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-   -   Time discretisation scheme for steady state case (http://www.cfd-online.com/Forums/cfx/26378-time-discretisation-scheme-steady-state-case.html)

Andrey September 11, 2008 13:34

Time discretisation scheme for steady state case
 
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

CycLone September 11, 2008 17:01

Re: Time discretisation scheme for steady state ca
 
What do you think?

Mehul September 12, 2008 02:20

Re: Time discretisation scheme for steady state ca
 
It depends on the application, but generally the second order scheme is used.

Andrey September 12, 2008 03:20

Re: Time discretisation scheme for steady state ca
 
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

Glenn Horrocks September 15, 2008 00:13

Re: Time discretisation scheme for steady state ca
 
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

Andrey September 15, 2008 03:59

Re: Time discretisation scheme for steady state ca
 
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


HekLeR September 15, 2008 15:20

Re: Time discretisation scheme for steady state ca
 
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?

Andrey September 16, 2008 04:40

Re: Time discretisation scheme for steady state ca
 
Yes. Thanks a lot :)

Andrey


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