Multi-Point Shape Optimization of a Horizontal Axis Tidal Stream Turbine

Round 1

Reviewer 1 Report

• Abstract

The abstract is concise yet provides a good and useful overview of the paper.

• Introduction

The introduction is well written and introduces the reader to the topic. The literature review is adequate. I suggest adding a concluding paragraph to further discuss the strengths and limitations of existing methods at the end of the introduction to justify the need for the present study and motivate the reader.

• The body part of the paper

The methodology is sound and solid and is clearly explained. The paper uses standard analysis tools.

• Conclusion

The results are well described and are of practical value. Discussions and conclusions are useful and solid.

• Reference

References are relevant.

General comments:

 The paper deals with the optimization of a tidal stream turbine. The NSGA-II genetic algorithm has been used. The topic is well within the scope of the journal. The paper is well written and well organized. My only suggestion for the authors is to clarify the scientific value of the paper and elaborate on the novelty of the paper other than the study has not been done before using this algorithm. Explain further what makes the present study distinguished competed with the available studies in the literature. As it stands now, it is more like a routine technical report with little novelty. Also, discuss further the choice of NSGA-II.

Author Response

General comments:

 The paper deals with the optimization of a tidal stream turbine. The NSGA-II genetic algorithm has been used. The topic is well within the scope of the journal. The paper is well written and well organized. My only suggestion for the authors is to clarify the scientific value of the paper and elaborate on the novelty of the paper other than the study has not been done before using this algorithm. Explain further what makes the present study distinguished competed with the available studies in the literature. As it stands now, it is more like a routine technical report with little novelty. Also, discuss further the choice of NSGA-II.

 

Answer: We would like to thank the reviewer for his constructive comments. They have been very helpful in the revision of this paper and allowed us to improve the technical contents and presentation quality. We have taken into consideration and complied with all of the comments and suggestions. We also added some of your suggestions in the paper.

 

1. We used a heuristic algorithm that has the ability to converge at global optima instead of a gradient-based method that has the limitation of getting stuck in local minima.
2. In many forms of heuristic algorithms, all the good solutions of a generation are selected for crossover, which might at times lead to the loss of the quality of the solutions. In the NSGA-II algorithm, the best solutions of a generation are not selected for crossover and they automatically go to the next generation. Selection, crossover and mutation are done for the remaining solutions.
3. One of the salient features of the NSGA-II algorithm is that if a situation arises wherein the optimization algorithm can select only one of the two good solutions for the next generation, it selects on the basis of its distance from its neighbours on the Pareto-optimal graph. The solution with the greatest distance from its neighbours is selected. This ensures that the optimized hydrofoils are diverse.
4. A lot of researchers used CFD as a flow-solver which is time-intensive. Here, we coupled our optimization code with the 2D panel code "Xfoil", which is computationally less time-intensive and accurate.

Reviewer 2 Report

This paper presents extensive results of the tidal turbine blade profile shape optimization process performed by a well-known method.
The authors have sensibly chosen the method and presented a justification for this.
Although the authors announced narrowing the search area with genetic algorithms by introducing stricter constraints, it is difficult to find a more precise description of them in the paper. 
Modifications of the profile came down to changes in the position of 10 control points of Bezier curves describing its geometry.
Analyzing the paper some questions arise due to not very precise presentation of some data. 
Fig. 5. Lack of legend. It is not clear which markers represent which method. One can only guess at it.
The scale of the drag coefficient does not allow for quantitative comparison of the differences in results.
The data presented in Fig. 5 indicate that the L/D values obtained for the panel method may significantly differ from the experimental data due to the observed smaller hydrodynamic resistance values that occur in the denominator of this parameter.
A plot of L/D value would reveal the large differences obtained by the panel method and CFD.
The authors, after the 3-D optimization phase (CL, Cd and Cpmin), move to 2-D optimization in which they use the glide ratio (GR) being the L/D equivalent.      
The information shown in Figure 5 demonstrates better agreement between the CFD results and experimental data for larger angles of attack.
The authors rather arbitrarily considered the fast panel method to be sufficiently accurate as a source of hydrodynamic data over the entire range of angles of attack studied. This choice appears to be not fully justified.

Specific comments.

line 27 drag error
Fig. 9 and 11 black not blue line
Fig. 12 The different ranges of GR values in the figures make them difficult to compare and analyze.
The descriptions of Figures 17 and 18 do not agree with their contents.

Optimization area 
Although the authors inform about the narrowing of the search area as one of their achievements, the boundaries are not further defined.
The paper contains a lot of comparative material, but because of the choice of panel method as a data source for optimization the results should be considered with some caution.

It seems that the paper contains a lot of interesting information and after some corrections it could be published. 

Author Response

Answer: We would like to thank the reviewer for his constructive comments. They have been very helpful in the revision of this paper and allowed us to improve the technical contents and presentation quality. We have taken into consideration and complied with all of the comments and suggestions.

 

Q1- Fig. 5. Lack of legend. It is not clear which markers represent which method. One can only guess at it.

       Answer :  We added the legend.

 

Q2- 

The authors rather arbitrarily considered the fast panel method to be sufficiently accurate as a source of hydrodynamic data over the entire range of angles of attack studied. This choice appears to be not fully justified.

 Answer:  A lot of researchers used CFD as a flow-solver which is time-intensive.  Here, we coupled our optimization code with the 2D panel code "Xfoil", which is computationally less time-intensive and accurate. The maximum difference between experimental and the XFOIL method is less than 5%, which is acceptable.

Q3- line 27 drag error.

Answer: We modified it.

Q4- Fig. 9 and 11 black not blue line

Answer: We modified it.

Q5- Fig. 12 The different ranges of GR values in the figures make them difficult to compare and analyze.

Answer: We modified the same ranges GR value.

Q6- The descriptions of Figures 17 and 18 do not agree with their contents.

Answer: We do not know which paper you mean. We reread the paper and found it agree with the figures. Could you please specify which lines your mean.

 

Round 2

Reviewer 1 Report

The authors have addressed my comments. I recommend publication of the paper.