Experimental and Numerical Analysis of a Novel Cycloid-Type Rotor versus S-Type Rotor for Vertical-Axis Wind Turbine

Round 1

Reviewer 1 Report

1.     Enhance the introduction by adding more studies on the optimization of blade configurations for S-type wind turbines to highlight the innovation of this research. Given the current interest in S-type wind turbine arrays, include relevant literature to support this context, such as the studies found at https://doi.org/10.1016/j.renene.2022.05.062 and https://doi.org/10.1016/j.oceaneng.2021.11.0020.

2.     Expand on the numerical calculation section by verifying mesh independence and detailing the criteria for selecting time steps and validating numerical methods.

3.     Clearly specify the dimensions of the internal concave wave-shaped surface.

4.     Given the limited performance improvement (1%) demonstrated in the structure, it is recommended to compare wind turbines with different wave sizes to identify structures that significantly enhance performance. Summarize the characteristics of beneficial configurations.

5.     Add an analysis of the internal flow field within the S-type wind turbine, particularly at the wave-structured sections. Add details on the flow field structure within the concave area and the blade pressure distribution curves.

6.     Thoroughly review the article for grammatical and formatting accuracy. Enhance the quality of figures using scientific drawing tools.

Improve and polish your English writing.

Author Response

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections changes.

Comments 1: In blue

Response 1: In red

2. You can find point-by-point responses to Comments and Suggestions.

Comments 1: Enhance the introduction by adding more studies on the optimization of blade configurations for S-type wind turbines to highlight the innovation of this research. Given the current interest in S-type wind turbine arrays, include relevant literature to support this context, such as the studies found at https://doi.org/10.1016/j.renene.2022.05.062 and https://doi.org/10.1016/j.oceaneng.2021.110020.

Response 1: Thank you for your comments, we enhanced the introduction and added your studies in lines 147 to 152.

Yunrui Chen et. al. (https://doi.org/10.1016/j.renene.2022.05.062) develop research related to the behavior of groups of Savonius turbines, which operate under the same rotation speed, optimizing the power coefficient 1.425 plus that of the groups than that of an isolated turbine, using the Taguchi method.

In the area of ​​hydrokinetic turbines, Aditya Kumar Nag et al. (https://doi.org/10.1016/j.oceaneng.2021.110020) analyzes Savonius-type helical systems, an improvement is obtained when the separation distance is four times the turbine diameter, and they are distributed in a triangular formation, the optimal way to distribute The turbines were staggered.

We most attentively request your understanding, since another reviewer has told us to reduce the introduction.

Comments 2: Expand on the numerical calculation section by verifying mesh independence and detailing the criteria for selecting time steps and validating numerical methods.

Response 2: Thank you for your advice, according to review 1, the computational domain was changed and new simulations were performed, so we used the Richardson’s extrapolation and added an explanation about the criteria for selecting time steps and validating numerical methods, As can you see, the changes are on:

Section 2.2.2 Mesh, and lines 300 to 335.

Table 1 Mesh size dependence study.

Section 2.2.3 Boundary conditions, and lines 321 to 329.

Comments 3: Clearly specify the dimensions of the internal concave wave-shaped surface.

Response 3: We agree with this comment, to clarify this point we changed Figure 2 and 3.

Comments 4: Given the limited performance, improvement (1%) demonstrated in the structure, it is recommended to compare wind turbines with different wave sizes to identify structures that significantly enhance performance. Summarize the characteristics of beneficial configurations.

Response 4: Thank you for your observation, however, we think maybe there is a slight confusion, the improvement among S-rotor and Cycloidal-rotor through simulation was 15%. In this work, we only demonstrated Cycloidal-rotor is better than S-rotor. We are working on the next investigation to improve the Cycloidal-rotor, evaluating different geometry sizes.

Comments 5: Add an analysis of the internal flow field within the S-type wind turbine, particularly at the wave-structured sections. Add details on the flow field structure within the concave area and the blade pressure distribution curves.

Response 5: Thanks for the remark, to solve this, we change the contours Figure 7 for the comparison of S_C1 and R_C, which have better performance than S_C2  and S_C3.

Comments 6: Thoroughly review the article for grammatical and formatting accuracy. Enhance the quality of figures using scientific drawing tools.

Response 6: Thank you for your comments, we have improved, Figures 1, 2, 3, 4 and 7.

