# Time discretisation scheme for steady state case

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

 September 11, 2008, 12:34 Time discretisation scheme for steady state case #1 Andrey Guest   Posts: n/a Hi, All! Tell me, please, what is the time discretisation scheme use for steady state problems in CFX: First Order Backward Euler or Second Order Backward Euler? Thanks Andrey

 September 11, 2008, 16:01 Re: Time discretisation scheme for steady state ca #2 CycLone Guest   Posts: n/a What do you think?

 September 12, 2008, 01:20 Re: Time discretisation scheme for steady state ca #3 Mehul Guest   Posts: n/a It depends on the application, but generally the second order scheme is used.

 September 12, 2008, 02:20 Re: Time discretisation scheme for steady state ca #4 Andrey Guest   Posts: n/a Mmm... I don't know, were in GUI I can check or change it. How it depends on the application? My usual application is subsonic and transonic flow over airfoils, wings, airplanes, etc. Thanks… Andrey

 September 14, 2008, 23:13 Re: Time discretisation scheme for steady state ca #5 Glenn Horrocks Guest   Posts: n/a Hi, Umm - in steady state flow the transient terms are zero so there is no discretisation. That is why you got a sarcastic reply from Cyclone. CFX uses a psuedo-transient approach to converge to a steady state simulation and that would use a simplified type of first order discretisation. But you should not take any notice of any time related stuff in a steady state flow as the underlying equations do not include all the transient terms. You need to do a transient simulation for that. Glenn Horrocks

 September 15, 2008, 02:59 Re: Time discretisation scheme for steady state ca #6 Andrey Guest   Posts: n/a Hi! Sorry for my English, guys. Sarcasm is not good. I understand difference in transient and pseudotransient problems. I interested in steady state (pseudotransient) problem. Would you say, that Eqn. 16-20 (p. 282 in CFX-Solver Theory Guide) are not important for steady applications, but Eqn. 7-9 (280) are right for them? So CFX uses for steady problems First order scheme in time (pseudotime . Is this right? But Mehul, why do you say: «generally the second order scheme is used»? Thank you for your time Andrey

 September 15, 2008, 14:20 Re: Time discretisation scheme for steady state ca #7 HekLeR Guest   Posts: n/a CFX uses the first order backwards Euler scheme to implement implicit relaxation. Since time accuracy does not matter: - no inner iterations are performed, so the flow is not forced to balance within a timestep, only at steady state convergence. - the physical timescale can be set per-equation, so sometimes this is called 'false' timestepping because different equations have different false time values. - the equations are assembled and solved once per iteration/time step. - some under-relaxations for explicit contributions (such as 2nd order corrections for advection) are increased, relative to the true transient values, to help with stability. Make sense?

 September 16, 2008, 03:40 Re: Time discretisation scheme for steady state ca #8 Andrey Guest   Posts: n/a Yes. Thanks a lot Andrey

 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 jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 05:24 zhajingjing OpenFOAM 0 April 28, 2010 04:35 Suzzn CFX 18 October 2, 2009 04:18 carsten OpenFOAM Bugs 11 September 12, 2008 11:16 Sanjay Padhiar Main CFD Forum 1 March 31, 1999 12:32

All times are GMT -4. The time now is 13:51.