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-   -   convergence criterion (http://www.cfd-online.com/Forums/fluent/42875-convergence-criterion.html)

Dominique November 16, 2006 15:31

convergence criterion
 
Hi I am wondering if someone can tell me what factors influence the atteinable convergence criterion. I am studying the turbulent flow over a descending step and am having a very hard time getting Fluent to converge. With a K-Epsilon model, and the Enhanced Wall Treatment, the results with a conv. criterion at 10^-4 are not acceptable. With 10^-5, it is better but not yet the right answer. The recirculation zone at the foot of the step is much too small. I even tried to apply a laminar model and still, no convergence with the convergence criterion at 10^-4. All these, with the same mesh.

Can anyone help?? Also, is it normal for k to be more or less 100 times smaller than Epsilon? Thank you in advance!

HSeldon November 17, 2006 23:55

Re: convergence criterion
 
Hi, Did you try with the under-relaxation coefficients? And did you try changing the discretization scheme?

Play a bit with them to see what happens.

Take care.

Dominique November 19, 2006 13:47

Re: convergence criterion
 
Hi, Well I am starting to play with the under-relaxation coefficients (which is what my professor suggested), but I'm looking for some guiding principles. I tried several discretization schemes and obtain the exact same results every time. Any idea why my laminar results aren't acceptable either? Could it be that I shouldn't be using a pressure outlet (of 0)? My inlet is a constant velocity inlet... Thanks again Dominique

RoM November 23, 2006 10:06

Re: convergence criterion
 
Is your inlet far enough away from the step? Maybe the velocity profile needs some more time to develop.

RoM

Dominique November 23, 2006 19:02

Re: convergence criterion
 
Well, for now I am using a constant velocity at the entrance. Thanks anyways, any other ideas? Dominique

RoM November 24, 2006 03:36

Re: convergence criterion
 
You can use a constant velocity if you dont have a profile but you have to make shure that the inlet is far enought away from the step so that the velocity profile has some time to develop. I think the distance should be at least 10*H where H is the channel height.

RoM


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