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

Strange values for Lift & Drag Coefficient

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 18, 2011, 13:20
Default Strange values for Lift & Drag Coefficient
  #1
New Member
 
Denis
Join Date: Jul 2011
Posts: 8
Rep Power: 14
recnice is on a distinguished road
Hi,

i'm trying to simulate the flow over a symmetric airfoil and computing the lift and drag coefficients. The Reynolds Number is 1e6 and i'm using the Spalart Allmaras turbulance modell. The angle of attack is 0


Here is the function to compute the coefficients:
Code:
functions
{
    forces
    {
        type            forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        outputControl   timeStep;
        outputInterval  1;

        patches
        (
           Finne-Wall
        );

        pName       p;
        UName       U;
        log         true;
        rhoName     rhoInf;
        rhoInf      1;
        CofR        ( 0 0 0 );
        liftDir     (0 1 0);
	dragDir     (1 0 0);
        pitchAxis   ( 0 0 0 );
        magUInf     22.22;          // [m/s] 
        lRef        0.11;                // [m]
        Aref        0.00036385;     // [m^2]
And this is the solution

Code:
SIMPLE solution converged in 319 iterations

forceCoeffs output:
    Cd = 789148
    Cl = -12015.9
    Cm = 0
The lift coefficient has to be 0, because it is an symmetric airfoil. Does anyone has an idea, what could be wrong ??


Denis
recnice is offline   Reply With Quote

Old   November 18, 2011, 15:15
Default
  #2
Senior Member
 
Dave
Join Date: Jul 2010
Posts: 100
Rep Power: 15
daveatstyacht is on a distinguished road
Denis,
Your Aref and LRef are likely the reason you are getting such huge lift and drag coefficients. It is not unusual for a very small lift coefficient at 0 degrees (your lift is 65 times smaller than the drag) due to not quite perfect symmetry of the mesh surface. It is pretty difficult to get it exactly 0 with most meshers.

Regards,
Dave
daveatstyacht is offline   Reply With Quote

Old   November 18, 2011, 15:40
Default
  #3
New Member
 
Denis
Join Date: Jul 2011
Posts: 8
Rep Power: 14
recnice is on a distinguished road
Hi Dave,

thank you for your reply!

Ok I understand that the lift-coefficient has not to be exactly zero.

The main problem is the extreme high value.
I attached a picture of the model. It looks like a NACA airfoil, but it is actually a fin. So I expected a lift coefficients between 0 and 4. So do you think that this is a "phyiscal" solution?

Denis
Attached Files
File Type: pdf meshing_fin.pdf (39.8 KB, 39 views)
recnice is offline   Reply With Quote

Old   November 18, 2011, 21:31
Default
  #4
Senior Member
 
Dave
Join Date: Jul 2010
Posts: 100
Rep Power: 15
daveatstyacht is on a distinguished road
Denis,
As I said in my prior post, I think the issue is your Aref. You could manually calculate the coefficients from your force output: "type forces" rather than "type forcecoeff" to confirm that your simulation is producing physically the right results.
Dave
daveatstyacht 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
lift & drag coefficient on airfoil n. natik FLUENT 8 March 31, 2015 20:02
Drag Coefficient for Ellipse Form (2D) Ketut Utama Main CFD Forum 8 December 11, 2014 12:03
Solidification drag coefficient atulverma FLOW-3D 1 May 14, 2009 10:24
strange node values @ solid/fluid interface - help JB FLUENT 2 November 1, 2008 13:04
Plot drag coefficient against Y(distance) edwin FLUENT 0 February 4, 2008 10:41


All times are GMT -4. The time now is 05:56.