# Is it possible to use PISO algorithm for steady case

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

 February 23, 2010, 10:42 Is it possible to use PISO algorithm for steady case #1 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 8 Hello, In fluent, when you are in a steady case you can choose SIMPLE or PISO for the solver. In openfoam, apparently, SIMPLE is for steady and PISO for transient. Is there a way to choose PISO even for steady state? Thanks for your answers

 February 24, 2010, 03:34 #2 Senior Member   matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 104 Rep Power: 9 well, steady state - strictly speaking is an average over time, or a transient with infinite time step. So this is also a way how to use PISO for steady state. What is you motivation for using PISO for steady-state? matej

 February 24, 2010, 04:43 #3 Member   Jérémy Bulle Join Date: Nov 2009 Posts: 93 Rep Power: 8 Hello Thanks for the answer My motivation was that with Fluent, PISO solver gave me better results than the SIMPLE solver so I wanted to try this in openFoam

 February 25, 2010, 05:39 #4 Member     Ulf Bunge Join Date: Mar 2009 Location: Wolfsburg, Germany Posts: 34 Rep Power: 9 Hello, in case the solution you find is steady, the temporal derivation should vanish and you could also deactivate this term in the code or use steadyState for ddtSchemes. However, alternatively and dependent on how good your initialization is, you might also want to compute the case as unsteady with a given ddtSchemes (other than steadyState). Dependent on the size of the time step you chose, you can interpret this as relaxation as you enhance diagonal dominance of your coefficient matrix. I.e., the smaller Dt, the larger the cell-centered coefficient becomes in comparison to the other coefficients. The procedure becomes stiffer but it might help in not so well-posed problems with a bad initialization. Best regards, Ulf.

 July 7, 2010, 04:03 #5 Member   David Join Date: Dec 2009 Location: Spain Posts: 61 Rep Power: 8 Hi Ulf Could you explain how to employ the PISO algorithm for a steady-state case so that it works as a relaxation? I am working with a SIMPLE algorithm without relaxation for the temperature and I am having problems due to that. So I have thought about use the PISO algorithm in the way you have suggested, but I don´t know exactly how to do it. My case problem is steady. Thank you very much Regards David

 July 7, 2010, 05:07 #6 Senior Member   matej forman Join Date: Mar 2009 Location: Brno, Czech Republic Posts: 104 Rep Power: 9 David, why don't you employ some underrelaxation for temperature? It seems to be much more simple solution than using PISO. The easiest way to use PISO for steady computation is to run transient solution, during the computation you may try lengthen the time-step. Once solution stops changing you have steady state solution. good luck

 July 7, 2010, 06:43 #7 Member   David Join Date: Dec 2009 Location: Spain Posts: 61 Rep Power: 8 Thanks for your help Yes I know that the best option is to apply a underelaxation for the temperature, but in the solver that I am employing (conjugateHeatFoam, in the 1.5-dev version) the underelaxation is not defined and I am not able to do it (I don´t have programming skills). Thank you again, regards David

 July 14, 2010, 06:59 Under relaxation factor #8 Senior Member     ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 322 Rep Power: 10 Hi David By changing the delta t you are changing under relaxation factor for a steady state case from one time step to an other not in solving equations in a single time step. Do you understand me? Good luck

 July 14, 2010, 09:40 #9 Member   David Join Date: Dec 2009 Location: Spain Posts: 61 Rep Power: 8 Hi Ata Thank you for your help. Well, I don't understand it very well. If I am solving a steady state case, how I can modify the time step? or, if the solver is transient, how I can specify that the case is steady? Do you mean that I can not employ the delta t of a transient solver as a underelaxation for a steady case? Thank you very much again. David

 July 15, 2010, 00:49 Under relaxation #10 Senior Member     ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 322 Rep Power: 10 Hi David Consider you have a fully explicit scheme for a unsteady case. In this case you can use delta t as a under relaxation factor ti achieve steady state solution but if you have implicit terms in your scheme you need an iterative method for solving system of equations in each single time step in its relation is separate from delta t. So, you can use a full explicit scheme for solving a steady state problem but you must have enough time to steady state solution after transient solution achieve. I hope my explanation is clear for you. Best regards Good luck lav likes this.

 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 vijay FLUENT 1 April 24, 2006 11:11 Constantinos Zervides (Zervides) OpenFOAM Pre-Processing 5 October 24, 2005 03:34 us FLUENT 0 July 6, 2005 10:43 jvn OpenFOAM Pre-Processing 2 June 20, 2005 21:19 Jonathan Castro Main CFD Forum 3 December 10, 1999 05:59

All times are GMT -4. The time now is 00:14.