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-   -   Already converged? (http://www.cfd-online.com/Forums/cfx/105968-already-converged.html)

Julien August 14, 2012 06:12

Already converged?
 
Hi!
I am new to Ansys/CFD and have some questions about my convergence plot shown below.

http://img845.imageshack.us/img845/5...onvergence.png

For the beginning an ideal gas in a pipe was simulated with following parameters:
Pipe:
length: 7cm
diameter: 0.1 cm
fluid:
Inlet speed: 125m/s
relative pressure outlet: 0 Pa
mesh:
elements: 15975
nodes: 17568

Is my result already "okay"?
Do I have to improve any settings, for example mesh, to obatin better results?
Are more iterations steps necessary?
I know, that the pipe is too short to reach a fully developed flow profile. Does this affect my convergence?

Thanks in advace,

Julien
http://imageshack.us/photo/my-images...nvergence.png/

murx August 14, 2012 08:43

Quote:

Originally Posted by Julien (Post 376988)
Is my result already "okay"?
Are more iterations steps necessary?

You cannot tell that from the values of the residuals. The best thing is to look at your values of interest, e.g. pressure loss, during the run by creating a monitor point. If those values do not change anymore, your problem ist converged.


Quote:

Originally Posted by Julien (Post 376988)
Do I have to improve any settings, for example mesh, to obatin better results?
I know, that the pipe is too short to reach a fully developed flow profile. Does this affect my convergence?
Julien
http://imageshack.us/photo/my-images...nvergence.png/

You specify a uniform inflow velocity, that means you have high gradients not only normal to the wall but also in flow direction. So the cells also need to have a small dimension in flow direction to capture these gradients. As far as I know, this can also be the reason for the fluctuating residuals. So maybe refining the mesh can smoothen your convergence.
But this is only a guess... I do not have much experience with turbulent flows. So you better wait for someone who is qulified to confirm this =)

ghorrocks August 14, 2012 08:49

Quote:

Is my result already "okay"?
That depends on what you are trying to get out of the simulation. A rough estimate requires a very different approach to somebody requiring 1% accuracy.

This FAQ describes some issues which cause CFD errors:
http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

the following FAQ has a link to a very useful piece on CFD accuracy.

siw August 14, 2012 14:18

Just to add to that above. You should also check the various imbalance percentages at the end of the out file.


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