Wake Width: Discussion of Several Methods How to Estimate It by Using Measured Experimental Data

Round 1

Reviewer 1 Report

In the present work, the authors have presented various experimental methods to define wake width. The work is having a quite interesting comparison demonstrating advantages and drawbacks of each method. My over observation is that authors have reorganize the manuscript as intention is not clearly spelt at the starting of the manuscript.  I would request a major revision for the following reasons.

1. It would be interesting to look at the methods look at in the present work, where these methods previously applied, especially for well-known bluff bodies like cylinders etc. Literature part of the introduction is not informative
2. It is stated that such analysis for wake width study using various experimental methods has been done for the cylinder. What is so unique when these methods are applied for an airfoil?
3. One of the main usages of such detailed experimental analysis is to help validation of CFD models. In that viewpoint, the experimental uncertainty is stated in the various method.
4. at certain places, the authors have used ‘C. Norberg’ which is supposed to be only ‘Norberg’
5. Will the uncertainty increase with the higher-order statistical moment, especially 3^rd and 4^th order ones?
6. The discussion on Figure 12 and 13 are not very proper as effect of each method has not been explained properly. For example Flat[u] method gives different results why?  Also position of min. WW is quite random why? 
7. In figure 14, the authors are explaining the repeatability, please state the uncertainty of the data plotted. Without this, these results will not be of any use.

 

  

 

Author Response

Response to reviewers

We thank to all reviewers for their work with reading our article. The main weakness pointed by all reviewers was the missing uncertainty analysis. It is not so straight forward to project the general rules of uncertainty propagation into a such non-linear analysis method, therefore we used an empirical approach of dividing the ensemble into multiple subenesmebles looking how the result varies. Hence we hope that You will be satisfied with this estimation.

The great thank belongs to the question about instantaneous wake width and the wake meandering. This approach has been added into the list of discussed methods and opens a new way insight into the problem: it has been found that the instantaneous wake is in average narrower than the average one and the wake meandering visible as the variation of centerline is then a new explorable feature of the wake.

We skipped the discussion about minimal wake position and the discussion about the effect of airfoil geometry modifications.

We added a paragraph into the introduction about the literature studies of wake past airfoils.

 

Reviewer 2 Report

Wake width – discussion of several methods how to estimate it by using measured experimental data

This paper made an experimental study for the flow behind the NACA airfoil to discuss about the width of the wake. The paper collects and analyze the data well. However, I cannot recommend the paper for publish in the current study due to the following reasons. 

- First of all. it is difficult to read this manuscript. The paper is like an essay, not the academic paper. Please do logical writing, rewrite, and recheck the entire the manuscript. 

- I cannot understand why does the author think that the ordinary definition of the wake would be changed? 

- Since the wake shows meandering motion in time, how is it included in several definition of the wake width. E.g. <u> means the averaged value of the streamwise velocity and the mean indicates the temporary averaging in this manuscript. However, the wave is obviously meandering and the temporal averging is not suitable. Accordingly, the wake width is the function of time. The reviewer think this point should be also analyzed and discussed in the paper. 

- The author does not discuss the accuracy of the measurement (not repeatability). The error analysis should be required. The ordinary PIV measurement has error in data and it is difficult to take high order statistics (i.e., flatness and skewness). The author also clarify this point. 

-The repeatability is discussed in the Sec. 5. I think that the repeatability should be checked in all experiment, but I can read this paper as that the author allows the errors for different experiment. The errors for different experiment is just "error" and it should be eliminated by the effort in the experimental setting and post processing and so on.  

- In the conclusion, the advantage and disadvantage are displayed, but they are very generic. Please write them quantitatively. 

Author Response

Response to reviewers

We thank to all reviewers for their work with reading our article. The main weakness pointed by all reviewers was the missing uncertainty analysis. It is not so straight forward to project the general rules of uncertainty propagation into a such non-linear analysis method, therefore we used an empirical approach of dividing the ensemble into multiple subenesmebles looking how the result varies. Hence we hope that You will be satisfied with this estimation.

The great thank belongs to the question about instantaneous wake width and the wake meandering. This approach has been added into the list of discussed methods and opens a new way insight into the problem: it has been found that the instantaneous wake is in average narrower than the average one and the wake meandering visible as the variation of centerline is then a new explorable feature of the wake.

