Performance of Semi-Active Flapping Hydrofoil with Arc Trajectory

Round 1

Reviewer 1 Report

This paper numerically studies the hydrodynamic performance of the semi-active 

flapping foil, where the swing arm undergoes the forced pitching motion and the 

hydrofoil undergoes simi-passive oscillation. Here are my comments,

1.      The language needs to be improved.

2.      What is the Reynolds number of the simulations? The authors did not specify Re throughout the paper. Aside from the spring constant K and L/c, there are many other important factors determining the dynamics of the system, like the moment of inertia, Reynolds number. Parametric studies on these variables are absent.

3.      What is the natural frequency of the mass-spring system? How does the resonance (the swing frequency resonates the natural frequency)  impact the hydrodynamics?

4.      The problem The research of semi-active flapping foil propulsion has been extensively explored over the past decades. What is the difference between the arc trajectory used in this paper and other trajectory forms in existing research?

5.      Validations. The authors employed the result from 2D experiment (Anderson) and a classical 2D flow of the soap film (Schnipper et al) to validate their numerical method. However, the authors performed the 3D investigations, while they validated the 2D numerical model. Can the corresponding 3D test results be found to improve the further validation of the numerical method?

6.      It can be imagined that this semi-active flapping foil driven by the swinging arm is a practical swinging wing form. However, from the results, the laws of its efficiency and thrust varying with parameters (such as arm length ratio, spring stiffness, and advance coefficient) are not the same. How to ensure this system works at the appropriate working point in application?

7.      Figure 9 and 11 provides the most interesting result of this work, it looks the optimal efficiency emerged at the specific maximal AOA. To some extent, it reveals the relationship between thrust and angle of attack, and how to design the angle of attack to make the flapping foil reach higher thrust. Further research or a discussion of the comparison to Read et al. need be carried out on this issue.

8.      From Figure 7, it seems that if the spring stiffness continues to be reduced, the wing swinging efficiency may still rise. The author can consider further improving the related work.

Author Response

Dear editors,

Thank you very much for your letter and advice. We have revised the paper and would like to re-submit it for your consideration. We have addressed the comments raised by the reviewers, and the amendments are highlighted in red in the revised manuscript. Please see the attachment. We hope that the revision is acceptable, and I look forward to hearing from you soon. 

With best wishes,

Yours sincerely,

Lei Mei

Author Response File: Author Response.doc

Reviewer 2 Report

This manuscript is important at present, and there is a real need for such studies. The following comments should address:

1. Many symbols are not defined. Therefore, the authors should make nomenclature to define the symbols and terms.
2. In the introduction section, the author wrote, " Due to the low efficiency of a single-degree-of-freedom flapping hydrofoil [18,19], a semi-active flapping hydrofoil with only one actuator is proposed and required [20]. "Is the semi-active flapping hydrofoil proposed in this study or in previous studies? The authors need to clarify that 
3. This manuscript has some typos. The authors should consider correcting the typos and grammar of some sentences.
4. On page 5, rows with numbers 168, 173, 179, 189, and 342 have typos. The authors should delete them.
5. The first paragraph of section 3 (Computational Method and Validation) needs to be written.
6. Have the authors considered the sensitivity of mesh analysis on the simulated results?
7. What is the range of inflow velocity used in this study?
8. What is the time of the overall simulation? At what did the authors start acquiring the results of the simulation?
9. Why are the different results obtained for different time steps, as presented in figure 4.
10. This manuscript has some typos. The authors should consider correcting the typos and grammar of some sentences.

Author Response

Dear editors,

Thank you very much for your letter and advice. We have revised the paper and would like to re-submit it for your consideration. We have addressed the comments raised by the reviewers, and the amendments are highlighted in red in the revised manuscript. Please see the attachment. We would like to express our sincere thanks to the reviewers for their constructive and positive comments. We hope that the revision is acceptable, and I look forward to hearing from you soon. 

With best wishes,

Yours sincerely,

Lei Mei

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

It has been improved a lot. I have no more comments.