Influence of Central Platform on Hydrodynamic Performance of Semi-Submerged Multi-Buoy Wave Energy Converter
Round 1
Reviewer 1 Report
The paper is clear and scientifically sound, but the content is shallow and brings few novel research results. I don't mind it being published as such but would have preferred if the authors had taken more time to go more in depth with their study.
Author Response
Thanks for your advice. In the future, a meaningful direction will be found from the results of this study for further research,and we will try our best to find the reasons behind the results of this study
Reviewer 2 Report
With respect to the previous version, the description of the result are more comprehensible and the final discussion is more complete.
However, the description of the used numerical model and the way by which it has been applied are not clear.
In section 2 (Materials and methods) it is not clear which is the geometry of the physical domain that has been numerically reproduced; it is not clear how much extended is the sea region in which the velocity and free-surface elevation fields have been simulated; it is not clear which scale ratio has been chosen for the numerical simulation of the WEC.
In section 4 (Discussions), the authors state that a full-size simulation should provide more insight for the comprehension of obtained results. The authors should explain which simplifications has been adopted in the reduced-scale simulation and which advantage they aspect from a full-size simulation.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
This paper uses the results of numerical modelling of a semi-submerged raft with three hinged heaving buoys to investigate the influence of certain geometric parameters. The report contains the results of investigations conducted at one instance of regular waves. It contributes to the knowledge of the Eagle-type WECs.
Broad comments
The work is interesting and represents useful steps in the development of WECs of this type.
Relevant literature is cited in the very first part of the manuscript and thereafter little/no literature is incorporated into the paper. Of the literature that is incorporated, about half is from the same authors or the same research project (the Eagle WEC). This lack of breadth or depth in referring to the literature, or positioning the results within a broader context, is a major weakness of this paper. More literature needs to be included in a meaningful way. For example, similar or comparable results on the topics of: WaveStar, floating bodies (especially floating structures including boats/ships), numerical modelling approaches and associated drawbacks can be used to discuss the validity/reliability of the results or to discuss how the design of the WEC might be altered.
The methods are incomplete. There is no mention of the numerical solver/method used, boundary conditions, time steps, simulation times or software used. What is the difference in the method for time domain versus frequency domain modelling? The description of the geometry does not describe the lower side of the support platform (what the curves may be and whether they are important for the function). Likewise, in the description of the results and the variables modified, information is lacking (see specific comments).
The experimental approach does not feel directed by an overall hypothesis, rather it seems that it is an investigation of varying multiple parameters. These are important steps in engineering development but this is not what journal publication is for.
The principal weakness of the paper is the lack of discussion of the results. I could only find one instance (line 289/290). The reasons behind the phenomena seen are not discussed at all, nor is the validity of the approach and so the reader is left wondering whether the results are true, why they occur or what they might mean. The simulations seem to be conducted at one instance of wave frequency and height and there is no discussion of the influence that these will have on the results or whether the trends observed would be applicable in other areas of the parameter space.
In general, many of the graphs are the wrong type or do not show the most important information of the tests. For example Figure 5 (and others like it) show a time series of the oscillation but the surrounding text highlights the amplitude of oscillation. It would be more effective to find the mean amplitude and to plot that against the value of B, thus a proper trend could be shown. While it is interesting to show the transient behaviour, this is not acknowledged or discussed. For several of these plots, the difference in mean oscillating position *could* be very interesting but this is never acknowledged.
The conclusions, that there is a relationship between the platform geometry and WEC performance and that there is an optimal geometry, are simplistic. Conclusion (3) is an interesting conclusion but it feels unfinished since its physical reason is unexplored.
The amount of work to rectify these issues is considerable - a good discussion takes a lot of time to construct - but I am confident the authors can do this.
The standard of English is generally ok but the manuscript would be improved with editing by a native English speaker.
Specific comments:
Results: Different weights and line styles might help to distinguish between series (Figure 6 has some of this) – don’t rely on colour.
