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/

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.


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 =)

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.

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