Experimental Investigation of Surface Waves Effect on a Ducted Twin Vertical Axis Tidal Turbine
Round 1
Reviewer 1 Report
This paper investigated the effect of surface waves on the performance and loads of the twin vertical axis tidal turbines by 1/20 scale model test. Seven wave conditions (different wave amplitude and frequency), regular and irregular waves are presented. The results show little or no effect of the waves on the average power and loads compared to the condition with current only, but the running of turbine becomes unstable under wave effect. The research provides guidance for design and optimization of the vertical tidal current turbine. The writing is well, and the description of experiments and data analysis method is in detail.
I have only one question about the study: “At the same surface wave condition, what is the effect of the depth above the vertical turbine on load and power fluctuation？“
I agree with accepting this paper in present form.
Author Response
Our answers to the reviewers comments (in black) are written in green.
Reviewer 1
Comments and Suggestions for Authors
This paper investigated the effect of surface waves on the performance and loads of the twin vertical axis tidal turbines by 1/20 scale model test. Seven wave conditions (different wave amplitude and frequency), regular and irregular waves are presented. The results show little or no effect of the waves on the average power and loads compared to the condition with current only, but the running of turbine becomes unstable under wave effect. The research provides guidance for design and optimization of the vertical tidal current turbine. The writing is well, and the description of experiments and data analysis method is in detail.
I have only one question about the study: “At the same surface wave condition, what is the effect of the depth above the vertical turbine on load and power fluctuation？“
Due to the experimental set-up, we could not carry out experiments at variable water depth to answer that question. That being, the wave theory predicts an exponential decay with the water depth of the amplitude of the velocity fluctuation caused by surface waves. Therefore, at a given distance from the bottom (say the turbine mid-height), we can expect bigger velocity fluctuation when the water level is low than when it is high, and so bigger load and power fluctuation, and vice versa.
I agree with accepting this paper in present form.
Reviewer 2 Report
Dear Authors,
the points of criticism reported in the following need elaboration to rise the quality of your paper to the Journal level.
1) State of the art is very concise and does not provide a detailed background on the technologies and the latest updates in the field of study. The lacks existing in the literature should be clearly stated to highlight the novelty of the proposed research. At present, the only statement is that twin vertical axis tidal turbines (2-VATT) were not studied yet.
2) The presentation of the experimental model and the piping and instrumentation should be improved as the replication of the study is currently impossible to obtain. The details allowing for the complete reconstruction of the experimental facility need to be defined using tables, images and text.
3) The results obtained require better explanation and physical motivation. Currently, the discussion is limited to a description of the data illustrated and lacks in detailing the motivations undergoing the obtained results.
4) A detailed discussion on the motivations for the differences of the outcomes of the Figures 3, 4 and 7 needs to be included in the text. The absence of an elaborated discussion signicantly hinders the level of the research.
5) The choice of the specific operating conditions of the turbine should be described for each configuration analysed. In addition, the reasons for the specific JONSWAP spectrum and extra peak enhancement factor chosen should be provided.
6) The key performance indicators of turbine should be discussed. It would be beneficial providing at least information regarding the possible electric power output, energy conversion and efficiency of operation for an average year and for the four seasons.
7) The Conclusions Section needs to be completely restructured. The statements should be supported by numerical data presented in the previous parts of the paper. Moreover, the achievements of the research presented need to be clearly stated. The inclusion of figures in this Section is not recommended.
8) The quality of English has to be improved to that of a scientific publication. The errors present in the text need to be corrected.
The quality of English has to be improved to that of a scientific publication. The errors present in the text need to be corrected.
Author Response
Our answers to the reviewers comments (in black) are written in green. The text added in the new paper version is highlighted in yellow.
Reviewer 2
Dear Authors, the points of criticism reported in the following need elaboration to rise the quality of your paper to the Journal level.
1) State of the art is very concise and does not provide a detailed background on the technologies and the latest updates in the field of study.
This sounds quite subjective as we do believe that presenting about 20 articles, mostly less than 3 years old, is already quite extensive and up to date… Regarding the technological review, we add a reference to an IEA OES note on tidal energy from this year that presents the main tidal energy projects and technologies currently developed in the world. However, providing a detailed technological review is out of the scope of the present paper.
The lacks existing in the literature should be clearly stated to highlight the novelty of the proposed research. At present, the only statement is that twin vertical axis tidal turbines (2-VATT) were not studied yet.
We found only three papers studying vertical axis turbines in presence of surface waves (2 experimental ones and 1 numerical) and none on 2-VATT, indeed. In addition, all of those focus on the power performance while disregarding the loads on the structure, and only in a few regular wave conditions. Therefore, the literature on VAT lacks studies in a wider range of wave conditions and the analysis of the loads on the structure, which is needed for the mechanical design. The lack of studies on 2-VATT is quite obvious and is sufficient to support the need of the present study, from our point of view.
Still, we add the following in the revised paper to highlight the lacks in the literature:
“However, the study only considers regular waves with a constant height and focuses on the power coefficient on a limited tip speed ratio range, disregarding the low tip speed ratios.”
…
“However, the numerical model with waves lacks a proper comparison to some experimental data for validation before studying the wave effects.”
...
“However, that study only focused on regular waves and on the power coefficient again, disregarding the load coefficients on the structure.”
2) The presentation of the experimental model and the piping and instrumentation should be improved as the replication of the study is currently impossible to obtain. The details allowing for the complete reconstruction of the experimental facility need to be defined using tables, images and text.
