
[Sponsors] 
November 5, 2012, 14:30 
non linear terms in OF

#1 
New Member
Join Date: Oct 2012
Posts: 15
Rep Power: 7 
Dear OpenFoamers
I understand that nonorthogonality is handled through deferred correction that solves the same equation repeatedly with different source term. But how are other sorts of nonlinearity such as those from higher order convection schemes as QUICK, explicit boundary conditions such as slip, turbulence models etc handled? I do not see a loop like the case for nonorthogonality for momentum prediction equation or ke turbulence models for instance. Are all of them handled implicitly? For steady state problems it seems to me that the outer loop takes care of all nonlinearity but for transient problems shouldn't there be another loop to make sure that we have a converged solution at each time step before going to the next. I appreciate it if someone can shed a light on this. Thanks 

November 8, 2012, 17:35 

#2 
Senior Member
Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
Posts: 438
Rep Power: 17 
Hi, the non linearities are treated by using the previous timestep information. This is the case for momentum predictor equation, TVD, schemes, etc.
Regards.
__________________
Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

November 21, 2012, 08:33 

#3 
New Member
Join Date: Oct 2012
Posts: 15
Rep Power: 7 
Hi,
I still don't understand how the new equation with improved corrections are iteratively solved just like the case of non orthogonal corrections in pressure. I give the outline of icoFoam below without nonorthogonal loop. I appreciate if you can point out the code takes care of other deffered corrections due to TVD schemes for instance. Code:
icoFoam() { for(timeSteps) { Predict U //has div(phi,U) for(pisoLoop) { //2 iteration Correct U Solve pressure equat } } } For steady state solution I can understand that the timesteps loop will take care of corrections in div(phi,U) but for transient solutions I just don't see it. I am definately missing something so I appreciate if anyone can point it out. 

November 21, 2012, 09:25 

#4 
Senior Member
Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
Posts: 438
Rep Power: 17 
Hi, much of this nonlinearities are treated but by the timestep, which must be such that Co<1. Nevertheless, you have solvers with "outer corrections" based in the pimple loop like pimpleFoam. In these solvers in each timestep you have an outer loop over all the equations as you want to find.
Regards.
__________________
Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

November 21, 2012, 09:46 

#5  
New Member
Join Date: Oct 2012
Posts: 15
Rep Power: 7 
Quote:


December 2, 2012, 06:38 

#6 
Member
Zifei Yin
Join Date: Sep 2012
Location: Shanghai & Ames
Posts: 33
Rep Power: 7 
Sorry to bother you. Why we need Co < 1 to get nonortho correction?


December 2, 2012, 16:39 

#7  
New Member
Join Date: Oct 2012
Posts: 15
Rep Power: 7 
Quote:
From what I understood, all explicit terms (including tvd shemes and nonorthogonality) are treated using deferred correction. For icoFoam and simpleFoam, the iteration over a timestep is done only once by default. Therefore if you use large time step, you accept inaccurate solutions at each time step. OTOH PimpleFOAM has an outer loop that does corrections multiple times. I guess that with Co<1, one deferred correction step is enough probably because time discretization term swamp the secondary corrections. I do not think there is a theory similar to why we need to specify Co<1 when use explicit time discretization schemes such as crankNicholson. Someone can correct if I am wrong as I am not knowledgeable in OF. Hope this helps. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
please help me which solver is better for my application  Ger_US  OpenFOAM  8  January 8, 2013 13:29 
[LaunderGibsonRSTMchannel]  tonkiplis  OpenFOAM Running, Solving & CFD  1  November 9, 2011 09:04 
solution diverges when linear upwind interpolation scheme is used  subash  OpenFOAM  0  May 29, 2010 01:23 
IdeasUnvToFoam Bug amp Fix  benru  OpenFOAM Bugs  42  November 13, 2009 08:59 
Water pump OpenFOAM 15 ANSYS CFX 110 comparation  waynezw0618  OpenFOAM Running, Solving & CFD  39  March 5, 2009 13:57 