CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM > OpenFOAM Running, Solving & CFD

kOmegaSST underpredicts cl compared to xfoil

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

Like Tree1Likes
  • 1 Post By haakon

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 23, 2013, 14:53
Default kOmegaSST underpredicts cl compared to xfoil
  #1
Senior Member
 
Klaus
Join Date: Mar 2009
Posts: 250
Rep Power: 22
klausb will become famous soon enough
Hello,

I am working on a case setup for airfoil optimization (cl/cd).

The case calculates cl and cd for a S809 airfoil at Re = 1.000.000 with alpha = 0, steady state and transient, using the kOmegaSST model.

xfoil predics a cl of 0.14 but OpenFoam underpredicts the cl =0.04.

kOmegaSST should produce good results for small angles of attac.

How can I improve the results?

Klaus
Attached Images
File Type: jpg S809-re-1e6.jpg (33.3 KB, 47 views)
Attached Files
File Type: gz airfoil.tar.gz (60.0 KB, 19 views)
klausb is offline   Reply With Quote

Old   March 23, 2013, 18:17
Default
  #2
Senior Member
 
Join Date: Dec 2011
Posts: 111
Rep Power: 19
haakon will become famous soon enough
As far as I can see, you use inlet velocity of 1 m/s, but in the forceCoeffs file, the value of freestream velocity, magUInf, is specified as 2.0 m/s.

This means that your coefficients of lift and drag is calculated to be one fourth of their correct value. If we take your example of 0.04 and multiply that by 4, we get 0.12. This is probably not that bad, considering that you most likely don't have done any mesh convergence studies or tuning of the case.

Good luck with your simulations.
majid fahim likes this.
haakon is offline   Reply With Quote

Old   March 24, 2013, 06:43
Default How to conduct a mesh convergence study?
  #3
Senior Member
 
Klaus
Join Date: Mar 2009
Posts: 250
Rep Power: 22
klausb will become famous soon enough
Thank you for the feedback!

How to conduct a mesh convergence study?

I am planning to add another turbulence model to the case which requires y+<1 hence the mesh gets more important.

Klaus
klausb is offline   Reply With Quote

Old   March 24, 2013, 14:12
Default
  #4
Senior Member
 
Join Date: Dec 2011
Posts: 111
Rep Power: 19
haakon will become famous soon enough
I don't like to be rude, but that is really something you should be able to find out on your own. Online discussion fora is (generally) not a place where other do the work for you or read the books that you should have read.

Anyways, I Googled, and this was the topmost hit: http://usa.autodesk.com/adsk/servlet...inkID=13806469 I think it illustrates the point pretty good, even tough it is written for solid mechanics FEM and Ansys, the general concept is the same for CFD.

If you are interested in lift and drag, start with a coarse mesh (too coarse), and gradually refine it. As you refine it, you note or plot the lift and drag coefficients. When they do not change substantially (say 1-3% from the last mesh) one usually say that one has found a mesh-independent solution, or that the mesh has converged or similar. If the converged result is significantly different from references, one might want to look at other aspects of the simulation, for example the numerical schemes or solution algorithm.
haakon is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
kOmegaSST OF2.1 Help needed! wiedangel OpenFOAM Running, Solving & CFD 0 May 9, 2012 11:01
Wrong calculation of nut in the kOmegaSST turbulence model FelixL OpenFOAM Bugs 27 March 27, 2012 10:02
Frozen Xfoil toto13000 Main CFD Forum 0 July 28, 2011 11:46
xfoil with matlab nikolaous Main CFD Forum 3 September 2, 2010 15:15
kOmegaSST in openfoam 1.6 Gearb0x OpenFOAM 2 March 3, 2010 07:02


All times are GMT -4. The time now is 06:42.