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-   -   Difference results between Steady and Unsteady? (http://www.cfd-online.com/Forums/fluent/33624-difference-results-between-steady-unsteady.html)

Giang T Vu April 30, 2004 05:51

Difference results between Steady and Unsteady?
 
Hi all! I got different results when I change my model from steady to unsteady state (both states are converged successfully). What makes the unsteady results differ from the steady ones with the same conditions? thanks for help. Giang T Vu

bob April 30, 2004 08:00

Re: Difference results between Steady and Unsteady
 
I think in steady State in the Navier stokes equation they do not have transient terms. While in unsteady State case they have it. When you are problem is steady you will get convergence whether you use steady State or unsteady State solver. Pratap

bob April 30, 2004 08:01

Re: Difference results between Steady and Unsteady
 
I think in steady State in the Navier stokes equation they do not have transient terms. While in unsteady State case they have it. When you are problem is steady you will get convergence whether you use steady State or unsteady State solver. bob

Giang T Vu May 1, 2004 05:15

Re: Difference results between Steady and Unsteady
 
Thanks Bob, yes, mine is a steady state problem, when I change to run it under unsteady state it should give me the same result, correct? What would you say if I got the same pressure and velocity distribution and DIFFERENT vapor product (I'm modeling cavitation) when I switch from steady to unsteady mode?

vladimir krejci May 4, 2004 03:24

Re: Difference results between Steady and Unsteady
 
hi giang

what about your time step size and time dicretisation scheme?

Giang T Vu May 6, 2004 09:07

Re: Difference results between Steady and Unsteady
 
Hi Vladimir, The time step size deduced from Courant number and I estimated around 0.001. I used 1st order implicit segregated solver with standard k-epsilon turbulent model. Giang.

Bob (a Differrent bob) May 22, 2004 06:37

Re: Difference results between Steady and Unsteady
 
I'm confused, how can an unsteady and a steady simulation give the same answer ? For an unsteady simulation you have transient effects, pertubations in the flow etc which if averaged over enough time, MAY give you a similar result to a steady state simulation, but more by luck than anything else ? or is my understanding of steady and unsteady not correct ?? Bob


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