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

Lift co-efs

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

Reply
 
LinkBack Thread Tools Display Modes
Old   November 23, 2000, 06:41
Default Lift co-efs
  #1
F.P. mie
Guest
 
Posts: n/a
Hi I'm trying to model (2D) flow around a NACA0009 foil, to compare with experimental results. I've drawn the foil from x/y co-ords, so I think its fairly accurate. However, when I try to compute lift and drag forces on the foil, they just oscillate like crazy, even after 10000 iterations! My flow speed is 10 m/sec, the attack angle is 10deg, and my fluid is water. I've specified a trapezoid area around the foil, with 3 inflows and an outflow. Any suggestions what might be wrong? In particular, should I have gravity on/off, and will it matter what operating pressure I use? Cheers FP
  Reply With Quote

Old   November 23, 2000, 18:18
Default Re: Lift co-efs
  #2
Trac
Guest
 
Posts: n/a
At 10deg, you could be getting unsteady flow from the separation, and you might need to switch to a transient analysis.

Gravity & operating pressure shouldn't matter, but you will need to make sure the reference values (under report) are correct, or your Cd and Cl will be calculated wrongly.

You also need to make sure the boundaries are far enough away (at least 10c) and you should probably check your y+ values since (assuming air and c=1) your flow is turbulent. You might want to consider a two-layer zonal model to pick up the separation.

It might be a good idea to run the model with a lower incidence, just to make sure you have everything working okay.

Tracie.
  Reply With Quote

Old   November 27, 2000, 00:13
Default Re: Lift co-efs
  #3
jangho.,lee
Guest
 
Posts: n/a
hi i got the same problem to compute the NACA00xx series. in my experience, NACA 0009 series oscillate at 14 degree angle of attat at Ma=0.085.

i use fluent5.1 and tubluent model is k-e model.coule implicit method.

i think that you should check your "courant number" and your time in the solver is steady..

if you want to any detail about the datum, write the mail to me...

ps.. fluent can not predict the exact stall angle...
  Reply With Quote

Old   November 27, 2000, 07:12
Default Re: Lift co-efs
  #4
F.P. mie
Guest
 
Posts: n/a
Guys Thanks for the help, I changed my turbulence model, and my lift co-efs are fine now. New problem- all my drag values are negative!! How is this possible-if Cd comes from 1/2(rho)(v^2)(Area), where is the minus coming from? Any ideas? FP
  Reply With Quote

Old   November 27, 2000, 10:22
Default Re: Lift co-efs
  #5
Peter Attar
Guest
 
Posts: n/a
Check your definition of the drag coefficient...you are missing the numerator.
  Reply With Quote

Old   November 27, 2000, 12:18
Default Re: Lift co-efs
  #6
FP Mie
Guest
 
Posts: n/a
Peter- Yeh, its Cd=D/0.5(rho)(v^2)(area), but my query remains, providing the drag force cant be -ve (which it can't, can it??), how are the Cd's coming out -ve? I'm new to Fluent, so forgive me if my questions seem simple! FP
  Reply With Quote

Old   November 27, 2000, 12:25
Default Re: Lift co-efs
  #7
Peter Attar
Guest
 
Posts: n/a
I'm not a fluent user at all actually..: ) just like to read the boards.I am not sure how things are defined but it would make sense to me that since the Drag opposes the motion of the airfoil that it's sign would be negative..this would give you your negative Cd..I would assume that in most aero books this definition is assummed and that is why you never see Cd defined as negative.

  Reply With Quote

Old   November 27, 2000, 21:30
Default Re: Lift co-efs
  #8
jangho,Lee
Guest
 
Posts: n/a
hi. guy. i think your drag problem based on your drag corrinate. first, check your force to drag.. i think that you write positive x cordinate at force. x cordinate should negative.

check your force...
  Reply With Quote

Old   December 13, 2000, 12:09
Default Re: Lift co-efs
  #9
P.venkataraman
Guest
 
Posts: n/a
Just accessed the group

By definition Drag and lift coefficients are defined parallel and perpendicular to flow. You have to transform those values using the angle of attack

Venkat
  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
Lift coefficient in Eulerian multiphase model hennas FLUENT 1 July 4, 2011 17:50
Lift distribution PB4 STAR-CCM+ 1 October 28, 2010 07:59
Thin foil analsis (sail) - Lift Coeff Problem Kelvin CFX 3 December 22, 2008 17:22
Lift force or coefficient of lift Rola FLUENT 1 November 12, 2006 14:29
Saffman Lift Force CFD MAN Main CFD Forum 0 November 26, 2002 19:26


All times are GMT -4. The time now is 13:31.