# Discretization of transient terms in steady state

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 November 4, 2021, 11:07 Discretization of transient terms in steady state #1 New Member   anonymous Join Date: Jun 2021 Posts: 10 Rep Power: 3 Hi, I know that CFX uses a pseudo-transient approach to reach a steady state solution. However, I could not find any information in the solver-theory manual or in this forum if CFX uses a 1st or 2nd order backward euler scheme to discretize the transient terms. Maybe I'm missing something but does anybody know where I could find this information? Thanks and have a good day!

 November 4, 2021, 12:29 #2 Senior Member   Join Date: Jun 2009 Posts: 1,688 Rep Power: 29 Your question confused two things: Pseudo transient discretization vs True transient discretization transient discretization order methods have no relevance in the pseudo-transient approach since the solution should be independent of how it got there. I think you are asking for something else. Let us see what that really is. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 November 4, 2021, 13:48 #3 New Member   anonymous Join Date: Jun 2021 Posts: 10 Rep Power: 3 Thanks for the quick response! Please correct me if I'm wrong in the following. The set of governing equations is discretized in time even for steady calculations in CFX with a "false" time step. So time proceeds until a steady state solution is obtained. In order to solve the transient term in the discretized equations I need a scheme or method to do it. In the theory guide two are presented namely 1st and 2nd order backward euler. And that's where my questions aims at. I want to know which scheme CFX uses for the steady state case. The Fluent theory guide explains everything pretty clearly, but I want to know how it is done in CFX.

 November 4, 2021, 14:17 #4 Senior Member   Join Date: Jun 2009 Posts: 1,688 Rep Power: 29 It may look for some like 1st order discretization, but it is not. If you thought from the Fluent theory that is a transient scheme, then the answer you want is: "the same as shown in the Fluent theory" Nothing different. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 November 4, 2021, 14:34 #5 New Member   anonymous Join Date: Jun 2021 Posts: 10 Rep Power: 3 So what is it then? Is there any paper/book/manual or something which describes properly what CFX does? Last edited by LuckyGuy; November 4, 2021 at 16:03.

 November 4, 2021, 16:30 #6 Senior Member   Join Date: Jun 2009 Posts: 1,688 Rep Power: 29 Exactly as described in the Fluent theory page __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 November 5, 2021, 03:11 #7 New Member   anonymous Join Date: Jun 2021 Posts: 10 Rep Power: 3 I missed things up yesterday and got confused, but I think now it's clear to me. So, CFX uses a implicit pseudo-transient under relaxation (see Fluent Theory Chap. 24.6 "Pseudo Transient Under-Relaxation", Eq. 24.99) to march to the steady state solution. Is there a general formula to describe this and how are the coefficients calculated in this equation?

 November 5, 2021, 03:37 #8 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139 The coefficients are the linearisation of the non-linear terms in the Navier Stokes equations. They are updated each coefficient loop. This is also in the Fluent Theory manual, section 24.2 (especially 24.2.1). LuckyGuy likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 November 5, 2021, 04:13 #9 New Member   anonymous Join Date: Jun 2021 Posts: 10 Rep Power: 3 Now everything is clear. Didn't know CFX and Fluent use the same method, that's what got me confused, I guess. Thanks for helping me!

 November 5, 2021, 05:12 #10 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139 A key reason Fluent and CFX came together under the ANSYS company was so they could share key bits of their respective technologies. Fluent did not originally have the coupled solver. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.