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Peer-Review Record

Novel Hybrid Aquatic–Aerial Vehicle to Survey in High Sea States: Initial Flow Dynamics on Dive and Breach

J. Mar. Sci. Eng. 2025, 13(7), 1283; https://doi.org/10.3390/jmse13071283
by Matthew J. Ericksen 1, Keith F. Joiner 2,*, Nicholas J. Lawson 1, Andrew Truslove 2, Georgia Warren 2, Jisheng Zhao 2 and Ahmed Swidan 2,3
Reviewer 1: Anonymous
Reviewer 2: Anonymous
J. Mar. Sci. Eng. 2025, 13(7), 1283; https://doi.org/10.3390/jmse13071283
Submission received: 29 May 2025 / Revised: 20 June 2025 / Accepted: 27 June 2025 / Published: 30 June 2025
(This article belongs to the Section Ocean Engineering)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a novel CFD analysis of a morphing-wing HAAV’s dive/breach dynamics, offering valuable insights into trans-media loads and structural requirements. While the numerical methodology is rigorously documented, the absence of experimental validation or comparative benchmarks significantly undermines the credibility of the predictions.

1.The study relies entirely on CFD simulations under idealized conditions (static water column). No pool experiments, scaled-model tests, or field data are provided to validate the predicted impact forces, accelerations, or breach heights.

2.Turbulence models (k-ω SST) may not capture cavitation/ventilation during high-speed entry.

3.Grid convergence only examines velocity—not forces, pressures, or free-surface deformation. Force predictions may be mesh-dependent.

4.Section 2.1 has only a subsection 2.1.1., which can be reorganized.

Author Response

Reviewer 1

Comment 1: The study relies entirely on CFD simulations under idealized conditions (static water column). No pool experiments, scaled-model tests, or field data are provided to validate the predicted impact forces, accelerations, or breach heights.

Response 1: Thank you for your insight and suggestion. We have provided this update ahead of pool tests. We have clarified within the final paragraph of the conclusion on page 17 that scaled pool experiments are currently underway to provide validation of the CFD results.

Comment 2: Turbulence models (k-ω SST) may not capture cavitation/ventilation during high-speed entry.

Response 2: Thank you for your insight and suggestion; we agree. We have expanded on the description for our use of k-ω SST to resolve the forces and flow separations from our surfaces of interest, while acknowledging its limitation to accurately capture cavitation during high-speed entry. Validation through plunge pool experimentation will also inform on the appropriateness of the turbulence equations utilised.

Comment 3: Grid convergence only examines velocity — not forces, pressures, or free-surface deformation. Force predictions may be mesh-dependent.

Response 3: Thank you for your insight and suggestion. Velocity was demonstrated in this paper as the variation of forces, while similar between the mesh refinements, weren’t as clear as velocity to the average reader. Unfortunately, we aren’t able to redevelop the graph due to technical issues at this time, but if we were, we would likely have an overlay of both velocity and forces. In the meantime, we have mentioned in the paragraph preceding figure 5, that the purpose of highlighting the velocity was “For ease of demonstration.” We hope this will suffice for now with a wider mesh validation as you suggest when we report the pool validation research results.

Comment 4: Section 2.1 has only a subsection 2.1.1., which can be reorganized.

Response 4: Thank you for your insight and suggestion. We have corrected this by removing the subsection title and combining it with ‘2.1. Numerical Setup.’

Reviewer 2 Report

Comments and Suggestions for Authors
  • The paper is well structured and written, the style is concise and clearly understood by the general reader. However, there are some technicalities that need more domain knowledge to be fully understood.

The references list contains 54 positions. While most of the references (over 90%) are recent, meaning the year of publication is after 2010, there are some positions that in my opinion are too old to be mentioned here, unless better justified: one dates from 1971, and the other from 1988 (positions 10 and 50)

  • Questions that the authors are recommended to answer in their conclusion section, mentioning which of these has been achieved in their research and which is to be set for future simulations and/or field tests:
    • Propulsion compatibility and source of energy/fuel
    • Weight versus buoyancy, as in the air: weight is the enemy, while in water neutral or slightly positive buoyancy is desired. Moreover, water salinity, temperature, and pressure could affect dynamic parameters of the trajectory that the HAAV employs.
    • Sealing and waterproofing: moving parts (e.g., servo joints for control surfaces or propulsion switching) need special attention to prevent leaks. That is a task for future live production and testing. However, excessive mechanisms could add to the weight of the HAAV, or introduce protuberances on the surface, that could affect the dynamics both in air and in water
    • Control and Navigation Challenges – are these clearly defined in the simulation model? Underwater dynamics are slow and damped; aerial dynamics are fast and reactive. Designing a unified control system that works across such different domains is a major challenge. The authors should emphasize more clearly if this issue has been solved completely.
    • Dynamics is seriously affected by Stability and Transition Control. Does the model comply with all these challenges?
    • Air-borne and underwater location, positioning, orientation and tracking. What other infrastructural elements are needed to obtain these features?

