Flow around a Rectangular Cylinder Placed in a Channel with a High Blockage Ratio under a Subcritical Reynolds Number

Round 1

Reviewer 1 Report

Many thanks to the authors for taking the time to address previous comments. I will focus my second review solely on section 2 at this stage, as believe there remain significant issues, which means the narrative and results cannot be reconsidered until these are addressed.

1. Figure 2 (a) shows the 'PIV measurement section' (typically named 'field of view') as being 200 x 140 mm. Later, the camera resolution is given as 2560 x 2048 pixels. These two measurements are inconsistent: the first image has a ratio of 1.43 (200/140), while the second has a ratio of 1.25 (2560/2048).
2.  The paper states that 'the processed window size is 140*200 mm'. This is not correct. This measurement refers to the field of view. The window size should be related to the processing of the data and be expressed in px x px. For example, for a multipass of decreasing size, I would expect to be told that X-many passes have been done on a AA x AA pixels wind with BB% overlap, with Y-many passes on a CC x CC pixels windows with DD% overlap.
3. The uncertainty is given as 1%, but no justification or evidence is provided. The uncertainty for PIV should be made of both a bias and an error. The bias may be know, in which case a reference to the bias for this particular facility should be provided. The error however must be quantified for your experiments. 
4. A spacial resolution of '0.07mm/pixel' is given in the paper. This is in fact the pixel size, which appears inconsistent with the previous field of view and camera resolution (See point 1), where pixels sizes would be respectively 0.078 and 0.068 mm/px. Spacial resolution refers to the distance between two adjacent vectors once data has been post-processed. It should be expressed as both a distance and function of the Diameter D.
5. The particle diameter of 0.2mm sound very high, can you confirm this is correct?
6. Could you clarify (though not needed in the paper) the result matrix size please?
7. Could you also specify the sampling rate (in Hz), the Dt (time between the two frames taken to constitute a PIV image), and the free flow speed(s). This will help ensure adequate post-processing has been achieved.
8. How many pairs of images were acquired?

Many thanks

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

I have no further comments.

Author Response

Dear reviewer:

Thank you for your precious comments and advice.

Reviewer 3 Report

In the paper entitled 'Flow Around a Rectangular Cylinder Placed in a Channel with High Blockage under Subcritical Reynolds Number', the current and interesting concept of generating electricity in the VIVACE device was developed. The work contains very rich experimental material, which is the result of non-invasive research on the flow around the rectangular cylinder using the PIV method and pressure sensors. The variable parameters during the experiments were the geometry of the flowing rectangular cylinder, the water velocity as well as the blockage ratio. The authors presented advanced methods of analysis of the obtained experimental results and a discussion of the obtained results against the background of the achievements of other researchers. In the summary and conclusions, however, there was no reference to the purpose of the work, i.e. to determine to what extent the reduction of the speed around the obstacle will affect energy generation in the VIVACE device. It is also very unclear which of the examined variable parameters has the greatest impact on the size of the generated turbulence. In the opinion of the reviewer, the article requires ordering from the editorial point of view. The introductory part lacks a clear definition of the purpose of the work and an explanation of how this goal will be achieved. The article also requires a thorough linguistic correction. Many sentences are grammatically and stylistically incorrect. Due to the large number of symbols used, I suggest adding a separate section under the name Notation.

While analyzing the content of the article, I came up with a few comments that are listed below:

1. Introduction

• Since the paper deals with issues directly related to fluid mechanics it is desirable that in the Introduction section the authors introduce the readers to the idea of energy generation from the VIVACE device. The current description is very poor and needs to be supplemented. For example, it is not known exactly where the so-called ‘Hidden generator’ is located in Fig 1.
• ‘It has been shown that as the Ar increases, the vortex shedding and reattachment take different forms’ – please add references
• ‘In addition, few papers have studied the flow around the rectangular cylinder with Ar<1.0.’ – please add references
• I suggest changing the notation of Ar, which may be confused with the Archimedes number (Galileo).
• ‘In this study, the flow field characteristics of a single rectangular cylinder three Ar in a restricted flow channel were investigated by PIV experiments under three Reynolds number conditions.’ – this sentence is difficult to understand and must be improved.

