The paper presents a wide set of numerical simulations of a virtual 1-blade Darrieus turbine using different airfoils. The amount of data presented is relevant and represents the main point of merit of the study. However, the results obtained for a single blade are scarcely representative of those a full turbine, since blade-to-blade interaction is lost, as well as the effect of turbine solidity on flow induction. This is reasonable for a first study but limits the validity of the conclusions. Double-checking the outcomes for at least 2 or 3 configurations in case of a 2- or 3-blade turbine could help understanding the validity of the analysis.

The term “morphing” is used throughout the paper. However, if this reviewer understood correctly, all simulations refer to an individual geometry tested under different conditions. The authors should more carefully alternate the terms “morphing” with “changing” or “modifying” for better transparency.

“Morphing” a blade during the revolution (second scenario considered) means that the flow induction changes at each azimuthal position, thus the behavior of the blade cannot be reconstructed as the sum of different “pieces” coming from different simulations. This part of the study must be re-thought carefully.

In addition to this main concern, a few additional points should be considered:

• Lumped references (like [23-33] or [42-46]) should be avoided. The authors should try to emphasize the contribution of each cited reference.

• The Reference section is well populated. However, the authors discarded a number of important references on VAWT design and simulation. The following readings are suggested since directly related to the present work:
  • http://dx.doi.org/10.1016/j.renene.2015.06.048
  • http://dx.doi.org/10.1016/j.energy.2015.12.111
  • http://dx.doi.org/10.1016/j.enconman.2014.10.038

• The choice of a blade-spoke connection at 0.5c makes all the analysis more complicated, since the pitching moment comes to play (see DOI: 10.1115/1.4034940). Moreover, the authors did not mention anywhere in the paper the impact of flow curvature effects (thicker airfoils in particular may result in ineffective virtual ones) – see: http://dx.doi.org/10.1016/j.enconman.2015.09.053. While these two phenomena are directly captured by CFD and are thus included in your analysis, the impact should be mentioned and possibly analyzed to help the reader interpreting the outcomes of the study.

• A literature study is referenced for the AoA calculation. However, that study has been recently overcome by a new one that is recommended for consideration by the authors: https://doi.org/10.1016/j.enconman.2020.113284

A discussion is present about the feasibility of a morphing blade system. However, it is strongly recommended to expand this section. Points to be addressed: 1) “time” for the actuators to move the airfoils in case of a variation along the revolution (it seems indeed unfeasible); 2) maximum change in thickness reasonably allowed by a constructive technology; 3) fatigue; 4) energy spent for morphing vs. increase in efficiency.

Introduction

The paper presents the results of significant modelling campaign completed with the aim of finding optimal aerofoil profiles to maximise production from a vertical axis wind turbine. This work in completed in the context of morphing blades, and the papers key stated aim is to ‘To pave the road towards smart blades for VAWTs’ through providing ‘a set of generalizable conclusions from 630 transient simulations… [to] understand the impact of different morphing blade scenarios on the turbine power performance CP as well as the thrust performance CT’.

The results presented represent a significant modelling campaign and the discussion of the results is thorough. However, the authors do not seem to draw generalizable conclusions from the work. The simulations are run for a single rotor geometry and whilst the optimum power increase achievable using morphing blades is found for this, single, rotor geometry there does not seem to be general conclusions put forward (or tested). Later, in the discussion section (line 381), it is stated that the main aim of the paper is to “prove the usefulness of the morphing technique to improve the power performance of VAWTs”. This aim can be said to have be achieved.

All simulations were completed with a fixed airfoil geometry. This is a valid approach for demonstrating the potential power performance enhancement achievable by scheduling the airfoil section on the rotor tip speed ratio, however the authors also use this simulation data to schedule the airfoil section on the angle of azimuth. This relies on a quasi-static assumption that is not valid as the aerodynamic forces on the blade are dependent on the time history. This limitation should be discussed thoroughly in the paper, and this section of the paper should be significantly altered or removed.

The following section discusses the 20 aspects put forward by Wind Energy Science for consideration by reviews and the Specific Comments section provides specific comments linked to the page/line number.

General Comments

This section responds to the 20 aspects put forward by Wind Energy Science for consideration.

• Does the paper address relevant scientific questions within the scope of WES?
  