For the sake of brevity, as the model was already described quite extensively in [23], we chose not to fully describe it again in the present paper and rather refer to [23]. In addition, given the model complexity, only the CAD drawings could allow the complete reconstruction of the experiments. Those can be accessed upon request and were already communicated to others. For example, numerical and experimental results on this turbine model are compared by Grondeau and Guillou (2023). High-fidelity modelling of a vertical axis tidal turbine model under realistic flow conditions. In Proceedings of the 15th EWTEC.
For precision, we add in the paper “as well as the complete drawings of the model” in the data availability statement.
3) The results obtained require better explanation and physical motivation. Currently, the discussion is limited to a description of the data illustrated and lacks in detailing the motivations undergoing the obtained results.
This point and the following are related. We answer them both below.
4) A detailed discussion on the motivations for the differences of the outcomes of the Figures 3, 4 and 7 needs to be included in the text. The absence of an elaborated discussion significantly hinders the level of the research.
References to those Figures 3 and 4 are added in the analysis of the Figure 7 to better link the turbine behaviour results to the flow characteristics:
l.302: “The limited effect of the surface waves on the average power and loads is consistent with the homogeneity of the average velocity profiles over the wave cases (Figure 3 (a)). The power performance decrease at the low tip speed ratios may reveal an increase of the dynamic stall caused by the wave-induced velocity fluctuation (Figure 4).
Besides, those wave-induced orbital velocities strongly affect the standard deviation of the power and loads coefficients”
l.353: “Indeed, the wave-induced velocity fluctuation in those wave cases is less than 6 % of the average velocity at the turbine mid-height (Figure 4)”.
l.364: “The waves direction is collinear with the current direction, which causes additional velocity fluctuation in the streamwise and the vertical direction (Figure 4). Therefore, […]. On the other hand, the waves collinear with the current barely modify the transverse velocity $v$, so […].”
5) The choice of the specific operating conditions of the turbine should be described for each configuration analysed. In addition, the reasons for the specific JONSWAP spectrum and extra peak enhancement factor chosen should be provided.
The choice of the specific wave conditions is clarified:
l.154: “The accessible wave frequency range narrows when the wave amplitude increases, and vice versa, due to the limits of the wave makers capabilities \citep{Magnier2023}. Therefore, to study the influence of the wave amplitude, we chose the frequency $f_\eta$ = 0.4 Hz that allows the largest amplitude range among the feasible wave conditions in the tank, and three arbitrary amplitude levels noted $A1$ (low), $A2$ (medium) and $A3$ (high). Similarly, to study the impact of the wave frequency, we chose the low amplitude level $A1$ to be able to cover a frequency range from 0.2 to 0.5 Hz. Finally, to compare the effect of regular and irregular waves on the turbine, we chose an irregular wave case with a similar peak period ($T_p = 1/f_\eta$) and significant wave amplitude ($A_{\eta}$, the average amplitude of the highest third of the waves) to the intermediate regular wave case $f04A2$. The irregular wave case is generated according to the standard JONSWAP spectrum with a peak enhancement factor $of 3.3 \citep{Hasselmann1973}.”
The JONSWAP spectrum and its peak enhancement factor were chosen as they are the standard in ocean engineering.
6) The key performance indicators of turbine should be discussed. It would be beneficial providing at least information regarding the possible electric power output, energy conversion and efficiency of operation for an average year and for the four seasons.
The performance indicators (hydrodynamic coefficients) are very classical and widely used in the literature. The power coefficient provides information regarding the energy conversion and efficiency. The absolute values cannot be communicated due to confidentiality restrictions by the company, which does not prevent studying the relative effect of the waves on the performance of the machine compared to the case without waves. The results extrapolation to some average power production at full-scale at sea is out of the scope of the present study regarding the surface waves effect on a reduced-scale model.
7) The Conclusions Section needs to be completely restructured. The statements should be supported by numerical data presented in the previous parts of the paper. Moreover, the achievements of the research presented need to be clearly stated. The inclusion of figures in this Section is not recommended.
We restructured the conclusion to better highlight the main conclusive statements and to make it shorter, as requested by the 3rd reviewer.
We agree with the last comment and no figures are presented in this section.
8) The quality of English has to be improved to that of a scientific publication. The errors present in the text need to be corrected.
The paper was proofread to improve the English and several sentences were rephrased to improve the readability.
Reviewer 3 Report
(1)The concluison should be shorten. It is too long now.
(2) The nomenclature should be given.
(3) Please explain the reason of the increase of drag fluctation when the wave was added.
The English is OK
Author Response
Our answers to the reviewers comments (in black) are written in green. The text added in the new paper version is highlighted in yellow.
Reviewer 3
(1) The conclusion should be shorten. It is too long now.
The conclusion is largely restructured and shorten.
(2) The nomenclature should be given.
The other reviewers did not require that and we made the effort to regularly recall the meaning of the symbols in the text, precisely not to have to add a nomenclature. We still believe that adding a nomenclature at the beginning of the paper would decrease the reading engagement.
(3) Please explain the reason of the increase of drag fluctuation when the wave was added.
The waves generate velocity fluctuation (the orbital velocities) in the whole water column. The Figure 4 displays the amplitude of theses velocity fluctuation in the streamwise (a) and the vertical (b) direction. Since the drag is proportional to the square of the velocity, the wave-induced velocity fluctuation causes drag fluctuation in a straightforward way.
Comments on the Quality of English Language
The English is OK
We also add some precision in the captions of the figures 3 and 4 for the copyright, as required by the editor.
