A Study on the Air Cavity under a Stepped Planing Hull

Round 1

Reviewer 1 Report

The paper is good but needs improvements. See the attacched file 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The authors replied to my requests in a sufficient way

Author Response

Thank you for your comments concerning our manuscript entitled ‘A study on the air cavity under a stepped planing hull’.

Reviewer 3 Report

Some small changes have to be performed prior to publication:

The paper is not provided with a list of symbols. Therefore, all the symbols used in the text should be defined. Here something is still missing, as |E| in table 3. Volumetric Froude number should be indicated in Figures 10 and 11, in the present version the nomenclature is the one of linear Froud number. Captions in Figure 13 should be modified, reporting only the volumetric Froude number. Volumetric Froude number nomenclature should be used through the whole text (e.g in lines 231, 279 Fr appears)

Author Response

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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

See file in attacched

Comments for author File: Comments.pdf

Author Response

Please see the attachmen

Author Response File: Author Response.pdf

Reviewer 2 Report

The article is interesting, well organised and consistent with the topics of the journal, however I have some little consideration and curiosity.

The experimental part is very poor and it lacks of several information as the towing tank facilities, the instruments, the model size, the accuracy of the measures etc…

It seems that in both numerical and experimental tests the trim was free to change. Consequently a part of the error in drag could depend on the difference in trim and sinkage (that are always more difficult to predict).  Maybe a solution could be to carry out some simulation with fixed trim and sinkage as coming from the experiments. This would lead certainly to a more accurate shape of the air cavity.

Do you have some picture of the shape of the cavity from the bottom in the tank?

The bibliography is a little poor and with a lack of the last studies. In the range of the numerical results references I suggest to read also:

A critical CAE analysis of the bottom shape of a multi stepped air cavity planing hull. DOI:10.1016/j.apor.2018.11.003. pp.130-142. In APPLIED OCEAN RESEARCH - ISSN:0141-1187 vol. 82
Cucinotta, Filippo; Guglielmino, Eugenio; Sfravara, Felice Numerical and experimental investigation of a planing Air Cavity Ship and its air layer evolution. DOI:10.1016/j.oceaneng.2018.01.071. pp.130-144. In OCEAN ENGINEERING - ISSN:0029-8018 vol. 152
Cucinotta, Filippo; Guglielmino, Eugenio; Sfravara, Felice; Strasser, Clemens Cucinotta et al. An experimental comparison between different artificial air cavity designs for a planing hull. Ocean Engineering Crossref DOI link: https://doi.org/10.1016/J.OCEANENG.2017.05.028

Barbaca, L., Pearce, B.W., Brandner, P.A., 2017. Numerical analysis of ventilated cavity flow over a 2-D wall mounted fence. Ocean Eng. 141, 143–153. https://doi.org/ 10.1016/j.oceaneng.2017.06.018.

In the formulas and graphs it is better to use the Froude number instead of the velocity.  

Author Response

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Reviewer 3 Report

Hereafter some comments/suggestions to improve the paper content:

(lines 46-47) Please clarify the statement 'few attention has been paid on the hull bottom flow details' since reference [16] is covering properly this kind of analysis. Other references in the literature highlight it, as e.g. A. Dashtimanesh, E. Esfandiani & S. Mancini. Performance prediction of two-stepped planning hulls using morphing mesh approach, Journal of Ship Production and Design 34(3), pp. 1-13. (lines 51-57) Please give more information with respect to the physical setup used by the CFD solver, e.g. is a segregated approach used? In case it is which interpolation scheme is used for the pressure-velocity coupling, etc... (lines 51-57) k-ω turbulence model is used in two-layers formulation? please specify. (line 58) Please specify the hull series number of the hull used for the study (is it hull C1 of the Taunton series?)    (line 69) Please clearly specify that only 2 degrees of freedom are considered in the simulation and not all the six. (Section 2.3) The choice of the domain dimensions should be justified at least giving some reference for the claimed 'conventional cases'. Moreover, ITTC recommendations suggest using the towing tank dimensions (for depth and breadth of the calculation domain) in case validation study is carried out. Please explain why other domain dimensions have been considered.  (Section 2.4) This section should be reworked. More details have to be given with respect to the adopted mesh (number of prism layers, location of the volumetric refinements, etc..), moreover, the choice to use tetrahedral cells in the inner domain and trimmed cells in the external one should be justified.  (Table 2) Please specify which region of the hull the y+ values refers to and why such high y+ values have been reached.  Separated sections should be dedicated to mesh sensitivity analysis and to validation study, describing all the quantities that are then reported in the tables. Please specify which mesh has been selected for the validation study. It seems that the authors considered that the total uncertainty of the process (U_V) is given by sqrt(U_G²+U_D²). Please explain why for the numerical simulation the uncertainty is given by the grid only. (Section 3.1) It is suggested to support the considerations upon the wave-making with figures highlighting the wave propagation at different speeds. (Figure 5) It is suggested to use in the text the same nomenclature A-A and B-B to identify the sections. (Figure 6) Please insert a legend with the colour-scale of the displayed quantities.     (Section 3.2) This section should be reformulated making more references to the explicative figures. In its current form, it is really hard to read and fully understand. (Line 219) Please explain the meaning of 'triangle planing stage'. (lines 226-228) The relative position between the centre of pressure and the centre of gravity is not reported. Moreover, in a calm water simulation/test the relative position between the centre of pressure and centre of gravity is influencing the dynamic trim of the hull. Dynamic instability detection requires the execution of test/simulations in waves; please comment on this. (Conclusions) Point 2 and 3 of the conclusions should be better supported by the data presented in the text. (line 244) please specify which is the speed for the presented case. (References) Please note that [4] and [13] refer to the same publication.