Results: Put axes in kN rather than N (kW not W) to ease legibility
17/18: Geometry (shape) is not the same as draught; please ensure you clarify which you mean here
32: I would say wave energy is at a *disadvantage* owing to a multitude of designs; please explain your reasoning
41: Citation should be Kramer (not Morten Kramer)
54: Please provide a brief explanation of what wedge angle is (a picture might be an effective way to do this)
55: “obviously” is incorrect – it is not obvious
57: by massive do you mean having a large mass or do you mean large in size? If the latter, please change, else clarify
86: dissertation – change for paper/manuscript/similar
96: Variables should be italic font, L and W are assumed to be on the top surface of the platform
112: Is there a better way to format the unit, eg: (Nms/°)?
124: Were all the results simulated (ie no use of a symmetry plane), and if so, did you check the results were the same?
129: What is the influence of the underside geometry of the platform; did you consider block coefficient?
136 (and others): added mass not add mass
Figure 4: what values of frequency were used? How was this modelling done?
137: What does this mean? Do the graphs not show the peak occurring at difference frequencies depending on the aspect ratio?
155: If B now stands for something else, please use a different letter
161: Please state value of density used (and rationale if appropriate)
164: “It can be seen that” – these graphs are very hard to interpret as to the values (see other comments on presentation of results).
165 (and all other instances apart from line 251): “opposite” is used incorrectly
171/172: This sentence needs to be clarified or rewritten as its meaning is not clear.
Figure 7 has a legend entry missing
Figure 8 (also Figure 13 and others): which values of B were simulated? if not many (<100) it would be more appropriate to display markers at these discrete values rather than continuous lines. Is this from a time domain model? How does frequency affect this result?
186: Are L and W taken at the top side of the platform or at the waterline and at what value of B (aspect ratio)?
194: Is this expected/typical?
Figures after this point are incorrectly numbered.
203: Why?
Figure 2 (page 8): there appears to be a bi-frequency response, especially of the S320 series, which does not appear to be transient. This is probably important and should be discussed/investigated.
Figure 3 (page 8): Colours of series change compared to previous graph
Figure 13 (page 9): Without information of the aspect ratio, S is not enough to define changes in the independent variable – please include more details in your method.
Figure 15 (page 10) exhibits interesting transient behaviour, oscillation with a bi-frequency response and very different mean oscillation values – ALL of these should be highlighted and discussed.
Figure 5 (page 12)
288: “Delicate” should probably be replaced with “small”
289: This seems in contrast to Figure 20 – which shows a large difference in platform oscillation (see comments on the wrong type of graph)
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 2 Report
The paper regards a study of the hydrodynamic performance of a wave energy converter.
In the paper there is no any description of the methodology used for the study.
From the abstract it seems that the study is a numerical study, but the only element of the numerical methodology adopted is Figure 3 (where there is a computational mesh that is not described or explained).
The paper is just a sequence of graphs that should describe the results obtained by the authors, but the description of the results is not comprehensible. Essentially the paper is a disclosure of the results of such a study and does not give any scientific contribution.
All the definitions of the hydrodynamic quantities are missing (e.g. wave force, diffraction force, add mass, …). Furthermore, it is not clear what the authors means with the quantity “wave frequency”. Is it a number of waves per unit time? Why is it measured in rad/s?
Many sentences in the text are not clear and must be must be rewritten. For example:
On page 1, line 41, the sentence is not well written. A correction is needed.
On page 2, line 85, the sentence is not well written. It must be rewritten.
On page 3, line 110, the sentence “,that is,” is not clear and seems to be a typing error.
On page 3, line 112, the expression in brackets is not clear: which are the dimensions?
On page 4, line 128, the damping layer width W is indicated as a vertical dimension (called vertical wave direction), while the in figure 2 W is a horizontal dimension.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
Summary
Numerical simulations of a wave energy converter have been conducted while varying some geometrical parameters for a single sea state. The power output of the device is computed for each geometry in order to find the optimal one.
The text is not always idiomatic English, but overall the paper is clear and easy to read. Unfortunately, many details of the methodology are missing. The many numerical results are presented in a clear way, but the authors only describe the curves without much discussion nor physical interpretation. Thus, the paper gives no better understanding of the physics of the system. Finally, the formulation of the main points of the conclusion is slightly misleading.
