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 Andrey September 11, 2008 12:34

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 16:01

Re: Time discretisation scheme for steady state ca

What do you think?

 Mehul September 12, 2008 01:20

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 02:20

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&hellip;

Andrey

 Glenn Horrocks September 14, 2008 23:13

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 02:59

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»?

Andrey

 HekLeR September 15, 2008 14:20

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 03:40

Re: Time discretisation scheme for steady state ca

Yes. Thanks a lot :)

Andrey

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