Re variation over span

Hi guys,
I didn’t know where to post this, so I’m sorry if it is misplaced. But I am testing turbulent effects on wind turbines using wt_perf and I am concerned over the fact that since I have only one set of airfoil data (under a single Reynolds number) and the Re varies over the span of the blade, my results may be way off.

I know that with the chord distribution I have, the Re varies from 1.5E5 to 4.5E5 (over the various wind speeds of interest) on the NACA4421. My question is, do you think that is a big enough variation to cause performance affecting changes in the Lift and Drag characteristics of the airfoil? (The data I have for it was taken under an Re of 2.75E5).

Thanks a lot

Pat, our crack aerodynamicist, may have better opinions, but my understanding is that airfoil data can vary quite a bit in the Re range you mentioned. I don’t know anything about the 4421 or whether there is more data for it. If there isn’t, you might look for some multi-Re data for some similar airfoils to see if you can synthesize some data for the 4421.

Another thing to consider is delayed stall due to 3D effects for the inboard sections of the rotor. You may want to consider using AirfoilPrep to adjust your airfoil data.


Thanks for the quick response. I’m afraid the NACA4421, S809 and the NACA0012 are the only options for me at the moment. What I have is data for these airfoils tested under grid generated turbulence of varying intensity and I have put this data into wt_perf (in the form of several airfoil files) to see the effects.

Please do post if Pat has anything to say on the matter.

Thanks again.

I’m not sure what you mean by those three airfoils being your only options.

We have S809 data for multiple Reynolds numbers. The 750,000-1,500,000 OSU data is available here:

I also have some stuff from higher Re tests if that helps. It’s not online.


You see, what I am trying to do is compare the effects of turbulence affected data to that of normal data. The grid generated turbulence data was taken under an Re of 2.75E5 using those three airfoils. There isn’t much point in using airfoil data under different Re or different Airfoils alltogether if I don’t have the equivalent data under grid generated turbulence to compare with.


I looks like you have two separate issues - the effect of turbulence and the effect of Reynolds number.

Your Reynolds number range is in the transition region for many airfoils and likely for the NACA 4421 as well. This means that there will likely be variation in lift and drag with Re. However, it will be most sensitive around airfoil stall. So, if you have a pitch regulated machine that doesn’t approach stall very often and doesn’t yaw (dynamic stall) then you will probably be safe using one Re number. Not sure how realistic such operating conditions are, but the error will be minimized.

The turbulence effect and surface roughness effects will also be most obvious near stall. If your airfoil is tripped and/or the turbulence intensity is high (locally on the blade), I would expect less influence on Cl & Cd with Re since these will increase the likelihood of a turbulent boundary layer.

There’s also the issue of dynamic stall if there is yaw in the turbine and I’m not sure what will happen. The uncertainty in this may be much larger than the uncertainty from Re and turbulence.

Without more data, it’s hard to speculate what will happen - I did find a couple reports by googling NACA 4421 that you may want to examine. Other than that I’m afraid I can’t be too helpful.

Good luck…


Does anyone know where I can find the NREL AMES UAE 3 test results for Power and Cp?

I’d start here…

Simms, D. A.; Fingersh, L. J.; Butterfield, C. P. (1995). NREL Unsteady Aerodynamics Experiment Phase III Test Objectives and Preliminary Results. Musial, W. D.; Hock, S. M.; Berg, D. E., eds. Wind Energy 1995: Proceedings of the Energy and Environmental Expo '95, the Energy-Sources Technology Conference and Exhibition, 29 January - 1 February 1995, Houston, Texas. SED-Vol. 16. New York: American Society of Mechanical Engineers pp. 273-275. Acc No. 16574.

Our librarian might be able to send you a copy - her email is tami_sandberg(at)

Thanks Pat. I will give your librarian an e-mail.

Perhaps it’s of no use to post to such an old thread, but in case this question comes up on a search (like it did for me), I’d like to suggest going back to the basics.

To me the obvious starting point is NACA test reports for their airfoils. This is public domain data which can be searched and downloaded from
Of particular interest to you, asking about NACA airfoils at extremely low Re, would be Tech Report 586, by Eastman Jacobs and Albert Sherman, 1936. The specific profile is not in the list, however its brothers and sisters are. The discussion on the effect of airfoil properties will lead you in the right direction. You may find, if you continue to search the NACA report website, that other reports in the series will turn up with the 4421.

Do you not have a copy of Theory of Wing Sections? NACA 4421 is there, Re from 3E6 to 9E6. I think a correlation between that book’s data and the Tech Report will get you pretty close.

Please don’t turn your nose up at such “old” data (I have other colleagues who do this) because the NACA research teams clearly knew how to deal with turbulence in their tunnel and dedicated a lot of work to isolating and reducing measurement errors. Don’t believe me - believe the number of pages in the report and other related reports on the subject dedicated to measuring and quantifying turbulence, in addition to other factors that affect the results. It may turn out that the old wind tunnel turbulence bears some relation to the turbulence that was deliberately simulated in the 4421 data you are working with.