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Airfoilprep Lift Spikes

Posted: Sat May 14, 2011 10:39 am
Hi guys,

I'm having some issue generating smooth lift curves that you guys have managed for the Delft airfoil (DU40_17) profile used in the NREL 5MW analysis. I have tried to replicate the process but failing to obtain such smooth variation of lift for high angle of attack. I have managed to get the DU40_17 airfoil lift, drag and moment data (Re=3 million) from TU Delft.

The stages on my calculation as follows:

1. Rotational augmentation
-Input 2D polar data in to the '3DStall' worksheet. I have chosen to input data from the angle of attack -10 to 50 deg
- Rotor RPM =12.1rpm, Rotor R= 61.5m, Wind speed = 12.1m/s, r/R= 0.192, chord= 4.557, Alpha End =30.
-Hit 'Calculate 3D table'

2. Extending the coefficient to full -180 to 180 deg
- Input the rotation augmented polar calculated from '3DStall' worksheet to the 'TableExtrap' worksheet.
- Aspect ratio=17, MaxCD= 1.416.
- Hit Update all

Which gives the polars the -180 to 180 deg angle of attack, unfortunately for my case with sharp stall transitions and poor drag values compared to the values published by J.Jonkman et al*. I have attached my AirfoilPrep Excel file and a picture of my results (spikes indicated with red circles). I hope someone can identify where Iâ€™m going wrong and help me fix this problem.

Results comparison
Airfoil Prep.JPG (78.43 KiB) Viewed 11327 times

* H Jonkman J, Butterfield S, Musial W, Scott G. Definition of a 5-MW reference wind turbine for offshore system development. Technical Report NREL/TP-500-38060, National Renewable Energy Laboratory, Golden, CO, 2009.

Re: Airfoilprep Lift Spikes

Posted: Mon May 16, 2011 10:41 am
Dear Kisorthman,

I've attached the AirfoilPrep v2.0 spreadsheet that I used to perform the rotational stall delay correction and Viterna correction for the DU40 airfoil.

AirfoilPrep_v2p0_DU40_A17.xls

From this spreadsheet, you can see that I entered the full -180 to 180 degree range of data into the 3DStall sheet. I only used the corrected data from 0 to 85 degrees.

I hope that helps.

Best regards,

Re: Airfoilprep Lift Spikes

Posted: Mon May 16, 2011 1:43 pm
Dear Jason,

This is exactly what I was looking for, thank you very much. From what I can gather is that for a best suited data, it is only necessary to apply 3D stall and drag (Viterna) correction within the positive 0-90 deg alpha(angle of incidence) range. This also suggests that stall or drag correction is not required for angle of attack range of 0>alpha>90. Is this correct?

Once again thanks for your prompt response, your help is greatly appreciated.
Kind regards.

Re: Airfoilprep Lift Spikes

Posted: Wed May 18, 2011 9:09 am
The 3D stall stuff is just to account for the fact that the radial flow delays the point at which the airfoil stalls. I doubt you need to go above 30 degrees or so. It should have no effect in the prestall portion of the lift curve.

Having a full 360 degrees of data is just so the software doesn't crash in an unusual situation. What happens at extreme angles of attack is poorly understood and the theories we use are invalid. One lift coefficient is probably as good as another in this situation. Before AirfoilPrep, I used to just make up numbers that gave me an aesthetically pleasing curve.

Use of the Viterna method or flat-plate theory allows us to generate data for extreme angles of attack. It is not related to stall delay due to rotation. It could be used for a nonrotating airfoil.

Please keep in mind that I am not an aerodynamicist--I just play one on the forums. If any Real Aerodynamicistsâ„˘ have anything useful to add, please do so.

Re: Airfoilprep Lift Spikes

Posted: Mon Sep 12, 2016 10:33 am
Dear Jason and Kisorthman,

Thanks for your informative comments from both of you. I have the following questions:
1. In the AirfoilPrep_v2p0_DU40_A17.xls provided ba Jason, the sheet Interpolate contains the data from angle of attack 0 - 90 degrees. Have you generated these data using Xfoil or Is it
experimental data?
I have generated the airfoil data Cl, Cd and Cm using XFLR5 for the airfoil DU40_17 with the help of coordinated from NREL 5M airfoil data, provided by FAST. I have used the RE:0.75 million as
mentioned in the airfoil data from FAST. However I did not the exact Cl, Cm and Cd values as there in the original file from FAST.
2. In the same excel file, the sheet 3Dstall contains the data from -180 to +180 aoa values. Did you first used the TableExtrap sheet to generate this data? As far as my knowledge we should the
we should follow the order of the sheet to generate the data. I'm very curious to know the source of the such data before using TableExtrap.
I hope I have explained it clearly to the best of my knowledge.
At present I'm generated the airfoil data with flap angles for all the airfoils used in NREL 5MW wind turbine to validate my ATEFlap model. I have used XFLR5 to generate the airfoil data Cl, Cd and Cm from -10 to 10 degrees aoa values. However, the resulted data is not in agreement with airfoil data of NREL 5MW wind turbine. Kindly provide your valuable suggestions and comments on this.

Regards,
Chaitanya Majjari

Re: Airfoilprep Lift Spikes

Posted: Mon Sep 12, 2016 11:22 am
Dear Chaitanya,

When I developed AirfoilPrep_v2p0_DU40_A17.xls, I did not use the Interpolate or BlendAirfoils worksheets. The data on these sheets is not for the DU40 airfoil.

As documented in the NREL 5-MW specifications report: http://www.nrel.gov/docs/fy09osti/38060.pdf, the airfoil data for the NREL 5-MW turbine was derived by applying 3D corrections to the 2D airfoil data provided in the DOWEC study (as detailed in Appendix A of Ref. [14]). This 2D data already included a full range of angle of attack from -180 to 180 degrees, so, we didn't need to use AirfoilPrep to due this extrapolation for us.

I hope that helps.

Best regards,

Re: Airfoilprep Lift Spikes

Posted: Wed Sep 14, 2016 3:05 am
Dear Jason,

Thank you for your quick response. I have managed to get the appropriate airfoil coefficients from XFLR5 and then used Airfoilprep to get corrected and interpolated data. I have referred to NREL 5MW wind turbine document and DOWEC 6MW document to find the appropriate reynolds number and other parameters required in ArifoilPrep. My lift curve doesnot exactly mimic the original lift curve, but it enough for me at this point of time.

Regards,
Chaitanya