A Study of Raindrop Impacts on a Wind Turbine Material: Velocity and Impact Angle Effects on Erosion MAPS at Various Exposure Times

Round 1

Reviewer 1 Report

This contribution deals with investigation on erosion of wind turbine blade material (G10 epoxy Glass Fibre) and determination of erosion maps. Despite generally being well prepared, it suffers from some weaknesses, which should be revised prior to publication. More detailed comments are listed below.  

1. It is suggested to change the title of the contribution, since it does not directly deal with erosion of wind turbine blades, but, however, with laboratory erosion testing of material which is used in such application.
2. It is suggested to include the investigated material into the abstract.
3. It is suggested to include an explanation why the properties provided in table 2 are taken from reference no. 14.
4. Referring to the erosion tests, how many specimens were used for each configuration of impact angle etc. (are the mass loss values averaged)? In case they are derived from more than just one specimen (which would be desirable), standard deviation should be provided.
5. Since the results section contains very few words and most description of results is following in the discussion it is suggested to merge the two sections and rename to “results and discussion”.
6. The spacing between value and unit needs to be checked and used consistently throughout the entire manuscript.
7. Please also refer to remark no. 1: which conclusion can be drawn regarding application of the investigated material in wind turbine blades?

Author Response

Thank you very much for spending your time on this document. Your comments were very constructive and I have tried to fully implement them. I believe the final document has been greatly improved by encompassing your comments.

Author Response File: Author Response.docx

Reviewer 2 Report

This study presents some interesting results and techniques to study wind turbine blade erosion. Whilst the results show promise and would be of interest to readers in this field, there is insufficient data provided that significantly advances understanding in this field. For example, there are actually more referenced figures from other people’s work included in this study than figures produced by the authors themselves, therefore I believe there is insufficient findings for publication and recommend major corrections before any resubmission.

Further to this, the following points should be addressed:

Abstract

Abstract could be improved – some statements sound vague e.g. ‘transition between wear states’ and ‘new insights into erosion were found’.

Introduction

Figure 1 and Figure 2 - potential copyright issues with figure directly copied from another source

Methodology

Line 103 – what is meant by issues cross referencing results? This perhaps should be discussed in the introduction rather than here

What justification is there for the statement that the WARER rig is the most appropriate for this study? Have different methodologies been compared?

Can a clearer image or diagram of the sample holder be provided? It is difficult to identify in the current figure

Was any characterisation of droplet size completed? Or calculated from needle size? Is it possible that any aerodynamic forces generated by the motion of the arm could influence this or influence impingement angle of the droplet?

Section 2.3 and Table 1 are probably unnecessary and could be described within the text discussing the test length on line 122.

How were the samples dried? E.g. in an oven? Left in ambient conditions? Under vacuum?

Were any controls completed to be sure that the drying period was sufficient? E.g. if the samples were left immersed in water for 24 hours (i.e. without erosion) and then dried for 24 hours, would any change in mass be observed?

It would be typical to show materials first in the methodology section – 2.4 should be moved to 2.1.

The flow velocity is lower than potential maximum (300 mph) stated in the introduction. Does this test method accurately represent the application?

Results

More detail is required in this section to explain what the figures show. Currently there is no mention of either Fig 6 or 7 in the text.

The methodology should be updated to include detail of SEM analysis.

SEM figures show good detail, however as they are at different magnifications it is difficult compare between all

Were repeat measurements completed for Fig 7?

Why is there such variation between the different flow velocities and not angles? Even though the differences are not large between mass loss results they are relatively significant. There appears to be no real trend, but as there are no repeats shown it is difficult to make this judgement.

There are also measurements which do not make sense, e.g. in Fig 7a at 30 deg, there is no increase in mass loss between 216 and 432 km (possibly even a decrease) but dramatic increases at the other measurements for this angle.  

Discussion

Line 243 – why would impingement angle influence whether the material is brittle or ductile? Does this mean deformation of the material at those impingement angles is similar to that found in brittle materials? Or is this the fundamental nature of this composite material? This could perhaps be reworded to improve clarity.

Figure 9 is fairly common knowledge in erosion – it does not add anything to the discussion in this study

Similarly Fig 8 and Fig 10 are just copies/modification from other studies – this doesn’t really add anything significant to the paper that isn’t discussed in the text.

Conclusions such as ‘flow velocity influences max impingement angle’ cannot really be made for this study. There is clearly some variability in results, typical of erosion testing, but repeat data is not shown to backup these statements.

Author Response

Thank you very much for spending your time on this document. Your comments were very constructive and I have tried to fully implement them. I believe the final document has been greatly improved by encompassing your comments.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper studied the erosion mechanism of GFRP by considering the effect of impac angle and velocity of rain. It was important for protecting the wind turbine blades. Actually, the blades will collide with raindrop at any angles when it rotates. So, the maxium destroy force was very important. The erosion rate should be accelerated if the surface of blade was destroyed. So, it shouldn’t increase linearly with distance in Fig.7. Besides, the raindrop just destroyed the epoxy matrix observed from Figure 6. Does it produce fracture on glass fibers? Or do you know the maximun carrying capacity of such materials? It must suffer more sever conditions, such as low temperature or hail weather before actual application of such materials. I hope you should consider other factors.

Author Response

Thank you very much for spending your time on this document. Your comments were very constructive and I have tried to fully implement them. I believe the final document has been greatly improved by encompassing your comments.

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Thank you for addressing my comments, however I do not agree that the results can be trusted as accurate without any repeats, due to the variation and lack of correlation that has been observed between different impact velocities and impact angles. Therefore it is recommend that until further repeats are completed this manuscript cannot be accepted for publication.

In addition to this, in response to point 14 - it is common to state which analytical techniques were used in the method. SEM analysis was completed in this study but has not been mentioned in the method, therefore this needs updating.

Author Response

Thank you again for your comments, we have ran a set of repeat experiments and formatted the results accordingly. I believe this has improved the research and this paper.

I have also addressed the response to point 14 and incorporated the SEM analysis into the methods.

Thank you again for your comments which I believe have significantly improved this paper.

Round 3

Reviewer 2 Report

Thank you for updating the paper. However, there are no error bars on the plots, other than just a comment saying 'Figures 7a) b) and c) have
been redone to incorporate repeat experiments' 

How does a reader know these were repeated? Or how as a reviewer am I expected to judge the quality of these results without knowing what all of the results are and the error in the results? 

Comparing to the previous reviewed version there are now significant differences between the mass loss values reported in the new plots. This was my original concern and gives me doubts about the accuracy of the results.

It should be very clear how many experiments have been completed, what the points on the plot show and the error present. Further repeats may be necessary if the error is significant. 

It is logical that error bars are quite large in erosion experiments, as experimental control is challenging. However, this should be reported regardless.