Aeroelastic load validation in wake conditions  using nacelle-mounted lidar measurements

Conti, Davide; Dimitrov, Nikolay; Peña, Alfredo

doi:https://doi.org/10.5194/wes-5-1129-2020

Articles | Volume 5, issue 3

https://doi.org/10.5194/wes-5-1129-2020

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

Special issue:

Wind Energy Science Conference 2019

https://doi.org/10.5194/wes-5-1129-2020

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

Articles | Volume 5, issue 3

Research article

|

25 Aug 2020

Research article |

| 25 Aug 2020

Aeroelastic load validation in wake conditions using nacelle-mounted lidar measurements

Davide Conti, Nikolay Dimitrov, and Alfredo Peña

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Final revised paper (published on 25 Aug 2020)
Preprint (discussion started on 13 Feb 2020)

Interactive discussion

Status: closed

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

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RC1: 'Referee comment', Anonymous Referee #1, 22 Mar 2020
- AC1: 'Responses to referee RC1', Davide Conti, 03 May 2020
RC2: 'Review of "Aero-elastic load validation in wake conditions using nacelle-mounted lidar measurements"', Anonymous Referee #2, 24 Mar 2020
- AC2: 'Responses to referee RC2', Davide Conti, 03 May 2020

Peer-review completion

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

AR by Davide Conti on behalf of the Authors (07 May 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 May 2020) by Julie Lundquist

RR by Anonymous Referee #2 (23 May 2020)

RR by Anonymous Referee #1 (06 Jun 2020)

Suggestions for revision or reasons for rejection

The manuscript has been significantly improved, but I suggest the authors address the following remaining comments:

• Original reviewer comment: "More motivation for the presented work should be included. For example, I was wondering how accurate current IEC-recommended methods for estimating loads in waked conditions are, and if there is a clear need to improve these methods using lidar-measured wind conditions. Additionally, I was wondering why the authors did not compare the accuracy of the load predictions to the accuracy when using these IEC standard wake modeling methods. It was not until the end of the paper (last paragraph of Section 5) that these topics were discussed. I believe this material should be moved to the introduction to better motivate the research and to explain the scope of the current work (i.e., that comparison with DWM, etc. is not part of the current work)."

Author response: "Following the comment of the reviewer, we now provide further motivation on the need to improve load validation approaches in wakes, and clarify the scope of the work along the manuscript. Further, "Sect. 2.1 Requirements for load validation in wakes" is fully dedicated to define the scope of the current work and introduce the reader with the elements of the load analysis.

The following text is added on page 2 line 9: …"

Moving this discussion to the introduction has strengthened the paper, but it is still repeated in the discussion. To avoid duplicating material, perhaps just refer back to the intro in the discussion section?

• Original reviewer comment: "Many of the paragraphs throughout the manuscript are very long, often an entire page. Organizing them into smaller paragraphs would greatly improve the readability. For example, pg. 2, pg. 11, pg. 14, pg. 17."

Author response: "To improve readability, we restructured the manuscript as following:…"

The changes by the author help improve the organization and readability of the paper, but there are still many paragraphs that are too long and should be divided into smaller pieces for easier reading. For example, Section 3.4 is one very long paragraph. Same with Section 4.2. The first paragraph of Section 5 should also be split into smaller parts.

• Original reviewer comment: "4) Table 1: Can you explain the difference between uhub (mean wind speed at hub height) and U (in the TI definition)? Or are these the same and could be written with the same symbol?"

Author response: "We now replace the U with u in Table 1 and throughout the manuscript."

This is a good change, but it makes it unclear if bar(u)_hub and bar(u) are the same quantity, or if bar(u) varies with height. TI is written as sigma_u/bar(u) (instead of sigma_u/bar(u)_hub), which makes it seem like TI varies with height in your wind field model. Can you clarify in the text if this is the case?

• Original reviewer comment: "8) Pg. 7, ln. 17: "The parameters (uhub,: : :" Because of the induction zone model, I imagine the induction factor is also a parameter that is estimated. Is this true?"

Author response: "This is correct, the Cind is now included in the sentence."

I think it makes more sense to list the induction factor a_0 as the parameter instead of the function C_ind.

• Original reviewer comment: "15) Pg. 10, ln. 8: "The filtered turbulence derived from CW and PL lidars are plotted: : :" Which beams are used to derive the turbulence?"

