
Member
Khaled Ahmad
Join Date: Dec 2015
Posts: 32
Rep Power: 9 
Quote:
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 

January 25, 2018, 05:19 

#8 
Senior Member
Join Date: Dec 2017
Posts: 153
Rep Power: 7 
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 

Tags 
code, pressurecorrection, solver 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Calculation of the Governing Equations  Mihail  CFX  7  September 7, 2014 06:27 
The correction on pressure equation of SIMPLE algorithm in MRFSimpleFOAM solver  renyun0511  OpenFOAM Running, Solving & CFD  0  November 10, 2010 01:47 
Pressure Correction Equation  morteza  OpenFOAM Running, Solving & CFD  2  September 4, 2007 06:16 
BCs for Pressure Correction Equation (SIMPLE)  Bharath Somayaji  Main CFD Forum  1  March 1, 2006 06:12 
what the result is negatif pressure at inlet  chong chee nan  FLUENT  0  December 29, 2001 05:13 