Review Reports - Self-Starting Improvement and Performance Enhancement in Darrieus VAWTs Using Auxiliary Blades and Deflectors

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Self-Starting Improvement and Performance Enhancement in Darrieus VAWTs Using Auxiliary Blades and Deflectors

Review report

The manuscript explores the impact of incorporating auxiliary blades alongside the primary blades in a Darrieus-type VAWT. Using CFD, the authors aim to enhance turbine efficiency at low tip speed ratios, thereby improving self-starting capabilities while reducing efficiency losses at higher tip speed ratios. Optimization of the auxiliary blade installation was pursued by examining the effects of pitch angle and positioning relative to the main blades. Additionally, two asymmetric deflectors were incorporated into the design to further improve overall efficiency. While the concept of auxiliary blades and flow deflectors is well-known, the integration of both within a single design framework presents a unique approach. The manuscript demonstrates the effects of these design elements both individually and in combination, which adds a valuable perspective for both research and industrial applications. However, the accuracy of the results appears closely tied to the quality of the numerical modelling. The current manuscript does not sufficiently clarify the implementation and validation of the model, which raises some questions regarding the fidelity of the findings. I recommend a major revision of the manuscript, particularly in the introduction and methodology sections to improve clarity and the credibility of the results. Specific comments and suggestions follow below.

Introduction

1- The literature review could be more targeted, focusing specifically on Darrieus turbine self-starting and enhancement methods. Currently, the discussion is somewhat generic and then shifts directly into design modification methods. The following articles are recommended for enhancing the review:

· Yunus Celik, Lin Ma, Derek Ingham, Mohamed Pourkashanian, Aerodynamic investigation of the start-up process of H-type vertical axis wind turbines using CFD, Journal of Wind Engineering and Industrial Aerodynamics, 204, 2020, DOI: 10.1016/j.jweia.2020.104252.

· Omar S. Mohamed, Ahmed M.R. Elbaz, Alessandro Bianchini, A better insight on physics involved in the self-starting of a straight-blade Darrieus wind turbine by means of two-dimensional Computational Fluid Dynamics, Journal of Wind Engineering and Industrial Aerodynamics, 218, 2021, DOI: 10.1016/j.jweia.2021.104793.

2- The sentence on lines 71–73 regarding the "j-shaped aerofoil" is misleading. While it does enhance efficiency at low TSRs, performance at high TSRs is significantly reduced. It would be useful to highlight the drawbacks of various aerodynamic design modifications more consistently.

Governing equations and numerical setup

3- The mesh quality is a concern; given this design, high mesh quality is essential. The authors report high skewness in all meshes, which introduces uncertainty, particularly if skewed elements are near the aerofoils.

4- Y+ values should be presented more precisely. Plotting instantaneous Y+ values along a particular blade span would convey critical information not currently evident in the contour provided.

5- The mesh independence test does not appear sufficiently thorough. Adopting widely accepted practices, such as using a convergence threshold based on Cp/Cp,final is recommended. This threshold may not always suffice, so additional metrics (e.g., the coefficient of determination) should also be calculated to assess instantaneous torque variations rather than just average torque. This approach is particularly relevant here, given the design’s sensitivity to boundary layer resolution on both main and auxiliary aerofoils. For further guidance, see:

· Francesco Balduzzi et al., Critical issues in the CFD simulation of Darrieus wind turbines, Renewable Energy, 85, 2016, Pages 419-435, DOI: 10.1016/j.renene.2015.06.048.

6- Lines 389–391 state that the variation between time step sizes is minimal, which is inaccurate; significant variation is still noticeable. The authors are advised to consult Balduzzi et al. (2016) for methodology guidance on this matter.

7- Validation results show limited agreement with experimental data. Although they compare favourably to other CFD studies, other research with the same VAWT model geometry and operating conditions has yielded closer results to experiments. The authors should discuss discrepancies in their model and justify deviations from experimental measurements.

Results and discussion

8- The authors indicate that auxiliary blades expand the operating range, which is not entirely accurate. The TSR operating range shifts toward lower values, though the range itself remains comparable to that of a conventional turbine.

9- The representation of the lift coefficient against the angle of attack could be improved, as the theoretical angle of attack does not match the observed one, particularly during downwind phases and at low TSRs. The following articles provide useful details on this topic:

· Pier Francesco Melani, Francesco Balduzzi, Giovanni Ferrara, Alessandro Bianchini, How to extract the angle attack on airfoils in cycloidal motion from a flow field solved with computational fluid dynamics? Development and verification of a robust computational procedure, Energy Conversion and Management, 223, 2020, 113284, DOI: 10.1016/j.enconman.2020.113284.

Author Response

Dear Reviewer,

Please find the revision letter attachment. Thank you for your time and consideration.

Regards

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper is not acceptable in its current form and needs a severe-major review for the following reasons:

1- Line 241, 242 and 243: The 2D CFD approach was chosen because it has a proven ability to simulate wake flow accurately while minimizing computational expenses [34]). Why an experimental study is cited here?
2: The Grid 4 is not acceptable: The transition between the prism layers and the tetrahedral mesh is not consistent
3- The convergence criteria is declared 10^-6, with 30 iteration by time step. But if we look at figure 1e, the residual value of the continuity are limited to 10^-3 ! How is it possible?
4- The periodicity of the solution is not addressed
5- All figures are reporting incorrect Cm values: The Cm value at 0° azimuthal position is different of the position 360° !! Unrealistic
6- The maximum value of Cm of 0.4 / 0.5 is very high !! usually the Cm maximum value for this machine is around 0.2
7- Table 3 is not completed
8- The quality of all figures is very poor
9- The validation section is not clear.
10- Figure 8: How to justify that the conventional machine is limited to 1.4 and cannot go lower?
11- the description Figure10: it's not clear the link between the pressure values and the self starting improvement
12- The transition from one topic to another and from one TSR to another makes the paper difficult to read and understand the connection between the various sections.
13- It's recommended to use additional values, for example distance/chord or distance/diameter.

