Review Reports

Round 1

Reviewer 1 Report

The paper "Wind tunnel study of low-frequency instability in near wake of vertical-axis wind turbine" of Zou et al.
proposes the results of wind tunnel experiments, executed to study vetical axis wind turbine, focusing on the the
low-frequency instabilities, already discussed in literature. As the authors said in the introduction, the physical
mechanisms of such low-frequencies instability are controversial.
Some of the results, such as the efficiency effifiency (sec 3.1) are not clearly related with the wake frequencies,
The wake effects are reported(L188-198), but are not sufficiently motivated in this section.
The wake are studied in the subsequent sections: first the velocity and turbulence profile are reported, and
subsequently the frequencies are analyzed with a wavelet spectral analysis.
Results are abundant, but they don't lead to a clear main conclusion. I suggest to select part of your results to
motivate conclusions.
The results of the spectral analysis is very interesting, but, even if the result are good, they should be more
discussed, focusing on the differencies with respect to other results in literature.
The author focused clearly nly on the efficiency of the wavelet method, but if this is a methodological paper, this
should be written clearly.
Finally, I suggest a major revision of the paper, before publication.

Other minor points:

L34-36: are you talking about a grid of turbines? Why do you talk about upstream and downteram turbine?

L48-72: In the introduction, a number of papers on HAWT are cited, while your work deal with VAWT. You should give
more space on the related literature. If it is not abundant (even if you cite a lot of papers in section 3.2), focus
on what was already knows and what is the novelty of your paper.

L161-164: the location of these points is not clear: report the label of the points in the figure, hence you can
remove their coordinates.

L180: specify the definition of the swept area for the VAWT.

L183-187: You should justify with related data the sentence "When the chord length increases, the chord Reynolds
number increases. However, when the chord Reynolds number exceeds a certain value, the influence of the blade on the
lift coefficient becomes weaker, and the interaction effects between the blade vortex shedding are enhanced.
Therefore, when the chord length of the blade continues to increase, the improvement of the power coefficient becomes
limited".

L188-190: You should justify with related data the sentence "When the number of blades increases, the influence of
the wake of the previous blade on the aerodynamic acquisition of the next blade increases".

L 192-194: You should justify with related data the sentence "With increasing number of blades, the fluctuation
period of the driving torque acting on the entire rotor decreases, which results in the acceleration of the
attenuation of the driving torque"

L196-197: You should justify with related data the sentence "When the solidity of the wind turbine is constant, the
blocking effect of the strut increases with increasing number of blades"

L201: What you mean with the sentence "The operating conditions of the wind turbine are different under different tip
speed ratios."?

L202: Explain why the rotation speed is fixed for the wake study.

L215-223: refers to figures when you discuss the results.

L216,L237: define the shear layer accurately.

L245: Report how the turbulence intensity was evaluated (in methods)

L260-261: you mean that the result are not in agreement or that one of them is too poor to reveal turbulent
structures.

L263-278:I think this long justification of the use of the spectrum analysis is not necessary. I suggest to
sinthetize it.

In figures with several images, in the caption the description of each panel should be included. Figures 6 to 10 are
somentimes badly referred. In addition, results are too spreaded in the paper and is difficult t verify the
discussion. please manage better the results and the figures

L324-327: This is the introduction of the subsequent section???

L326: How you identify the different operation states?

L330: why configuration D and E are equivalent?

L346: define the yaw angle

Fig 11: this figure (and also figure 4) is difficult to read. Can you manage the representation? This poor
representation hinder the comprehension of lines 346-356

L362-365: since you missed a clear identification of the operation states, this part is not clear.

L382-387: this seems part of the conclusion.

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Dear Authors,

congratulations for a well-written paper. It could be beneficial for the paper to scope down the presented information, so you can focus more on the low-frequency instability. I have added some suggestions in the review report that, in my view, would improve the manuscript.

Best regards

 

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I thank the authors for following my suggestions on their paper.

I think that now the paper is more clear and focused on the object, then it is good for publication on the journal JMSE.

Author Response

Thank you very much for your suggestions. See the attachment for the final version of the paper.

Author Response File: Author Response.pdf