CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Cl/Cd problem which has been troubling me for months

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 5, 2010, 07:58
Default Cl/Cd problem which has been troubling me for months
  #1
New Member
 
Khor Yee Shin
Join Date: Jan 2010
Location: Glasgow UK
Posts: 6
Rep Power: 7
khor_albert is on a distinguished road
Hi there

Part of my project requires modelling with airfoil. I used C-grid, NACA 4424 and k-e turbulence model and strange enough, the Cl/Cd keeps increasing from 15 to 30 degrees. The 30 deg Cl/Cd value is quite close to experimental value but the trend is totally different. I tried rotating the model instead of the flow but it didn't work although it helped a bit. I made small modifications on the airfoil, but Cl/Cd should get smaller as angle of attack gets too large, shouldn't it?

Please advise. This problem has been troubling me for months. What could be the cause of this inaccuracy? All my other simulations produce value within 20% difference from experimental value - only this particular case with the largest modifications. Just give me anything - it just might help!

Thanks mates!
khor_albert is offline   Reply With Quote

Old   April 6, 2010, 02:39
Default
  #2
New Member
 
Arda Ersan
Join Date: Aug 2009
Posts: 13
Rep Power: 7
cfdman is on a distinguished road
Dear Shin

I think you should check near wall treatment. is Y+ values appropriate for k-e turbulence model? Near wall treatment is very important for accurate solutions on aerodynamic calculations.
Std. k-e model is known to be insufficient in regions where flow seperation occurs.
You should check both y+ values and seperation while using k-e model as these two are the main sources of erroneous results.
Maybe you can use Spalart Almaras which gives accurate solutions on external aerodynamic. If you have fine boundary layer mesh you can use realizeable k-e.

Good luck
Arda
cfdman is offline   Reply With Quote

Old   April 6, 2010, 11:34
Default
  #3
Senior Member
 
Join Date: Nov 2009
Posts: 411
Rep Power: 10
DoHander is on a distinguished road
Check these:

1. That your projection vectors (for calculating Cl and Cd) are properly calculated, you must manually change these vectors when you change the incidence.

2. Check your y+ value. This should be about 100 or smaller then 1 (for enhanced wall treatment). Intermediary values for y+ will give you a bad estimation for Cd. However your Cl should decrease after a certain incidence (typically between 12-20 degrees).

3. Try with a different turbulence model (k-omega maybe) or Spalart-Almaras

Do
DoHander is offline   Reply With Quote

Reply

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
UDF compiling problem Wouter Fluent UDF and Scheme Programming 6 June 6, 2012 04:43
Incoherent problem table in hollow-fiber spinning Gianni FLUENT 0 April 5, 2008 10:33
natural convection problem for a CHT problem Se-Hee CFX 2 June 10, 2007 06:29
Adiabatic and Rotating wall (Convection problem) ParodDav CFX 5 April 29, 2007 19:13
convergence problem Trushar Phoenics 5 August 27, 2002 23:40


All times are GMT -4. The time now is 21:49.