Overall, despite an interesting subject, this is a very weak paper. At the very least, the presentation of the methodology needs to be improved before it can be considered for publication.
Detailed comments
Please provide the full geometry of the device (size of the floating layer and damping layer, size of the buoys, ...).
Which model are you using? How is it discretized? I'm guessing potential flow and boundary elements method, but this needs to be stated more explicitly.
With which software did you run the simulation? Did you check for mesh convergence of your results?
Would you consider releasing the meshes and the source code / parameter files as supplementary material in order to make this work more easily reproducible?
Only one curve of added mass (resp. radiation damping) is presented. Is the central platform assumed to move along only one degree of freedom?
Are you using the term "Wave force" to refer to the Froude-Krylov force or to the total excitation force (diffraction + Froude-Krylov)?
Section 3.1: Why are you focusing on this aspect ratio? It does not seem that the problem is invariant when W and L vary while B remains constant. You should then give the values of L and W (and H) that have been used in each computations.
Section 3.2: Which value of H has been used?
Section 3.3: Which values of L and W have been used?
It is not clear to me how relevant is the inclusion of the plots of the forces, added mass and radiation damping of the central platform. Do you draw any conclusion from them?
Please define the capture width ratio for all buoys (average of the three ratios?).
How did you chose the range of parameters that you are using? The best damping layer seems to be the largest on Fig. 13, so it is important to know why you chose this upper bound.
320 m² seems quite large to me. Could you comment on the economical constraints on the dimensions of the device?
In the conclusion, item (2) is misleading: finding the optimal values while varying one parameter at the time does not mean that you have found the best parameter in the full space of admissible parameters.
Item (3) is also misleading: implying that 5.6 m is the second best value is very misleading since there a wide range of better choices between 4.3 m and 5.1 m.
Typos and presentation
"add mass" -> "added mass"
l.13: "have" -> "has"
l.138: "is decreases" -> "is decreased"
l.153: "for better reflects" -> "to better reflect"
Please check the numbering of the figures.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I think the authors for addressing the specific comments, however, my broad comments were not addressed and these are more significant.
No extraliterature has been included in discussion or contextualisation of the results. There is still very little discussion of the results - this is a major flaw. I suggest the authors split the section into disctinct Results and Discussion sections and follow a dicsussion template. It may help to reduce the number of graphs presented to focus the aim of the paper - this can be done by consolidating results (fewer time series graphs) or omitting some.
English still needs editing.
Author Response
I have to admit that this paper just presented some results of the numerical simulations, and only a few discussion has been presented owing to lack of adequate research.
The purpose of this paper is trying to proved that the geometry of the central platform do have great effect on hydrodynamic performance of the WEC (It appears to be the first study of its kind, which may resulting a lack of reference in the discussion section ), and try to explain the physical reasons for these results. There are some results in the study that we haven't been able to figure out the physcial reason yet, and which is needed to do more research.
Hence, I'm afraid that to split the section into results and discussion is not very appropriate. Nevertheless, We will explain it appropriately in the conclusion
Reviewer 2 Report
The modifications of the text that I suggested have been accomplished.
Author Response
The modifications of the text that the reciewer suggested have been accomplished. Thanks for your advice!
Reviewer 3 Report
Thank you for this revised version. The improved description of the methodology makes the paper much clearer.
The paper is clear and the methodology is fine, but the novelty and impact of the paper are very low. It would have been appreciated if the authors had tested more cases (linear potential flow is not very computationally expensive) and had gone a bit deeper in the analysis of their results.
Nonetheless, I can agree on the publication of the paper, once the minor presentation issues below have been addressed.
Comments
During copy-editing, please improve the presentation of the math formulas (especially the exponents).
Please fill up the "Acknowledgments" section.
Please format the bibliography according to the publisher guidelines: https://www.mdpi.com/authors/references
Typos:
- divisions in Table 3?
- l. 163: "reflects" -> "reflect"
- l. 165: "waver"?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
