Optical Detection of Underwater Propeller Wake Based on a Position-Sensitive Detector

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Summary:

The authors propose a method for measuring the wake of an underwater propeller based on a position-sensitive detector. Based on the principle of laser beam deflection, a theoretical model of the relationship between the laser spot displacement and the change of refractive index of the wake field is established. A prototype experimental underwater propeller wake measurement setup was constructed using the optical measurement method. Furthermore, the propeller wake flow field simulation with strong and linear density stratification was conducted based on the experimental setup. These experimental results are found to be in general agreement with the simulation results of the propeller wake field. A comparison of the experimental device-based wake measurements with the CTD-based wake measurements reveals a consistent trend. The authors claim that in the future, the proposed method can aid in detecting the wake fields of ships and underwater vehicles. Consequently, ships and underwater vehicles can be positioned and tracked.

The manuscript is well organized and could be a good contribution to the field of underwater optical imaging. The manuscript should also interest readers in this field. Overall, the reviewer recommends the publication of this article in MDPI Photonics. However, certain areas can be enhanced and minor issues should be addressed before acceptance. Therefore, the reviewer suggests the manuscript be ACCEPTED AFTER MINOR REVISIONS.

Detailed Comments:

1. The abstract can be more precise and condensed. Here, it is too long and very specific. The authors may make it more general without going deep into the technical specifics and also talk more about the expected impact, potential applications, and novelty of the presented work. This will make it more readable to the audience.

2. The abbreviations like RUI and CTD should be used only after they are explained. It isn't easy for non-experts to understand what they mean without appropriate expansions.

3. The first paragraph of the introduction section could benefit from more explanations regarding the significance of the method described. Please provide a more general introduction regarding the "tracking, detection, and localization" of underwater objects and their significant technical uses.

4. In lines 49 - 50: What are the limitations of non-acoustic methods? Please explain.

5. In section 3: On what basis are the experimental parameters chosen regarding propeller speed and the tank dimensions? What are the limits of the proposed method? What is the maximum and minimum propeller rotation speed that can be detected?

6. What are the minimum and maximum widths between the laser and the PSD at which this system can be implemented? 

7. Why is 532 nm of light used? What is the effect of wavelength on such a system? 

8. In section 5.3, Please also add a discussion regarding the following:

a) The novelty of the proposed system compared to existing mechanisms and a quantitative and qualitative analysis with existing literature.

b) Limitations of the proposed method and future perspectives on the development of this technology

c) Potential applications.

9) The reviewer would like to know the viability of the proposed method when translated into real-life settings. The waters at the sea are generally turbulent, which causes heavy distortions to the optical signal. Moreover, the water is impure and contains a high concentration of soluble substances, leading to significant scattering and absorption of light. Furthermore, the turbulent water conditions may contribute to considerable background noise, which may be higher than the wake field, making detection impossible. The authors should address these issues and demonstrate the scenarios described above through simulations or experiments. The manuscript can be significantly enhanced by another section regarding this issue and issues noted in comments 5, 6, and 7.

10. Another minor issue is using several abbreviations without properly using the expansions first. This must be addressed.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This work report a propose a method for measuring the wake of an underwater propeller based on a position-sensitive detector. The authors demonstrate that the experimental device-based wake measurements reveals a consistent trend with the CTD-based wake measurements. This research may have potential applications in the wake fields of ships and underwater vehicles. In general, the structure of the manuscript is clear, and experimental data are abundant. The paper could be published after minor revision addressing the following points.

1.line 42: “……the straightforward utilisation of acoustic detection of underwater vehicles is no longer sufficient to meet the demand for underwater vehicle detection” 

In fact, the acoustic detection technology is advancing with the advancement of noise reduction technology.  At present, acoustic detection technology is still the main method for detecting underwater vehicles. The relevant expression in the manuscript is inappropriate.

2.line 179: The size of the testing system is relatively small. Have you considered the instability of the flow field caused by the reflection of the wake on the wall?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf