# non linear terms in OF

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

 November 5, 2012, 14:30 non linear terms in OF #1 New Member   Join Date: Oct 2012 Posts: 15 Rep Power: 6 Dear OpenFoamers I understand that non-orthogonality is handled through deferred correction that solves the same equation repeatedly with different source term. But how are other sorts of non-linearity 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 non-orthogonality for momentum prediction equation or k-e 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 non-linearity 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: 436 Rep Power: 17 Hi, the non linearities are treated by using the previous time-step 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: 54-342-4511594 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: 6 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 non-orthogonal 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 } } }``` The momentum predictor has div(phi,U) term say discretized with linearUpwind that has explicit term. So that is solved once but to get better results shouldn't this be solved multiple times just like other deferred corrections? Or is it just that since this is a prediction, we take what we have and do corrections of U and P in pisoLoop. Does increasing the number of piso iterations (>2) improve deferred corrections from div(phi,U) ? I don't think that happens because the discretization of momentum equation is outside the pisoLoop? 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: 436 Rep Power: 17 Hi, much of this non-linearities are treated but by the time-step, 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 time-step 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: 54-342-4511594 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: 6
Quote:
 Originally Posted by santiagomarquezd Hi, much of this non-linearities are treated but by the time-step, 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 time-step you have an outer loop over all the equations as you want to find. Regards.
Thank you very much. I finally see what you mean after looking in pimpleFoam. So if I understand correctly, for Co<1 the ddt term dominates most terms including source contributions from deferred corrections, thus there is no need for outer corrector loop to do repeated corrections. But I guess in principle the loop should be there for both small or large time steps.

 December 2, 2012, 06:38 #6 Member   Zifei Yin Join Date: Sep 2012 Location: Shanghai & Ames Posts: 33 Rep Power: 6 Sorry to bother you. Why we need Co < 1 to get non-ortho correction?

December 2, 2012, 16:39
#7
New Member

Join Date: Oct 2012
Posts: 15
Rep Power: 6
Quote:
 Originally Posted by yzf1215 Sorry to bother you. Why we need Co < 1 to get non-ortho correction?
Hi,
From what I understood, all explicit terms (including tvd shemes and non-orthogonality) are treated using deferred correction. For icoFoam and simpleFoam, the iteration over a time-step 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 crank-Nicholson. Someone can correct if I am wrong as I am not knowledgeable in OF.

Hope this helps.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Ger_US OpenFOAM 8 January 8, 2013 13:29 tonkiplis OpenFOAM Running, Solving & CFD 1 November 9, 2011 09:04 subash OpenFOAM 0 May 29, 2010 01:23 benru OpenFOAM Bugs 42 November 13, 2009 08:59 waynezw0618 OpenFOAM Running, Solving & CFD 39 March 5, 2009 13:57

All times are GMT -4. The time now is 20:33.