CFD Online Logo CFD Online URL
Home > Forums > FLUENT

Help! Convergence of Separated Flow on 2D airfoil.

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   September 19, 2005, 05:22
Default Help! Convergence of Separated Flow on 2D airfoil.
Posts: n/a
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!


  Reply With Quote

Old   September 20, 2005, 03:36
Default Re: Help! Convergence of Separated Flow on 2D airf
Posts: n/a
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
  Reply With Quote

Old   September 25, 2005, 00:24
Default Re: Help! Convergence of Separated Flow on 2D airf
Posts: n/a
Thankyou Razvan,

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


  Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Poor Convergence for Transient Subsonic Diffuser sims with flow separation TWaung CFX 1 April 16, 2009 08:25
simulation of a turbulent flow on airfoil Dante FLUENT 3 June 8, 2007 14:54
Problem with Convergence at high flow rates Syed CD-adapco 1 April 10, 2007 15:18
Convergence for inviscid flow over sylinder Rjakk Main CFD Forum 2 March 21, 2007 11:53
simulating separated flow past the airfoil Yusoff CFX 7 January 16, 2001 20:19

All times are GMT -4. The time now is 13:49.