  
  
  This paper aims to identify optimal aerofoil shapes for vertical axis wind turbines (VAWT) based on both rotor tip speed ratio and azimuthal position through a large number (630) of 2D URANS simulations. The aerofoil sections are based on modifications to the symmetric NACA 00 series with the leading edge radius, thickness to chord ratio, and the location of maximum thickness treated as design variables. This work is presented in the context of morphing (smart) blades, and the main objective of the paper is to provide generalisable conclusions based on these simulations to understand the impacts of morphing blades of VAWT performance. The aims of this paper refer to relevant scientific questions that are within the scope of Wind Energy Science journal.
  
  
  
  
• Does the paper present novel concepts, ideas, tools, or data?
  
  
  
  The simultaneous evaluation of turbine performance based on 3 aerofoil design parameters with the intention of finding globally optimal values and the significant amount of data provided indicate that the paper is sufficiently novel.
  
  
  
  
• Is the paper of broad international interest?
  
  
  
  The potential interest in this paper is sufficiently broad as it both evaluates the potential impact of morphing blades on both the power and thrust coefficients of VAWTs (of interest to a broader range of researchers interested in the effects of the design innovations), and provides details of the optimal symmetric aerofoil sections to maximise performance alongside thorough technical descriptions of the behaviour (of interest to more specialised researchers interested in the aerodynamics of morphing blades).
  
  
  
  
• Are clear objectives and/or hypotheses put forward?
  
  
  
  Two objectives are put forward by the paper: “(i) To pave the road towards smart blades for VAWTs, having the capability of adaptation to different operational conditions” and “(ii) To provide a set of generalizable conclusions from 630 transient simulations for 126 identical airfoils, generated with different values of maximum thickness t/c, chordwise position of maximum thickness xt/c, and leading-edge radius index I at 5 different tip speed ratios λ; and thus, understand the impact of different morphing blade scenarios on the turbine power performance CP as well as the thrust performance CT”. The first objective is not concrete and appears to be a motivation that is inherently completed through the completion of the second objective and should be understood as a research motivation rather than a direct objective of the paper.
  
  
  
  A third objective is mentioned in the discussion section: “ to prove the usefulness of the morphing technique to improve the power performance of VAWTs”. This object is the one that is most readily met by the paper, and it is advised that this should be mentioned in section 1.2.
  
  
  
  
• Are the scientific methods valid and clear outlined to be reproduced?
  
  
  
  The simulation methodology is clearly described and the paper provides references to previous, more thorough, descriptions of the simulation methodology and validation. The wording in both the abstract and introduction imply that the validation of the numerical model is presented in the current work which is misleading, it is advised to change the description of the model validation to the past tense.
  
  
  
  The methods by which aerofoils are parametrically generated is clearly defined and easily reproducible.
  
  
  
  
• Are analyses and assumptions valid?
  
  
  
  The discussion of the analyses and assumptions is split into 3 sections: (a) the modelling paradigm, (b) the rotor configuration, (c) the analysis of optimal aerofoil shape based of tip speed ratio, and (c) the analysis of optimal aerofoil shape based on azimuthal position
  1. The simulation model has been validated in previous work referenced by the paper and is a valid method of analysing the performance of vertical axis wind turbines. There is, however, no discussions of the limitations of the modelling paradigm or any justification of why the specific modelling paradigm was used. A short paragraph discussing what phenomena are (and are not) effectively modelled by 2D URANS simulations (i.e. tip losses) and why the results presented here are valid should be included.
  2. The paper uses a single bladed configuration and a fixed solidity over the entire study. In terms of the blade number and previous work is cited to evidence the claim that the blade number does not have a significant effect on rotor performance. However, maintaining a constant solidity, the chord length changes significantly with blade number. This directly impacts both the effective Reynolds number, and the non-dimensional frequency of the aerofoil motion which have impact performance, a short discussion of this would be beneficial. Additionally, the use of a single rotor configuration over the study makes it hard to draw generalisable conclusions (a stated aim of the paper). A discussion/ demonstration of how generalisable conclusions can be drawn from the specific simulation campaign should be included, or the limitations of using only a single rotor configuration should be discussed.
  3. The analysis of the optimal aerofoil shape based on tip speed ratio is valid
  4. The analysis of the optimal aerofoil shape based on azimuthal position is based on the extraction, at each azimuthal coordinate, of the aerofoil shape that corresponds to the maximum local power/torque coefficient. The previous analysis of the aerodynamic behaviour of blade discusses the inherently time-dependent nature of the aerodynamics as the blade rotates, however the extraction (and reconstitution) based on azimuthal coordinate is based in a quasi-static assumption that the aerodynamic characteristics at a given azimuthal coordinate are not dependent on the time history, which is not a valid assumption. This limitation is not discussed in the paper and should be.
     