Author Response

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Reviewer 4 Report

The paper presents a numerical investigation of the air cavity created under a stepped planning hull. CFD simulations are conducted using the commercial CFD software ANSYS CFX. In the specific study not only the aero/hydrodynamic part is taken into account but also the dynamics of the hull are considered. In that sense (and taking into account the complexity of the problem itself), the problem considered is both interesting and challenging. The paper is well organized and the use of language is good. However, there are some points that should be made clear prior to recommending this paper for publication.

 

1)The authors state that they employ a 6-DOF dynamic model. It is not clear how this is utilized together with the symmetry condition. Either they remove some dof’s to ensure symmetry(for example the roll motion) or it is ensured by some other means? This should be made clear in the manuscript.

2)The grid convergence study conducted is very interesting and the analysis of convergence of the various characteristics is thorough. However, I was surprised to see Y+ values of order of 100. In Section 2 of the manuscript they state “To precisely capture the adverse pressure gradients caused by the flow separate at hull bottom, the SST (Shear Stress Transport) turbulence model is introduced to enclose the aforementioned equations”. However, it is argued that wall functions are not an option when separated flows are considered. A simulation with a smaller y+ (<=10) would be interesting, otherwise, a comment must be made , on how using wall function in separated flows can affect the results. Indeed the comparison with the experimental data is fair, however, the use of wall functions might affect the dynamic characteristics.

 

3)Finally regarding the numerical results section their analysis is good and it covers a lot of aspects of such a complicated problem. However, an additional paragraph or table is needed to report the time-step employed, the simulation time and maybe some convergence indications. In the previous section they identified that they do not achieve convergence of the dynamic characteristics. It is important to add a figure presenting some convergence characteristic (Maybe some dynamic response over time for). In the reviewers opinion , this should complement their very interesting analysis.

 

Overall, it is a well organized paper that addresses a very complicated problem. The numerical results and the conclusions drawn are quite interesting. However, the above issues must be addressed prior to publication.

Author Response

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Round 2

Reviewer 2 Report

The Authors addressed all my requests and consequently the paper is ready, in my opinion, for publication.

Author Response

Thank you for your comments. The revised manuscript is ready for publication.

Reviewer 3 Report

The authors partially addressed reviewer comments:

Please add the notation C1 stepped hull form through all the text, including figures and tables (e.g. Fig. 2 and Tab. 1). (Referred to comment 5 of the first review) the fact that domain dimensions have been enlarged to avoid wave reflection has sense, for validation purposes, only if the tank data are corrected for wall effect. Did you check if this is the case? (Referred to comment 6 of the first review) not all the quantities reported in table 3 are defined in the text. Readers familiar with validation studies and procedures will have no problem to identify the meaning of the symbols, but other readers may have some difficulties. Means, U_D, U_V and |E| should be defined in the text.

Hereafter some new minor comments:

1. In the revised document, the authors introduced a Froude number. It should be specified that this is the volumetric Froude number and identify it as Fn

_∇

  2. Please remove lines 129,130 and 131. They are part of the reply to the reviewer and not of the final text. Moreover, a reference to the new figure 5 is missing in the text.

Author Response

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Reviewer 4 Report

The authors have answered my questions and thus I propose the paper for publication

Author Response

Thank you for your comments. The revised manuscript is ready for publication.