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

Help! lift/drag coefficients do not match experimental data!

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 17, 2012, 13:52
Default Help! lift/drag coefficients do not match experimental data!
  #1
New Member
 
Ali
Join Date: Apr 2012
Location: India
Posts: 17
Rep Power: 13
ali.qasim is on a distinguished road
Hello

I came across this paper which contains lift and drag coefficients for a few airfoils at various Reynolds numbers.

I'm trying to get these results in FLUENT, but my values are way off.

The airfoil I've chosen is NACA 0012. Reynolds number is 80,000. The airfoil model used in the experiment had a chord length of 15.24 cm. I got the coordinates online, and scaled them to this length.

Now, assuming values for density (1.225 kg/m^3) and dynamic viscosity (1.7894e-5 kg/m-s), I arrived at a free-stream velocity of about 7.668 m/s. I used this as my inlet boundary condition.

For the reference values in FLUENT, I set the reference length = chord length, depth = 1m, and therefore area = (chord length) * 1.

I chose the Inviscid model. I know this is not perfect, but for now I just want to see if I get a rough approximate for the lift coefficient.

Here's a link to a spreadsheet showing the values I got, and the percentage difference from the experimental values. As you can see, they are not even close. I get a constant difference of around 40% (I expect it to be within 5% at least for the first 8 degrees), which tells me I could be missing a factor here.

Can anyone see what I'm missing? Please help me out, as I've been stuck on this for many days!

Thanks

Ali
ali.qasim is offline   Reply With Quote

Old   May 17, 2012, 15:40
Default
  #2
Senior Member
 
Join Date: Mar 2011
Location: Germany
Posts: 552
Rep Power: 20
cfd seeker is on a distinguished road
your lift results are ok and drag results are bad?? isnt it the case with you?
cfd seeker is offline   Reply With Quote

Old   May 17, 2012, 15:44
Default
  #3
New Member
 
Ali
Join Date: Apr 2012
Location: India
Posts: 17
Rep Power: 13
ali.qasim is on a distinguished road
Quote:
Originally Posted by cfd seeker View Post
your lift results are ok and drag results are bad?? isnt it the case with you?
No, both are.

I'm not concerned about drag right now, as I'm using the inviscid model.

The problem is, the lift coefficients are not what they should be. The approximation of inviscid should not really affect that very much.
ali.qasim is offline   Reply With Quote

Old   May 19, 2012, 12:11
Default Problem solved.. but doesn't make sense!
  #4
New Member
 
Ali
Join Date: Apr 2012
Location: India
Posts: 17
Rep Power: 13
ali.qasim is on a distinguished road
I'm now getting pretty decent results for the lift coefficient. But the weird thing is, it was because of the symmetry boundary condition.

I am using a C-mesh for the airfoil, and had defined the arc of the semicircle as the inlet, and the side edges (on top and bottom) as symmetry. This time, I removed that symmetry BC, and made these side edges also as part of the inlet.

I have absolutely no idea how that would make such a big difference. The percentage deviation from wind tunnel data is now within 5% compared to the 40% deviation I had before. Can someone please explain why this happened?
ali.qasim is offline   Reply With Quote

Old   May 19, 2012, 12:32
Default
  #5
Far
Super Moderator
 
Sijal
Join Date: Mar 2009
Location: Islamabad
Posts: 4,553
Blog Entries: 6
Rep Power: 54
Far has a spectacular aura aboutFar has a spectacular aura about
Send a message via Skype™ to Far
Are you sure? Please post this problem on ANSYS meshing forum to get detailed discussion
Far is offline   Reply With Quote

Old   May 19, 2012, 12:57
Default
  #6
New Member
 
Ali
Join Date: Apr 2012
Location: India
Posts: 17
Rep Power: 13
ali.qasim is on a distinguished road
Quote:
Originally Posted by Far View Post
Are you sure? Please post this problem on ANSYS meshing forum to get detailed discussion
I'm pretty sure about that.

Started a thread in ANSYS meshing.
ali.qasim is offline   Reply With Quote

Reply

Tags
airfoil, coefficient, drag, lift

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
Experimental data mapping. paka OpenFOAM 5 January 13, 2011 17:27
Step Changes in Roughness Experimental Data syler3321 Main CFD Forum 0 October 20, 2010 01:45
UDF and experimental data in Fluent Rogerio Fernandes Brito FLUENT 0 November 25, 2008 10:32
Multiphase experimental data James McLaren Main CFD Forum 1 September 14, 1998 07:35
Finding Experimental Data Mattias Jaran Main CFD Forum 0 August 10, 1998 08:35


All times are GMT -4. The time now is 04:08.