     
     
     
• Are the presented results sufficient to support the interpretations and associated discussion?
  
  
  
  The discussion of the sufficiency of the results to underpin the interpretations will be split between (a) the discussion of the λ dependent blade morphing and (b) the θ dependent blade morphing
  1. The results from the λ dependent blade morphing are valid and provide ample data to underpin the conclusions drawn. A minor detail is that, during the discussion, there is often reference to plots of the aerofoil lift, drag and skin friction coefficients which are not graphically presented. Referring to the behaviour in terms of the coefficients, rather than plots of the coefficients would make the lack of inclusion of these plots less jarring to the reader. Otherwise including a few example plots to show the general behaviour may be of benefit to the reader.
  2. The results from the θ dependent blade morphing are not be valid, as the scheduling of the aerofoil shape is dependent on a quasi-static assumption that is not valid. Presenting these results under the caveat of this limitation may still interesting to the reader, however this limitation must be discussed in the text.
     
     
     
     
• Is the discussion relevant and backed up?
  
  
  
  Points of note in reference to the discussion are presented in response to section 7
  
  
  
  
• Are accurate conclusions reached based on the presented results and discussion?
  
  
  
  Points of note in reference to the conclusions are presented in response to point 7
  
  
  
  
  
  
• Do the authors give proper credit to related and relevant work and clearly indicate their own original contribution?
  
  
  
  The original contribution of the paper is clearly stated, however the reviewer is not familiar enough with the smart-blade concept to comment on the completeness of the literature review in the introduction. A more full discussion of the key results from the papers cited in the literature view should be included.
  
  
  
  
• Does the title clearly reflect the contents of the paper and is it informative?
  
  
  
  The title is clearly reflects the contents of the paper
  
  
  
  
• Does the abstract provide a concise and complete summary, including quantitative results?
  
  
  
  The abstract provides a concise and generally complete summary. Presenting the change in peak power coefficient as well as the average change in the power coefficient may be useful. Additionally, presenting the changes in power coefficient as relative (rather than absolute) values would be preferred.
  
  
  
  
• Is the overall presentation well structured?
  
  
  
  The paper is well structured
  
  
  
  
• Is the paper written concisely and to the point?
  
  
  
  Generally, the paper is written concisely, however specific comments on writing style are given in the specific comments section of the review.
• Is the language fluent, precise, and grammatically correct?

Generally, the paper is written precisely, however specific comments on grammar are given in the specific comments section of the review.

• Are the figures and tables useful and all necessary?

Generally, the figures and tables are useful and necessary, however specific comments on figure presentation are given in the specific comments section of the review.

• Are the figures and tables useful and all necessary?
  
  
  
  Specific comments of mathematical formulae are given in the specific comments section of the review

• Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?
  
  
  
  

Any elements to be clarified are given in the specific comments section of the review

• Are the number and quality of references appropriate?
  
  
  
  The number of references is appropriate however a greater discussion of the references given in the literature review is encouraged.

• Is the amount and quality of supplementary material appropriate and of added value?
  
  
  
  There is no supplementary material

Specific comments

This section lists specific comments about the work referencing the line and page number

Line 14 page 1: Use the past tense for validated as the validation is not presented in the current paper. Additionally include that the CFD simulations are 2D.

Line 16 page 1: Give the relative improvement in power coefficient rather than absolute value and provide the increase in peak power coefficient as well as the increase in mean power coefficient.

Line 17 page 1: Wind energy not capitalised in key word list.

Line 25 page 2: When was “of the day”? Date the previous reference in the text to give the reader context.

Line 29 page 2: “In contrast” to what? The operational conditions of aircraft, helicopters, UAVs, MAVs, were not discussed in text so the operational conditions of a wind turbine blade are not being defined in contrast to anything.

Line 31 page 2: Increasing weight and complexity is an issue for wind turbine blades as well.

Line 34 page 2: “Through this quasi-sinusoidal variation of α, it exceeds the static stall angle” Replace with “Through this quasi-sinusoidal variation, α often exceeds the static stall angle”.

Line 36 page 2: This is the first reference to the tip speed ratio so λ should be defined here (rather than on line 60.

Line 38 page 2: replace ‘sophisticated’ with complex.