Comments on the Quality of English Language

English language must be improved.

Author Response

Dear Reviewer,

Please find the revision letter attachment. Thank you for your time and consideration.

Regards

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1) The state of the art should be improved, and the discussion of previous findings in relation to yours should be more detailed.

2) The novelty and contribution of the work should be clearly outlined at the end of the introduction, highlighting the key points.

3) What are the practical implications of your findings for the design and deployment of Darrieus VAWTs in real-world applications? Can you discuss how your results might influence future research or engineering practices in this area?

4) How do you justify the selection of the optimal configurations for the auxiliary blades and deflectors?

5) The quality of the figures should be improved.

6) Ensure that the language used is clear, concise, and free from ambiguity. Technical terms should be well-defined, and complex ideas should be broken down into easily understandable segments.

7) The conclusion should provide directions for future work.

Comments on the Quality of English Language

The English could be enhanced to more effectively convey the research.

Author Response

Dear Reviewer,

Please find the revision letter attachment. Thank you for your time and consideration.

Regards

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

This research investigates the enhancement of self-starting capabilities and overall performance in VAWTs through the implementation of auxiliary blades and deflectors. The following points require attention:

In the introduction, the motivation of this paper should be clearly stated, and the innovation points should be listed one by one.
The information on the far right of Table 3 is obscured and needs to be readjusted.
Is Figure 5a) incomplete, or should the details within it be expanded?
The explanation of the experimental process figures is still insufficient. The images are rich but lack adequate introduction and analysis.
There are inexplicable spaces in the middle of sentences, such as why is there a prominent space between "of" and "deflector" in "Figure 17. Effect of deflector installation on turbine"?
How exactly are the performance indicators 47% and 73% derived? Detailed explanation and calculation are needed.
The references lack research on relevant literature from the MDPI database.

Comments on the Quality of English Language

can be improved

Author Response

Dear Reviewer,

Please find the revision letter attachment. Thank you for your time and consideration.

Regards

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

The manuscript reports the research on optimization of auxiliary blade arrangement with CFD simulation to study the design parameters. The operational efficiency of a conventional Darrieus vertical axis wind turbine is upgraded by the installation of deflectors in various configurations. These findings can provide detailed information on the auxiliary blade arrangement for Darrieus vertical axis wind turbines. Overall, the engineering application value of this manuscript is high and meets the requirements of the journal. There are, however, some problems I would like to see addressed before the paper reaches its mature form.

1. The abstract section needs to provide an appropriate simulation model for selection.

2. With regard to the introdution section, some of the sentences in the middle and at the end are on the long side. It is recommended that they be described in different sentences.

3. Figure 1 provides a broader description of the turbine,I suggest to add to Figure 1 the location of the auxiliary blades in the exact parameter of the Darrius wind turbine schematic.

4. The image in Figure 5 is not very clear.I suggest to clarify the favaorable values of 𝑦+ by labeling the picture with the appropriate parameters.

5. Please provide the reason for choosing the three values of “-5”, “0”, and “+2” for the auxiliary blade pitch angle "𝛽".

6. Please provide a detailed explanation of the criteria and reasons for the selection of the numeric values for horizontal distance "𝑋"and vertical distance "𝑌".

7. Figure 16 position comparison is not clear, please enlarge the vortex contour plot of the same position of conventional rotor and optimum modified rotor to make a comparison.

8. Figure 7 shows a partial discrepancy between the present 2D simulation with experimental at TSR = 2.5. I suggest to explain specifically the rationale for why this partial discrepancy is not an issue for the simulation accuracies.

Author Response

Dear Reviewer,

Please find the revision letter attachment. Thank you for your time and consideration.

Regards

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have thoroughly addressed all comments from the previous review, and the article is now suitable for acceptance in its current form.

Author Response

Dear Reviewer,

Thank you for your feedback.

Regards

Reviewer 2 Report

Comments and Suggestions for Authors

The work has been improved. Last recommendation: Use dimensionless values, for example ( ... horizontal distance of 44.2mm, and vertical distance of 34.9m ..) in the Abstract could be expressed by Distance/Chord or Distance/Diameter. This should be applied to the whole manuscript.

Author Response

Dear Reviewer,

Thank you for your kind compliment. We are pleased to receive your positive feedback. In response to your insightful comment, we have divided the vertical and horizontal distances along the airfoil's chord to transform them into dimensionless parameters. We have provided these in the form of two dimensionless ratios: the vertical ratio and the horizontal ratio, as detailed throughout the article. We hope these amendments will meet your expectations.

Regards

Farzad Ghafoorian

Reviewer 3 Report

Comments and Suggestions for Authors

All my comments have been addressed by the authors, and the paper has improved considerably, so it merits publication.

Author Response

Dear Reviewer,

Thank you for your feedback.

Regards

Reviewer 4 Report

Comments and Suggestions for Authors

no further questions

Author Response

Dear Reviewer,

Thank you for your feedback.

Regards