Author Response

Reviewer 2.

Comment 1: The references list contains 54 positions. While most of the references (over 90%) are recent, meaning the year of publication is after 2010, there are some positions that in my opinion are too old to be mentioned here, unless better justified: one dates from 1971, and the other from 1988 (positions 10 and 50)

Response 1: Thank you for your insight and suggestion. Few studies through the decades have been dedicated to the sole examination of the breaching abilities of avians. Penguins, in particular those that were able to launch a considerable distance, such as Emperor Penguins, were of interest to this study given their size and weight. Remote nature studies are not as frequently advanced as theoretical and engineering research but the biomimicry remains an inspiration. Kooyman et al [10] and Sato et al [11] were both referred to for their in-depth and corroborative data, while Kooyman was also a source study for other papers. Korobkin and Pukhnachov [50] were referred to in other papers as being the comprehensive source study for the development of splash curtain data. While not recent, none of the papers examined regarding the development of splashes sought to dispute their findings, indeed, they expanded in ways that weren’t usable to this paper at the time.

Comment 2: Propulsion compatibility and source of energy/fuel

Response 2: Thank you for your insight and suggestion. Agree this is a medium risk, no testing of EDFs in water has been attempted of the size and scale required, only a conceptual consideration that it should be feasible. Underwater testing would need to be arranged with a manufacturer such as Schuebler. We have added Table 2 to document known high and medium risks, adding those you have suggested to the table.

Comment 3: Weight versus buoyancy, as in the air: weight is the enemy, while in water neutral or slightly positive buoyancy is desired. Moreover, water salinity, temperature, and pressure could affect dynamic parameters of the trajectory that the HAAV employs.

Response 3: Thank you for your insight and suggestion. Agree, incisive suggestions. We rated this risk as medium as well because we had access to a fast-rate buoyancy engine from deep sea research vessels and our HAAV has the size to adopt it. We are researching the dive and breach speeds necessary both to clear sea state but also to achieve sufficient time in the new medium to have the buoyancy engine perform the exchange.

Comment 4: Sealing and waterproofing: moving parts (e.g., servo joints for control surfaces or propulsion switching) need special attention to prevent leaks. That is a task for future live production and testing. However, excessive mechanisms could add to the weight of the HAAV, or introduce protuberances on the surface, that could affect the dynamics both in air and in water

Response 4: Thank you for your insight and suggestion. Again, incisive comments, especially to depths below the thermocline. Again, we rated this risk medium based upon our deep-sea research vessel’s work, but fully acknowledge such sealing adds to weight. Our use of syntactic foam for its compressive strength is hoped to provide the necessary weight offset.

Comment 5: Control and Navigation Challenges – are these clearly defined in the simulation model? Underwater dynamics are slow and damped; aerial dynamics are fast and reactive. Designing a unified control system that works across such different domains is a major challenge. The authors should emphasize more clearly if this issue has been solved completely.

Response 5: Thank you for your insight and suggestion. Agree, slender underwater vessels are under-actuated. We rated this a high risk per the new table.

Comment 6: Dynamics is seriously affected by Stability and Transition Control. Does the model comply with all these challenges?

Response 6: Thank you for your insight and suggestion. Transition control was identified as being a challenge for vehicle development. However, to develop the idealised data that would precipitate a 6-dof simulation, vehicle motion was constrained to a single axis. This was a known limitation intended to gather data that would inform material strength requirements, such as compression requirements at the interface of the Titanium nose cone, and syntactic foam fuselage. Additional words and sentence have been included into the paragraph preceding “2.2.2. Grid Generation and Boundary Conditions” on page 8.

 

 

Comment 7: Air-borne and underwater location, positioning, orientation and tracking. What other infrastructural elements are needed to obtain these features?

Response 7: Thank you for your insight and suggestion. Agree, we rated this risk medium, as the control for airborne vertical flight has advanced considerably, as has control for underwater gliders. Many of our references also examine the challenge of the transition control, where our design's challenge will be accentuated by the high sea states it has to transit. To assist control in vertical airborne flight we placed torque-sensitive EDFs on either side of the fuselage. For control under the parent aircraft and underwater, the tail surfaces are larger than an aircraft alone would require, with the future option where necessary for all-moving tail surfaces.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

It is somewhat acceptable.

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