Line 40 page 2: Give more information about references 42-46: What airfoil parameters were changed? What did these papers find? What did they miss?

Line 41 page 2: Again expand on the references 47-49, What results were found in these studies, and how is this piece of work different?

Line 42 page 2: The connective “In other words” doesn’t make sense here

Line 45 page 2: ‘resulting in improved aerodynamic and power performance‘ How is aerodynamic performance different to power performance here?

Line 47 page 2: Past tense “have shown” rather than “show”

Line 58 page 2: ‘126 identical airfoils’ implies the same airfoil section was used, perhaps amend with ‘126 unique airfoils’.

Line 126 page 6: “, not for all the studied airfoils the same event is observed.” Replace with “. This behaviour is not observed for all of the studied airfoils.”

Line 138 page 6: “can be due to the following reasoning:” replace with “can be explained by the following:”

Line 144 page 6: Describing the trend as polynomial isn’t very accurate language as any line can be represented by a high enough order polynomial (including a polynomial of order 1 which is linear), this description is repeated multiple times through the results section and should be replaced with more exact language, perhaps contrast monotonic to non-monotonic or to a line with a defined maxima. References to polynomial are repeated on: Line 145 page 6, Line 191 page 9, Line 203 page 9, Line 208 page 9, Line 209 page 9, Line 211 page 9, Line 215 page 10, Line 216 page 10.

Line 149 page 7: Replace “Overall” with “mean”

Line 192 page 9: “on the one hand” this connective is unnecessary, can just start the sentence, the same goes for Line 203 page 9 “on the other hand”

Line 199 page 9: “This can also be recognized from the Cl and Cd plots, where by increasing xt/c…” This sentence is maybe better to just remove as you have already describing stall onset occurring earlier and without the Cl/Cd plots this description is jarring for the reader.

Line 205 page 10: It is hard to visualise this without a plot of the skin friction coefficient.

Line 227 page 10: “either” This word should be removed

Line 228/229 page 11: Don’t need to reference the Cf contour plots if it is not plotted. Can replace with “This is because increasing xt/c higher than xtopt/c promotes…”

Line 240 page 11: Can just say ‘selected’ rather than “selected and optimal”

Line 245 page 11: Reference figure 12 when talking about the moment coefficient

Line 283 page 12: Don’t need to say “Nevertheless”

Line 290 page 13: “Morphing the airfoil shape-defining parameters is thought to have a fully coupled impact on the turbine CP”  replace with “The airfoil shape-defining parameters have a coupled impact on turbine performance”

Line 295/296 page 13: “although the combination of topt/c and xtopt/c remains invariant, the local optimum area is found to morph into thin airfoils with low xt/c.” It is unclear if the maximum does remain the same, perhaps have a point on the plot shown the maximum would be useful. Also what is meant by the “local optimum area”? this could be edited for clarity

Line 302 page 14: “In other words, the higher λ is, the less dependent the local optimum is on I.” This is really saying the same thing twice and the sentence can be removed.

Figure 13 page 14: This represents some of the key, and most interesting, results from the paper and is a great way to visualise a lot data at once. However:

1. The colormap could be held constant for set of results at a given tip speed ratio, as this would allow easy comparison between all 3 airfoil design parameters that are being varied
2. Each surface/colormap is made up of only 42 points, and some of the features, such as the ‘staircase’ appearing at λ=2.5, I = 6, might be due to the interpolation algorithm used to fill the space. Perhaps contour plots would look better? Or perhaps a different plotting function could be used.

Line 315/316 page 14: “optimal” regions of CP and CT is tricky language as optimal value of CT could have multiple meanings, perhaps better to use “regions of maximum CP and CT”

Line 324 page 15: My problem with this section is discussed in 6 d) and 7 b) in the general comments section.

Line 334 page 16: Presenting the relative difference in CP max values is more relevant than the absolute.

Line 361 page 17: Describing the airfoils as “morphing” implies that a dynamic process is taking place, however they were modelled with a constant shape. It would be clearer to describe them as ‘morphed’ or ‘optimal’.

Line 382 page 18: The structural limitations were not thoroughly discussed in the paper

Line 392 page 19: Previously validated with experiments (reference)

Line 396 page 19: Reference to “morphing airfoil shape” could be replaced with “optimal airfoil shape” for clarity

Line 405 page 19: “A morphing blade” or “morphing blades”

Line 409 page 19: “the rest of the geometrical parameters” Define what these parameters are.