Aerodynamic effects of Gurney flaps on the rotor blades of a research wind turbine

Alber, Jörg; Soto-Valle, Rodrigo; Manolesos, Marinos; Bartholomay, Sirko; Nayeri, Christian Navid; Schönlau, Marvin; Menzel, Christian; Paschereit, Christian Oliver; Twele, Joachim; Fortmann, Jens

doi:https://doi.org/10.5194/wes-5-1645-2020

Articles | Volume 5, issue 4

Wind Energ. Sci., 5, 1645–1662, 2020

https://doi.org/10.5194/wes-5-1645-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Wind Energy Science Conference 2019

Wind Energ. Sci., 5, 1645–1662, 2020

https://doi.org/10.5194/wes-5-1645-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 5, issue 4

Research article 26 Nov 2020

Research article | 26 Nov 2020

Aerodynamic effects of Gurney flaps on the rotor blades of a research wind turbine

Jörg Alber et al.

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Final revised paper (published on 26 Nov 2020)
Preprint (discussion started on 21 Feb 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Reviewer's comments', Anonymous Referee #1, 02 Mar 2020
- AC1: 'Reply to Anonymous Referee #1', Joerg Alber, 31 Mar 2020
RC2: 'Article review', Athanasios Barlas, 10 Mar 2020
- AC2: 'Reply to Referee #2', Joerg Alber, 31 Mar 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Joerg Alber on behalf of the Authors (15 Jun 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Jul 2020) by Mingming Zhang

RR by Galih Bangga (19 Jul 2020)

Suggestions for revision or reasons for rejection

Dear Authors,

The investigated studies are of interest for wind turbine community and have been conducted thoroughly. After evaluating your paper and previous remarks from the reviewers, I agree with most of your answers but further detail shall be given concerning the angle of attack. Below please find some minor critics that I wish to be considered before the paper can be published:

1. If I follow your paper correctly and by reading your answers to one of the reviewers (including the paper from Annette), the angle of attack was obtained in the measurement by assuming 2D flow. This was done by comparing the rotating rotor with a 2D polar data. This relatively simple approach should work well if no flow separation or unsteady effect presents. However, as you introduce the Gurney flaps, there will be little unsteadiness occurring near the trailing edge of the blade. How did you take this into account?
2. The source of the 2D polar data for the AoA calculation is also unclear. Was the Gurney flap also included here?
3. Furthermore, the analysis was done mostly at 0.45R, and you claimed that the AoA evaluation has been confirmed with FLOWer calculations in the other paper. Despite that, the comparison done in Annette’s paper was at the spanwise positions no less than 0.65R. How did you ensure the 2D assumption will be correct especially for the lower TSR case? I do agree that lift will not be affected significantly for little inaccuracy in AoA calculation in the mid-span region, however drag will still be influenced as I have seen for example on the AVATAR rotor (see Fig. 7 page 13 https://aip.scitation.org/doi/pdf/10.1063/1.4978681).
4. It is interesting to note that you state the drag penalty is still at reasonable value (line 115) but this has never been proven in the paper.
5. Lastly, I would recommend to add a plot showing the lift over drag to really assess the performance of the rotor. The drag might be estimated by the pressure integration, although friction drag is also important for small AoAs.

Small typos
Line 275 i.e. 0.65R < r < 0.45R -- > should be other way around.
Line 13 Reynold --> Reynolds. It is a person name and please check for some others….

God luck!

Kind regards,
Galih Bangga

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RR by Anonymous Referee #4 (10 Aug 2020)

Suggestions for revision or reasons for rejection

The manuscript presents an experimental study on a 3 m diameter horizontal wind turbine rotor. The focus was on the effects of Gurney flaps on the aerodynamic performance of the blade. Two Gurney flaps heights were tested on a clean blade and with turbulator tape.
Experiments studies are always welcome, whether for the knowledge provided and/or for data generated that can be used to validate numerical models.
Only few attempts to physically comment the experimental observations reported. This should be improved in the revised version of the manuscript.
My comments and suggestions are presented as follows:

1. A linguistic revision of the manuscript is recommended.
2. The introduction needs to be improved to show the relevance of the presented experiment. Can author briefly mention what are the wide knowledge gaps and what portion of that current study is trying to fill. List point-by-point objectives and map the achievements of objectives in conclusion.
3. Line 38: add (c) after chord-length.
4. Line 48: correct the range (1.3 %c < GF <3.5 %c).
5. Line 87: the authors stated a blockage ratio of approximately 40 %. Could authors elaborate on this (how did they evaluate this value; what impact will have this blockage on the results...)
6. Line 109: mention that XFOIL is a program.
7. Section 2.2.1-2.2.2: To adjust the dimensions of the ZZ tape and the GF, the authors used XFOIL to evaluate the BL thickness. XFOIL is a 2D airfoil calculator, could authors explain how can exploit this result to a blade flow which is 3D?
8. Please rearrange Table 1 for clarity. Tripped case is missed. Measurement method part could be arranged in another table.
9. Lines 138-139: the authors stated that their findings are relevant beyond the Re-numbers of the BeRT blades, as long as the GF/BL ratio is kept constant. Are you suggesting that 2 flows with different Re numbers are comparable? Please explain more about this statement and define the ratio (GF/BL) in the text.
10. Line 217: RMBs#RBMs
11. Curves in graphs with 2 vertical axes must be correctly identified. For example, in figure 7a, indicate which curves correspond to axial and tangential velocities.
12. Line 245: free flow conditions may be confusing (flow without turbine or without wind tunnel walls).
13. Lines 245-253: The authors showed that the axial wake velocity is more sensitive to the wind tunnel blockage than the tangential velocity, could you please explain this phenomenon?
14. Line 276: Define ΔαGF
15. Line 277: Please what do you mean by ‘’ to a more favourable level in terms of the BeRT rotor ‘’? I think it is better to reformulate lines 277-278.
16. Is there a reason to evaluate the angle of attack at r=0.56R and the pressure coefficient which depends on this angle at a different radial position (r=0.45R)?
17. Line 285: Define ΔCp.
18. Lines 285-290: Description of Figure 10 needs to be rewritten more clearly to show the effect of GF on the pressure coefficient.
19. Line 312: change Δcl to cl.
20. Regarding the conclusion, please see comment #2.

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (10 Aug 2020) by Mingming Zhang

AR by Joerg Alber on behalf of the Authors (13 Sep 2020) Author's response Manuscript

ED: Publish as is (21 Sep 2020) by Mingming Zhang

ED: Publish as is (21 Sep 2020) by Joachim Peinke(Chief Editor)

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Joerg Alber on behalf of the Authors (19 Nov 2020) Author's adjustment

EA: Adjustments approved (19 Nov 2020) by Mingming Zhang

Short summary

The aerodynamic impact of Gurney flaps is investigated on the rotor blades of the Berlin Research Turbine. The findings of this research project contribute to performance improvements of different-size rotor blades. Gurney flaps are considered a worthwhile passive flow-control device in order to alleviate the adverse effects of both early separation in the inner blade region and leading-edge erosion throughout large parts of the blade span.