I propose that the Introduction section be re-edited to read as follows:

• description of the idea of the VIVACE device,
• description of the achievements of other researchers in research on the VIVACE device,
• a description of the elements which have not yet been investigated,
• formulation of the purpose of the work

2. Experimental Apparatus

• ‘This experiment's self-circulating damping water sink was 4 m long, 0.6 m wide, and1.8 m high, as shown in Fig. 2.’ - Unfortunately, this information is not shown in Fig.2 because it is simply not there.
• ‘The sink was made up of an upstream tank, a diffusion section, a contraction section,’ – where are those sections in Fig. 2?
• ‘each rectangular cylinder will be evaluated’ – is evaluated
• ‘To imitate the restricted VIVACE device, the flow channel with a blockage ratio of 0.25.’ – this sentence has no sense
• ‘PIV was utilized to gather flow velocity’ - Usually PIV aparatus is used to analyze flow field and not gather flow velocity
• In the process of preparing images for analysis, the authors used multiple image filters. Since image data filtering usually involves loss of information, please comment on what part of the information was lost during image preparation for analysis and how it affected the final result.
• The authors unnecessarily use the future tense in the description. Typically, the present or past tense is used to describe prepared experiments.

Several pieces of information need to be completed in the description of the test apparatus:

• What type of PIV system was used in the study?
• What kind of pressure transducers were used?
• What kind of laser was used?
• Was the data from the PIV system analysed in real time?
• What type of material is used as a seeding?

3. Signal Pre-Processing

• ‘PIV is a non-contact measurement technique for two-dimensional flow fields basedon the correlation analysis’ – cross-correlation
• ‘Because the original signal and noise have distinct characteristics at different wavelet coefficient scales.’ – this sentence in icorrect

4. Results and Discussion

• Are the values 0.02, 0.04, and 0.1 the values of Uo velocity?
• Authors define Ar as B/D (length-to-width). Please explain why in Fig. 5 Ar = 0.2, 0.3 and 0.5? It seems to be 5, 3.33 and 10.
• Please include in the text the determination of Re number.
• Do the values of Reynolds numbers 1000, 2000 and 5000 correspond to velocities 0.02, 0.04, and 0.1? It is not clear.
• ‘Reynolds shear stress is an interchange of turbulent fluid masses between flow layers between adjoining flow levels.’ – this sentence is wrongly formulated

5. Conclusion

From the point of view of the purpose of the whole work, the last conclusion seems very important. The problem is that it does not clearly result from the results of the analysis. Authors should make sure that each conclusion that appears in chapter 5 is formulated in advance when analyzing the obtained results. Therefore, I am asking for this deficiency to be completed. 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Many thanks to the authors for taking the time to further clarify the details of their PIV setup and analysis, and reflect this in the manuscript.

Looking at the numbers provided, I remain worried about the ability of the experimental work to support the conclusions.

At 0.02 m/s, for a 10 ms dt, a particle would be expected to travel 0.02 m/s * 0.01 s = 0.0002 m = 0.2 mm, or 2.3 pixels. Since the light point given in the paper is 2-3 pixels (0.2mm), the dt would be too low to achieve the desired particle motion. Furthermore, this assumed the flow speed as U_inf, but figure 5 shows much lower velocities than that, and therefore an even lower particle displacement. Results at 0.02 m/s and 0.04 m/s should therefore be removed from the study. At 0.1 m/s, the particle displacement would be 11.77 px, which while still being low (typically, 1/4 of the processed window size is recommended), could yield acceptable results. Some results could be salved by omitting some pictures in the post-processing to yield a higher dt and greater particle displacement. Combined with a new refined multipass post processing, this may yield more acceptable results.

