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-   -   Help! Convergence of Separated Flow on 2D airfoil. (http://www.cfd-online.com/Forums/fluent/37933-help-convergence-separated-flow-2d-airfoil.html)

Daniel September 19, 2005 05:22

Help! Convergence of Separated Flow on 2D airfoil.
 
Dear All,

I am using Fluent 6.1 to model the flow about the NACA 0009 airfoil from zero angle of attack to stall. I am trying to obtain a set of data that look something like the experimental set for use in my honours project on control of AUVs.

I seem to get good results from 0 to 9 degrees angle of attack using the k-eps RNG model setting the turbulent dissipation rate to 1% and the length scale to 0.1. I'm using a C-mesh out to about 20 chord lengths away with quadrilateral cells (150 nodes on the upper surface of the foil).

As soon as I reach 12 degrees, the solution will not converge and stopping the simulation every 100 or so iterations will show a different velocity profile above the foil each time. Usually this shows anything from a tiny separation bubble to fully separated flow.

Can anyone shed any light on how this can be fixed. I have tried reducing under relaxation values, varying the intensity and length scale and the pressure velocity coupling method, yet i still have problems.

Thankyou very much in advance!

Regards,

Daniel

Razvan September 20, 2005 03:36

Re: Help! Convergence of Separated Flow on 2D airf
 
Simply changing underrelaxation factors (URF) will never help you in this case.

Switch to UNSTEADY solver and record Cl and Cd data. Iterate until these become "stable" and take the medium value (of course they will still be oscillating because of vortex shedding behind the airfoil).

Use PRESTO and PISO alghoritms and try to start with very high URF: say, 0.9 for pressure and momentum. If this does not work, lower to 0.7 both. If it's still not working, lower pressure URF until iterations converge within your chosen time step.

Best wishes, Razvan

Daniel September 25, 2005 00:24

Re: Help! Convergence of Separated Flow on 2D airf
 
Thankyou Razvan,

I'll give that a shot. I'm assuming I am using the right Turbulence Model still.

Cheers,

Daniel


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