Comments on the Quality of English Language: Improve and polish your English writing.

Response : In response to your comment, we have received support from a colleague to review the English writing.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper analyzes and compares the performance of a new vertical-axis wind turbine rotor based on the mathematical equation of the cycloid with a semicircular or Savonius-type rotor, which is widely used. Three cases are studied: equalizing the diameter, chord length, and the area under the curve. Computational Fluid Dynamics (CFD) simulations evaluate force, moment, angular velocity, and power, while experimental validation is conducted in a wind tunnel. Results show that the cycloid-type rotor outperforms the semicircular or Savonius-type rotor by 4.7% in simulation and demonstrates superior performance in all three cases experimentally, achieving an efficiency of 10.8%.

The literature review provides a comprehensive overview of research related to Vertical-Axis Wind Turbines (VAWTs) and their optimization, focusing on various aspects such as aerodynamics, structural dynamics, and performance enhancement techniques. Numerous studies have been conducted using computational simulations, experimental testing, and numerical analyses to explore different parameters affecting VAWT performance, including airfoil design, blade shape, chord length, aspect ratio, solidity, and rotor geometry. Additionally, the review highlights the absence of studies investigating the use of the cycloid geometry in VAWT rotors, which is the focus of the present research. The paper proposes a novel blade design based on the parametric equations of the cycloid and presents a numerical and experimental analysis comparing the performance of a cycloid-type rotor with a Savonius-type rotor for VAWT development.

In my opinion, the literature review is impressive, but it seems disproportionate to the overall length of the article. The introduction spans 5 pages, while the entire text is 22 pages long. I believe this is a bit excessive, and some condensation is warranted. I understand that the Savonius topic is extensive, but let's not overdo it!

If the computational domain is as depicted in Figure 2, then it is incorrect.

Te rotor analyzed by these authors is not a Savonius rotor.

The methodology has been described insufficiently.

Author Response

Response to Reviewer 2 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections changes.

Comments 1: In blue

Response 1: In red

2. You can find point-by-point responses to Comments and Suggestions.

Comments 1: This paper analyzes and compares the performance of a new vertical-axis wind turbine rotor based on the mathematical equation of the cycloid with a semicircular or Savonius-type rotor, which is widely used. Three cases are studied: equalizing the diameter, chord length, and the area under the curve. Computational Fluid Dynamics (CFD) simulations evaluate force, moment, angular velocity, and power, while experimental validation is conducted in a wind tunnel. Results show that the cycloid-type rotor outperforms the semicircular or Savonius-type rotor by 4.7% in simulation and demonstrates superior performance in all three cases experimentally, achieving an efficiency of 10.8%.

Response 1: Thank you for your comment, the objective of the research was the design, construction and experimentation with a new geometry based on the equation of a cycloid, then perform the simulation using Computational Fluid Dynamics, and compare it with an S-type rotor without overlap ratio.

RMS power simulation results are as follows: 338.4 mW for the Cycloid-type rotor and 294.1 mW, 32.39 mW, and 29.1 mW, for S_C1 , S_C2 , and S_C3 , (Rotor Type S) respectively.

Therefore, a wind tunnel and 3D printing of the rotors are built to verify the performance.

Comments 2: The literature review provides a comprehensive overview of research related to Vertical-Axis Wind Turbines (VAWTs) and their optimization, focusing on various aspects such as aerodynamics, structural dynamics, and performance enhancement techniques. Numerous studies have been conducted using computational simulations, experimental testing, and numerical analyses to explore different parameters affecting VAWT performance, including airfoil design, blade shape, chord length, aspect ratio, solidity, and rotor geometry. Additionally, the review highlights the absence of studies investigating the use of the cycloid geometry in VAWT rotors, which is the focus of the present research. The paper proposes a novel blade design based on the parametric equations of the cycloid and presents a numerical and experimental analysis comparing the performance of a cycloid-type rotor with a Savonius-type rotor for VAWT development.

Response 2: Thank you for your comments, and exactly the research proposes a new blade design based on the parametric equations of a cycloid.

Comments 3: In my opinion, the literature review is impressive, but it seems disproportionate to the overall length of the article. The introduction spans 5 pages, while the entire text is 22 pages long. I believe this is a bit excessive, and some condensation is warranted. I understand that the Savonius topic is extensive, but let's not overdo it!

