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-   -   Airfoil lift and drag understimated (http://www.cfd-online.com/Forums/openfoam-pre-processing/121306-airfoil-lift-drag-understimated.html)

Kio July 25, 2013 11:58

Airfoil lift and drag understimated
 
Hello everybody, I am quite new in CFD and in particular in OpenFoam.
I use the 2.1.1 version and I try to simulate the behaviour of a 4415 airfoil at different angle of attack.
There were no problem to obtain a "correct" trend of Cp (so pressure) according to experimental values, but lift and drag are understimate of 50%.

I guess the error can be associated to the BC of k and omega in k-w model and nuTilda in SA, because I choose them according to the parameters used in OpenFoam tutorial.
I have a freestream velocity U=100 m/s, solver sonicFoam, k=150 and omega=462 at the inlet fot k-omega, and nuTilda=0,14 for SA, wall_function for patches type wall.

Another question is about the function for calculate forces in controlDict file, I set this for angle of attack=0:
functions

{ forces

{

type forceCoeffs;

functionObjectLibs ( "libforces.so" );

outputControl timeStep;

outputInterval 1;

patches (profile_wall)

pName p;

UName U;

log true;

rhoInf 1.225;

CofR ( 0 0 0 );

liftDir ( 0 1 0 );

dragDir (1 0 0 );

pitchAxis ( 0 0 1 );

magUInf 100.0;

lRef 0.1254;

Aref 0.01292;

}

}

For angle of attack >0 I have to set the correct directions for lift and drag or the function calculates the force resulting on the airfoil and from that split lift and drag?

Thanks everybody

Kio July 25, 2013 13:54

2 Attachment(s)
something that could help to understand the problem with k and epsilon (or omega, with all models are the same..)

bscphil July 27, 2013 03:42

Quote:

Originally Posted by Kio (Post 442003)
something that could help to understand the problem with k and epsilon (or omega, with all models are the same..)

Hey Kio,

is your case 2D or 3D, because there are different definitions for lift and drag coefficients?
Just set the "lref" and "Aref" values both to 1 and after simulating calculate your coefficients with your definition.

For angle of attack > 0, i think you don't have to set new directions for liftDir and dragDir, beacause you want the forces in global coordinates and not in local.

To your boundary conditions for the turbulence values:
  • what is your turbulence intensity and turbulence length scale ??
You can upload your case, then we can look at your settings!!!



Greetz, Phil.

Kio July 27, 2013 04:42

1 Attachment(s)
Hi, sorry I forgot it. It is a 2D case, the turbulence intensity is 10% of the velocity and turbulence lenght scale is 0.0965 .
Here the case setup for the k-epsilon model.
thanks to all

bscphil July 27, 2013 09:27

Quote:

Originally Posted by Kio (Post 442277)
Hi, sorry I forgot it. It is a 2D case, the turbulence intensity is 10% of the velocity and turbulence lenght scale is 0.0965 .
Here the case setup for the k-epsilon model.
thanks to all

Thanks for the case. I've two more questions
  1. What is your airfoil chord length? --> i think 0.1254m?
  2. Do you have theoretical or experimental values for lift and drag coefficients?

Kio July 27, 2013 13:20

Quote:

Originally Posted by bscphil (Post 442317)
Thanks for the case. I've two more questions
  1. What is your airfoil chord length? --> i think 0.1254m?
  2. Do you have theoretical or experimental values for lift and drag coefficients?

Yes, the chord length is 0.1254 m. The values are experimental, from a report NASA. I check that the flow conditions are the same.

Naruto July 29, 2013 13:45

Hello Kio,
You did not provide the /constant folder. I faced a similar kind of problem like you a few months ago. The solution of my problem was quiet obvious. I made mistake in system/forceCoeff file. I did not take into account the length in z-direction. What I am meaning is that you have to find out the reference area like this: Chord_length * Height of extrusion in z-direction. :)
Anyway check the small detail and keep us posted. Normally a tiny misplaced setting plays the main role in this kind of error. Good luck.

Kio July 30, 2013 03:51

No the reference area is correct, I calculated area in the way you tell me.
The flow simulated is correct, I cannot understand.
The cd and cl coefficient are non dimensional, so I could compare with the coefficients from the report also if the airfoil scale is different

I cannot provide you the constant folder because is too big

Kio September 11, 2013 04:18

3 Attachment(s)
Hello everybody, after one month I do not find the problem.
I try different meshes and turbulence models but the problem is still the same, pressure field is correct but cl and cd are understimate.
I do not understand if the direction of lift and drag are simply (0 1 0) and (1 0 0) or inclinated with the flux. However also with an angle of attack of 0 cl is understimate.
There is another problem with an angle of attack > 0 with the coefficient cd that is negative. I attach an image to show you the reference system, how can I set the lift and drag direction? I think (0 -1 0) and (1 0 0) but the cl is positive and understimate of 20-30% and cd is negative and in any case too high. I attach also the log file of a case with angle=6 and k-omega model.
Please help me! I have to present the case within two weeks!

Kio September 11, 2013 07:15

A little doubt: I read all the threads about airfoil simulation on the forum and I observed that in all simulations the airfoil mesh is divided in two patches, top and bottom, but only one coefficient is shown in the log file. How the library choose on which patch calculate forces?

Naruto September 11, 2013 07:57

Hello kio. I am uploading my casefiles. I have validated the results with real data. Although I have simulated for incompressible flow using simpleFOAM. But still I think you might get an idea. Good luck.
https://dl.dropboxusercontent.com/u/...8/NACA0015.rar

Kio September 11, 2013 09:56

thank you very much Naruto, I tried also with simpleFoam and you boundary conditions (my original solver was sonicFoam) but the result remains the same, the cl is too low and cd too high.
Are you sure your results are right? I find a forceCoeffs.dat in your case uploaded, but with an angle of attack of 13 specified in the file 0/U also your cl was too low!
For example the coefficient with xfoil are here:
http://airfoiltools.com/airfoil/deta...il=naca0015-il

Naruto September 11, 2013 12:45

Yes I am sure that my data are ok. I checked them. :)
But I am saying again they are for incompressible flow.

Kio September 11, 2013 13:37

ok, thanks again. Probably the problem is related to the compressible flow

Kio September 13, 2013 06:41

I try with a mesh generated automatically by a script found here
https://www.hpc.ntnu.no/display/hpc/...l+Calculations

Although the pressure field is not accurate like that of the previous mesh, cl and cd are calculated exactly! y+ is between 1000 and 5000, I think my previous mesh was better because y+ was between 10 and 30, but the forces were not right.
I cannot explain that...

Freestream October 1, 2016 13:48

Hello Kio, do you have any insight to this problem?

I'm doing a validation for NACA 0012 and 4412, i am facing the same problem as well.

Hope you may give you advise...


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