Inlet Velocity for DU 35 and DU 40 Airfoils

Wind & Water Airfoils
1

Good day everyone,

Please I need help with the inlet velocity for the DU 35 and DU 40 airfoils used in the NREL 5MW wind turbine.
First, I have made an airfoil from the data made available elsewhere on this forum.
Secondly, I made a 3m meters extrusion of the 2D airfoil to make a 3D wing.

I want to run a CFD simulation of the case using the wind speeds for the NREL 5MW (9.0, 11.4m/s) but I didn’t get the CL and CD values specified in the airfoil data.

on a second check at the airfoil data, I found that the calculations were done at 7E06, this gives an approximate inlet velocity of 109m/s… ?? if this is true, is this wind speed not too high for a supposed wind turbine airfoil?

I would like to know if my assumption is correct.

Thanks for your response in anticipation.

Victor S.
Master’s Candidate,
HIT, china.

2

Dear Victor,

I’m not sure how you are calculating 109 m/s. With:

Re = u*L/nu

then:

u = Re*nu/L

I calculate u = 34.2 m/s with Re = 7E6, nu=1.464E-05 m^2/s, and L = 3 m.

Best regards,

3

Thank you so much for the Reply Dr Jonkman.

I found my mistake. I was taking the length as 1m.

4

Dear Victor,

Actually, you never mentioned what chordlength you are using in your original post. In the Re calculation, you should use the chordlength for L. You mentioned 3 m in your original post, but that is in the context of extrusion length, which I assumed you meant as the chordlength. If the chordlengh is 1 m, u = 102.5 m/s.

Best regards,

5

Dear Dr Jonkman, Thank you for your replies.
However, I still haven’t been able to get the cl, cd values specified in the document “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. Perhaps I’m doing something wrongly… Below are the steps I took;

I downloaded the airfoil coordinates from elsewhere on this site, After I generated an airfoil geometry from the coordinates, I found that the chord length is 0.999. So I scaled it up to 4.65 (to match up with the chord length found in literature) and made a 3m extrusion.

Then I specified u = 22.03 m/s with Re = 7E6, nu=1.464E-05 m^2/s, and L = 4.65 m. I supposed that this configuration is for AOA=0, but I didn’t get the values (cl=0.196, cd 0.0094) specified for Zero AOA.

I would like to hear your perspectives sir.

Best regards,

6

Dear Victor,

Are you asking why your CFD solution is not matching the airfoil data provided by NREL for the DU35 and DU40 airfoils? I’m not sure I can really answer that question, but your CFD solution likely depends on the solution method (2D or 3D CFD). discretization, and sub-grid scale turbulence model.

Best regards,

7

Dear Dr. Jonkman,
I am sorry for asking so many questions but I still can not pinpoint where the problem is … Even if there’s going to be a difference in the values due to the solution method and discretization, they would not be so much.

Maybe I’m working an a wrong coordinates file?

please, Doc

(1) Are the airfoil coordinates here " github.com/old-NWTC/FAST/tree/m … e/Airfoils " the correct ones?

**I seem to find a lot of different data for the same airfoils on the internet.
(2) Are there any liberties i’m supposed to take (like any modifications?) after making the geometry from the downloaded coordinates, any change in Origin/airfoil center???

BR

8

Dear Victor,

Yes, those are the correct airfoil coordinates. These coordinate files are also available along with the airfoil data in the FAST v8 archive and OpenFAST r-test.

I’m not sure what differences you are seeing, but please be aware that the airfoil data NREL has provided includes 3D corrections such as rotational augmentation and deep stall.

The aerodynamic center is specified in the FAST / OpenFAST model, but would only influence the pitching moment, not the lift and drag coefficients.

Best regards,

9

Thank you so much, Dr Jonkman. But, I’m still unable to find out why I couldn’t get similar results. Also, I cannot find literatures that have carried out CFD tests on these airfoil sections.

In the real sense, what I was trying to do is to do an analysis of the turbine cross sections (DU 35, 21,40) cut from the turbine but, I couldn’t get past specifying the correct inflows and angle.

I guess if there’s anyone on the forum that has done these things before , I’d really appreciate their input and guide.

Finally, i would like to find out the pitch angles for each of these sections (in order to calculate the local AOA).

Best regards.
Victor.

10

Dear Victor,

It may help to share the results you’ve generated to understand what level of difference you are talking about.

I’m not sure what you mean by pitch angle, but in the NREL 5-MW turbine, the blade-pitch angle is wind speed dependent and determined by the pitch controller. I would not think this would matter for the standalone airfoil with uniform inflow, where the local AoA can be specified geometrically directly .

Best regards,