Response 3: Thank you for your observation, the introduction has been compacted by reducing from 243 lines to 208. The number of references and outstanding quantitative data have been maintained. Additionally, we shared that the other reviewer requested that a few references be added, which was also fulfilled.

Comments 4: If the computational domain is as depicted in Figure 2, then it is incorrect.

Response 4: Thank you for your point, to solve this issue, the computational domain was changed and new simulations were performed. As can you see, the changes are on:

Figure 3, Figure 7,  and lines 250 to 256.

Comments 5: The rotor analyzed by these authors is not a Savonius rotor.

Response 5: This observation contributes a lot to the research, we appreciate it, a novel blade design based on the parametric equations of the cycloid, is compared with an S-type rotor without overlap ratio. Due to the above, the name has been updated throughout all paper.

Reference to rotor name:

Hussain H. Al-Kayiem, Bilawal A. Bhayo, Mohsen Assadi, Comparative critique on the design parameters and their effect on the performance of S-rotors, Renewable Energy, Volume 99,

2016, Pages 1306-1317, ISSN 0960-1481,

https://doi.org/10.1016/j.renene.2016.07.015.

Comments 6: The methodology has been described insufficiently.

Response 6: Thank you for your excellent comment, Figure 1 has been added, which more clearly expresses the design, construction and experimentation, as well as the value of the results obtained.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

1. Regarding the format of literature citation: ichange "[57] suggests" to "Author name [57] suggests".

2. Improve the quality of the use of symbols, grammar, and other basic content.

3. Carefully check the format of the entire text to enhance its readability.

Improve the grammar

Author Response

Round 2 

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections changes.

Comments 1: In blue

Response 1: In red

2. You can find point-by-point responses to Comments and Suggestions.

Comments 1: Regarding the format of literature citation: change "[57] suggests" to "Author name [57] suggests".

Response 1: Thank you for your comment. We have corrected the citation and other similar issues in the complete manuscript. For example line 29 to line 32:

For example, research related to the use of CFD includes studies by Day et al. [7], Zhang et al. [8] and Ye et al. [9]. They investigated the influence of two parameters on 30 the performance of a VAWT using CFD: the airfoil’s maximum camber and its position along the chord.

Comments 2: Improve the quality of the use of symbols, grammar, and other basic content.

Response 2: Thank you for your observations. We have reviewed and enhanced the use of symbols, grammar, and other basic content in the complete manuscript. 

Comments 3: Carefully check the format of the entire text to enhance its readability.

Response 3: We appreciate your feedback and thank you once again. We have reviewed and improved the manuscript to enhance its readability.

Comments on the Quality of English Language: Improve the grammar.

Response: Thank you for your recommendation. We used Writing Assistance's grammar tools to improve the manuscript's grammar and readability.

Thank you for your assistance with the manuscript. We have noticed a significant improvement. 

Author Response File: Author Response.pdf

Reviewer 2 Report

1/ In the conclusions, there shouldn't be as many annotations. General statements should be presented. A significantly lower level of detail is needed.

2/ Figure 6: What does "wind generator" mean? This term suggests that the flow in the wind tunnel was generated using such a small wind turbine. Simply use "wind turbine".

3/ The authors should revise Figure 7. Case "c" is missing in the description. Additionally, I suggest using uniform color scales, separately for pressures and velocities.

Author Response

Round 2

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections changes.

Comments 1: In blue

Response 1: In red

2. You can find point-by-point responses to Comments and Suggestions.

Comments 1: In the conclusions, there shouldn't be as many annotations. General statements should be presented. A significantly lower level of detail is needed.

Response 1: Thank you for your recommendation. Taking this into account, we have rewritten the conclusions.

Comments 2: Figure 6: What does "wind generator" mean? This term suggests that the flow in the wind tunnel was generated using such a small wind turbine. Simply use "wind turbine".

Response 2: You are correct. We apologize for the error and have made the necessary corrections. 

Comments 3: The authors should revise Figure 7. Case "c" is missing in the description. Additionally, I suggest using uniform color scales, separately for pressures and velocities.

Response 3: Thank you for your observation. The case “c” is not missing. However, we have improved the wording of the description and adjusted the contour plots based on your recommendations.

Thank you for your assistance with the manuscript. We have noticed a significant improvement. 

Author Response File: Author Response.pdf