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Range of NREL Airfoil Data

Posted: Thu May 11, 2017 6:14 am
by Andrea.Bracchitta
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!

Re: Range of NREL Airfoil Data

Posted: Fri May 12, 2017 5:10 pm
by Jason.Jonkman
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