Publisher’s note: a supplement was added to this comment on 26 February 2024.

General comments

The paper describes a wind tunnel experiment about the aerodynamic response of swept wind turbine blades. The experiment uses a scaled wind turbine with rotating blades and focuses on measurements of the velocity field around blade that were obtained by means of particle image velocimetry.

Blade sweep is an interesting technology for passive load alleviation. To be adopted more widely in future wind turbine designs, numerical tools must be developed to predict the blade aerodynamic response. The experiment presented in this article, as well as the dataset released to public, support the development of this technology, therefore the article is relevant to the international scientific community. The article falls within the scope of WES because of the focus on aerodynamics and the pioneering experiment presented by authors.

The methods are valid, the analyses presented by authors and the results the obtain are sound. The article is generally well structured, but there is room to improve it. For these reasons, the article deserves to be considered for publication in WES after being revised.

As a general comment, I would like to see (at the beginning of the methodology or in the introduction) an explanation of what is expected to happen in a swept blade from the aerodynamic point of view, especially if compared to a straight blade. Authors decided to measure and present in the article some quantities (velocities, flow angles, section loads) and I ask them to explain why these quantities are important to study (e.g., because they see a significant change passing from a straight blade to swept blade, or because they are difficult to predict with current engineering models for rotor aerodynamics, …).

The scale model blades show small pitch offsets and a bend-twist coupled elastic response. At line 146 it is said that blades were designed to be stiff, thus I suppose that the bend-twist deformation is unwanted, but it seems to affect results. I ask authors to clarify this aspect and explain which effects in the results are wanted and which are not, but are a consequence of manufacturing difficulties (that I think are normal at this scale).

Specific comments are reported below, and technical corrections are in the attached document.

Specific comments

• “Such values would be unrealistic on a full-scale, operational wind turbine”. Can you provide typical values for an operational wind turbine?
• “To maintain the same tip radius as the unswept reference blade, the swept blade axis coordinates are scaled by…”. I think this is not clear. You should explain what happens if you do not scale the blade axis coordinates.
• “was mounted rigidly on a traversing system”. I suppose the traversing system moves the PIV plane in a radial direction. Please add this information for clarity.
• Figure 4. This figure is not explained clearly. Please explain the difference between the green shape and the blue shape. The meaning of the different line styles is explained in the figure caption. I think it would be better to explain it inside the figure with a legend.
• 114-115. Which the implication of this distortion? A few lines before you said that measurements are set up to have comparisons with a straight blade. Is it possible to make comparisons if the PIV plane is not aligned with the airfoil?
• 122-124. It seems contradictory that you remove the induction and then you compute the induction. I suggest explaining briefly how the method works.
• Do you have an explanation on why the Noca’s method does not work for tangential force? Maybe it is worth to report the KJ’s and Noca’s methods in the article appendix and use the appendix to explain where the Noca’s method fails.
• Add a short introduction at the beginning of the Results section where you explain the content of the next subsections.
• “The blades used in this experiment were manually manufactured…”. I suggest moving this sentence in section 2.1. The orientation of fibers plays a role in the bend-twist coupling of blades. Explain if this was controlled in the manufacturing process and if you expect it to influence the results.
• 152-158. Please explain why you correct the airfoil to align it to the illuminated cross section.
• 163-164. “All three blades exhibit twisting behaviour…”. This is measured by the variation of DeltaBeta between at the blade tip and root. DeltaBeta at r/R = 0 is instead the blade pitch offset (right?)
• Table 3. I suggest putting the equations in the text and avoid the use of a table.
• “Non-dimensionalized”. How did you normalize measurements? Can you recall how you computed V_rel?
• Figure 7. I suggest to add in the figure a line for reference straight blades.
• “where the twist deformations vary strongly” and where measurements are more uncertain due to the small dimensions of the airfoil?
• 209-210. “This corresponds to the forces in the coordinate system spanning the measurement planes…”. Add a reference to Fig. 4 if you think it's useful.

Technical corrections

See attached document.

This experimental work consists in a wind tunnel study using stereoscopic particle image velocimetry among several planes to reconstruct the flow around rotating swept blades. A rotor composed by three of them is studied, and blade-level aerodynamics quantities are measured, allowing to a comparison to a BEM model generalized to include swept blades, that was recently proposed by the authors in another publication. Furthermore, the differences in terms of aerodynamics and shape between the three blades are discussed.

I agree with the authors that swept blades present some advantages for horizontal axis wind turbines that deserve to be studied and quantified. Furthermore, the experimental setup in the manuscript is very well conceived, allowing to obtain high-quality aerodynamics quantities that can be used to test different blade models, and that has also been made available to the community via an open database. I therefore consider that the manuscript is apt to be published in Wind Energy Science, provided that the following remarks are addressed:

- Overall, I found section 2.2 confusing. It gives a lot of information very densely and it is hard to understand the experimental setup properties. First, as other referee remarked, figure 4 is nor properly explained. Furthermore, I find the perspective of the laser sheet from figure 3 ambiguous, and it does not allow to see the field of view extend and direction.

I understand the difficulty in explaining the very large amount of SPIV planes covered, but in its present state the reader requires some time to understand them (for example, the so-called blade 1 was tested in 22 planes while 2 and 3 were tested in only 4). The authors may consider adding an extra table better detailing such measurements. I also propose that the laser planes are defined in terms of fixed cartesian coordinates instead of relatively to the blades.

- Connected to my previous comment, if the authors have a film of the experimental setup running, that can bed added to the public dataset, may help to support section 2.2.

- I also agree with another reviewer about adding to the manuscript which parameters from equation 1 correspond to a realistic shape. While I understand the authors plan to do a further manuscript in this topic, the present work would greatly benefit from testing the BEM correction model for swept blades (Fritz et al. (2022)) to test the sensibility of the lift coefficient from the blade to the parameters from equation 1.

- Related to the last comment, the authors say in line 147: 'Experience from previous experiments taught that the stiffness properties of the three blades can vary considerably'. How much they change? Furthermore, figures 10 and 11 show significant differences in terms of performance between blades. A sensitivity analysis would help to see if this is indeed due to the manufacturing of the current blades or an actual limitation for the application of swept blades in wind energy.

- 120 phase-locked images were recorded at each plane to extract the average velocity field and its standard deviation. Can the authors comment about the convergence of the fields?

- Why figures 2 and 3 do not expand the full range of the blade (0<r/R<1)?

- While they can be deduced from available data, the Reynolds number of the blades and rotor should be better specified.

- In line 82, the term 'three-dimensional velocity field' is confusing. It is the stitching process, detailed later, that allows to reconstruct three dimensional vector fields in space.

- The DOI towards the dataset is the manuscript is outdated (a v1 is missing).

- In figure 6, the velocity V and the relative velocity V_rel are not defined appropriately (the reader has to go to subsequent sections to find definitions).

- Do figures 5 and 7 have error bars? It looks like they are within the markers. If that is the case, it should be mention it in the captions.

-The phrase from line 45 ' By basing the scaled blade geometry on the aerodynamic characteristics of the IEA 15 MW reference wind turbine (Gaertner et al., 2020), relevance for state-of-the-art wind turbine designs is ensured', could be better sustained.