Indeed, further post-processing would be required: a single pass with 50% overlap would not be deemed accurate enough for published work. At the very least 2 (if not 3-4 for good practice) passes with a smaller window size and higher overlap would be needed. This is particularly crucial considering the very small particle displacement.

Showcasing the cross-correlation values in the flow fields for the experiments conducted would be beneficial to give the reader confidence in the accuracy of the results. This could take the form of a new figure as part of the uncertainty discussion, or local addition to existing figures.

It would also be recommended to review the manuscript for typos and check for consistency (eg: space between quantity and unit not always present, 'Re of D' used when Re is already defined as a function of D, etc)

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper still requires an extensive editing of English language and style.

Author Response

Dear reviewer：

Because it took too much time to reply to the comments of another reviewer, there was no time to find an English editing service. As a result, the English editing was conducted by me together with my colleagues. 

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

The paper deals with an extremely interesting topic, probably with great application potential. Sophisticated experimental methods are described in detail, including methods for evaluating the results. For these reasons, it would be desirable to publish the paper. However, in order to be published, it is absolutely necessary to make substantial adjustments. A set-up experiment is given. Unfortunately, the exact definition of the coordinate system and other quantities, such as c, t, Br, e.g. a suitable scheme, is missing. Among other things, Ar, sometimes AR is mentioned in the text.

It is necessary to solve the problem, which is often marked in the text as (Error! Reference source not found.) In the discussion of the figure, the beliefs that are probably identified in the press by eg (# 1 and # 2) are discussed. There are 5 vortices; there are only 4 in the picture.

The opposed manuscript must be modified graphically. Some paragraphs have a different beginning, etc.

It is interesting to apply the problem to energy production with the help of VIVACE equipment. Recommendations for the design of this device with the help of the obtained results would help to improve the contribution.

Reviewer 2 Report

Many thanks to the authors for submitting this manuscript on the PIV experiments undertaken on a rectangular cylinder. Please see after my comments and suggestion:

Abstract:

• There is a punctuation issue in the first line, which should feature a comma instead of a full stop after aquatic.
• two different spelling of the AR abbreviations are presented, namely Ar and AR, which appears inconsistent
• There is inconsistent use of capital letters when introducing abbreviations (eg; PIV v POD)
• Very little context is provided, and no information on 'VIVACE' is provided

Introduction:

• Considering the emphasis on VIVACE (4 mentions on the 1st page), context and explanations should be provided to the reader.
• The 2nd paragraph on page 2 needs a reference after the 1st statement. Same after the 1st statement of the next paragraph
• St is not defined
• There are numerous typos to correct (eg: 26and; none researches have) - the paper requires a thorough proofreading
• There are also numerous cross-references that appear as 'error' throughout, which would need to be addressed.
• There is a lot of seminal work not cited in the introduction.
• The research questions should be made more explicit, and the structure of the paper outlined
• Pages 3 after figure 1 belongs in the methodology section, not the introduction

Experimental Appartus

• There should be a space between the value and its unit in all instances
• More background on the PIV is needed, including the spacial resolution achieved and uncertainty analysis, but also particle sizes and density, processed window sizes, etc

Signal Pre-Processing

• CLAHE is defined after its first use in the text, and the use of all caps text in this paragraph is not relevant

Results

• This is where the lack of information on spacial resolution and uncertainty affects the reader ability to fully accept the results
• More discussion on the individual subplots of figure 5 would be welcome
• A stronger case should be made for the use of the RSS in this instance
• Figures are not cross referenced in the text, which makes following your narrative very difficult. There are references to 'the figure' but without number, eg in section 4.2
• equation display, alignment and nomenclature to be improved throughout
• The whole POD section cannot be followed due to the numerous 'errors' (up to 6 in 6 lines below figure 10). This critically needs addressing.

Conclusion

• Conclusion 1 is inconsistent with the abstract (70% v 75%)
• Because of the limitations highlighted in the results section, the reader would not be convinced of all the findings listed at this stage. A stronger narrative is needed in the paper.

It is hoped the above comments will prove helpful in refining the publication