Range of NREL Airfoil Data

Discuss acquisition, use, and manipulation of airfoil data.

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Andrea.Bracchitta
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Joined: Wed May 10, 2017 4:19 pm
Organization: Politecnico di Milano
Location: Italy

Range of NREL Airfoil Data

Postby Andrea.Bracchitta » Thu May 11, 2017 6:14 am

Good morning to everybody,

I would like to know if data from Eppler simulations of S818, S830 and S831 profiles provided here:

https://wind.nrel.gov/airfoils/AirfoilData.html

…are stopped at stall conditions (e.g. for the S818 at Re=1.5e06, the last incidence angle is 11°: is it the stall angle or not?).
I am doing a preliminary design of a HAWT (variable RPM, feather pitch-regulation) for a university course’s project, and I am deciding what profiles I have to choose. One option is to select the family S818, S830 and S831: for this reason, I am interested in public data available at NREL website.

I know that I will not need to use high incidence angles (since the maximum L/D is near 8-9° for the S818, for instance, and anyway I will use a feather pitch-regulation). Anyway, I am curious to know if provided data are stopped at stall conditions (juts because in the final presentation, I would like to show CL-polar diagrams with a large range of incidence angles in order to highlight where stall is).

Thanks for replies… and sorry for my bad English.

ps: I have made some simulations using Xfoil, since I read that it provides smart results (even if it seems to over-predict maximum lift coefficients). However, I have a serious doubt about the following results (CL vs incidence angle for the S831 profile). As you can see from the attached figure (http://imgur.com/a/JrsBg), the maximum lift coefficient is at much higher values than the one provided by wind.nrel.gov/airfoils/AirfoilData.html. Moreover, there is a “strange hill” at more or less 7.5°. From one side I say “ok, the max. lift coeff. goes up increasing Reynolds, while in the linear region I don’t see Reynolds influence, and this is OK”, but from on the other side I say “well, the hill it’s quite strange and I am not such confident with the simulation, since the stall angle is much more bigger than the final value provided by NREL so I don't have a comparison”. This is another reason why I would like to know if provided data from NREL are stopped at stall or not: if yes, I will completely discharge my results!

Jason.Jonkman
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Re: Range of NREL Airfoil Data

Postby Jason.Jonkman » Fri May 12, 2017 5:10 pm

I asked Pat Moriarty of NREL, and here is what he said:
My informed guess is that the simulations are stopped near the stall angle, but not necessarily at the stall angle - the Eppler code will start breaking down near and before full (or partial) stall, but it cannot be used to predict the actual stall angle.

Cheers,
Pat
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
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