We skipped the discussion about minimal wake position and the discussion about the effect of airfoil geometry modifications.

We added a paragraph into the introduction about the literature studies of wake past airfoils.

 

Reviewer 3 Report

In this manuscript, the authors systematically summarized several estimation methods of wake width by using measured experimental data. It is a topic of interest to the researchers in the related area but the manuscript is not adequate for publication in Energies. The reviewer suggests the authors to rewrite the manuscript carefully. Discussions and figures should be corrected and improved accordingly. The followings are the questions in the manuscript:

• The biggest problem of this paper is that it is very poorly written, which makes the evaluation and interpretation of the main contributions of the paper obscure. This manuscript requires a complete rewrite to improve the grammar, style and readability. Some problems are presented hereinafter:

1. Nomenclature is needed.
2. Spelling mistakes and inappropriate words, e.g., “turbulize” in Line. 35, “result into” in Line. 39, “wkae” in Figure. 13(a), “farer” in Line. 395.
3. “…wake past a …” should be corrected as “…wake of a…” or “…flow past a…”.
4. “splashing” is not adequate for the aerodynamic issues.
5. A large number of grammatical errors, e.g., sentence structure errors in Line. 25, Line. 158, Line. 176, morphological errors in Line. 38, Line. 39, Line. 40, Line. 167, Line. 174, Line. 192, Line. 285.
6. Abuse of punctuation marks, e.g., 16, in Line. 18, Line. 46, Line. 48, Line. 49, Line. 80, Line. 109, Line. 253, Line. 259.
7. There are too many rhetorical questions in this paper, which reduces the accuracy and conciseness of the main contributions.
8. There are too many ambiguous and poorly readable sentences in this paper, so a complete rewrite is necessary.

• The introduction is too short to demonstrate the investigations of wake characteristics, which makes it more like a list of opinions and lacks in-depth discussion.
• In Section 2, please provide the sampling frequency, the spatial resolution and the interrogation window of the PIV system.
• The definition of “span-wise” should be clearly elaborated, because in many papers the “span-wise” is defined as the direction perpendicular to the cross-section of the test model.
• In Line. 317, Figure 9(b) is not a “Histogram”.
• Figure 12-13 is so vague that it is difficult for readers to identify the useful information. The combination of line and symbol is recommended.
• In Section. 6, the expanding boundary layer approach should be included.
• The uncertainty of the experimental results and their calculation methods should be discussed in the manuscript in details, which is a concern about the accuracy of results.
• The aspect ratio of the test model is 125/80=1.5625, which is too small to ensure the two-dimensional flow condition. However, all the data processing in this manuscript is based on two-dimensional flow assumptions, so a verification should be carried out.

Author Response

Response to reviewers

We thank to all reviewers for their work with reading our article. The main weakness pointed by all reviewers was the missing uncertainty analysis. It is not so straight forward to project the general rules of uncertainty propagation into a such non-linear analysis method, therefore we used an empirical approach of dividing the ensemble into multiple subenesmebles looking how the result varies. Hence we hope that You will be satisfied with this estimation.

The great thank belongs to the question about instantaneous wake width and the wake meandering. This approach has been added into the list of discussed methods and opens a new way insight into the problem: it has been found that the instantaneous wake is in average narrower than the average one and the wake meandering visible as the variation of centerline is then a new explorable feature of the wake.

We skipped the discussion about minimal wake position and the discussion about the effect of airfoil geometry modifications.

We added a paragraph into the introduction about the literature studies of wake past airfoils.

 

Round 2

Reviewer 1 Report

I am really happy that the authors have taken comments so seriously and work on them. Their response is really good. I would recommend this manuscript be published in the present form.

 

 

Author Response

Response to reviewers

We thank to the reviewer for accepting our article. 

Reviewer 2 Report

The authors do not answer my questions sincerely. I require the rebuttal that answers all my question properly. 

Author Response

Response to reviewers

Thank you for your efforts. Please see details in the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The authors have revised the manuscript accordingly, and I can recommend its publication.

Author Response

Response to reviewers

We thank to the reviewer for accepting our article. 

Round 3

Reviewer 2 Report

Thank you for the letter. When you submit the revised paper, the letter should be required to show where the author changes and how the author thinks.