Author response: "To derive the filtered turbulence estimates from both the CW and PL lidars, we used all the beams and ranges. To derive the unfiltered turbulence from the CW lidar, we use only measurements at 1.3 D.

We replaced the text in the manuscript on page 10 line 8:

From: "The unfiltered turbulence derived from the CW and PL lidars are plotted respectively in Fig. 4 (left and middle), whereas the unfiltered turbulence derived from the CW lidar is shown in Fig. 4 (right)."

To: "The unfiltered turbulence derived from CW and PL lidars, using all the ranges and beams, are plotted respectively in Fig. 4 (left and middle), whereas the unfiltered turbulence derived from the CW lidar measurements at 1.3 D are shown in Fig. 4 (right).""

Thanks for clarifying, but please also explain in the text how the multiple measurement points are combined to estimate TI. For example, are the TI values derived from each measurement point averaged together to form a single TI estimate?

• Pg. 10, ln. 23: "The second approach avoids…" Since you are switching topics and talking about the 2nd approach after a few paragraphs focusing on the 1st approach, it makes sense to start a new paragraph here.

• Pg. 16, ln. 12: "When compared to the filtered turbulence, the unfiltered estimations show a significant reduction by ~6%..." In Figs. 8 and 9 when comparing the filtered and unfiltered turbulence estimates, it appears there might be an error in the plots. The filtered and unfiltered cases should be for CW measurements, but in the plots the differences between blue and black are along the pulsed lidar ("y") axis, while the corresponding values along the CW axis remain the same. I think the opposite should be true. Is this an error?

• Original reviewer comment: "21) Pg. 14, ln. 32: "It can be noticed that broadening effects are present only in b3: : :" Where is the evidence of broadening effects? Wouldn't this require the velocity spectrum in freestream conditions for comparison?"

Author response: "We added the Doppler spectrum for a wake-free case with similar inflow conditions."

This is a nice addition to the plots. But in the text, please describe in what sense the inflow conditions are similar for the measurement periods of the different Doppler spectra.

• Original reviewer comment: "22) Pg. 15, ln. 18: Please compare with the coefficient of determination equation in Dimitrov et al. 2019. It appears there are some typos in the equation listed here.

Author response: "The coefficient of determination is now corrected"

There is still an error in the coefficient of determination equation. Both expectations should be the expected value of the measured statistics (y hat).

• Original reviewer comment: "23) Pg. 16, ln. 5: Why is only MXBCmin investigated, as opposed to MXBCmax?"

Author response: "This is due to the convention used in the strain gauges. The increasing
flapwise bending moment results in negative loading. We keep the same convention as in (Dimitrov et al., 2019)."

It would be good to provide this explanation in the text and also explain what direction negative loading is in and why this is the direction of the maximum loading.

• Original reviewer comment: "27) Pg. 18, ln. 7: "The obtained linear regression coefficients for Power: : :" This is a nice analysis. Are the trends the same for MXBCmin and MXTBDEL (which aren't shown)?"

Author response: "We show the linear regression coefficients for all the channels in the figure below."

It would be good to briefly mention in the text if the trends for the coefficients for for MxBC min and MxTB DEL are the same or different as the other channels since they are not shown in the plots.

• Pg. 23, ln. 17: "Although we demonstrate a low sensitivity of the loads to the shear exponent…" It is hard to understand the meaning of this sentence. Are you trying to say that "horizontal shear" or a "horizontal velocity gradient" is envisioned to more appropriately account for wake-affected velocity gradient profiles…?

Pg. 25, ln. 9: "…load predictions of the order of 4% compared to the free-wind case" Kind of confusing. Do you mean 4% "higher" compared to the free-wind case?

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ED: Publish subject to minor revisions (review by editor) (08 Jun 2020) by Julie Lundquist

AR by Davide Conti on behalf of the Authors (11 Jun 2020) Author's response Manuscript

ED: Publish as is (09 Jul 2020) by Julie Lundquist

ED: Publish as is (10 Jul 2020) by Joachim Peinke (Chief editor)

AR by Davide Conti on behalf of the Authors (16 Jul 2020) Manuscript

Short summary

We propose a method for carrying out wind turbine load validation in wake conditions using measurements from forward-looking nacelle lidars. The uncertainty of aeroelastic load predictions is quantified against wind turbine on-board sensor data. This work demonstrates the applicability of nacelle-mounted lidar measurements to extend load and power validations under wake conditions and highlights the main challenges.