An Experimental Study of Turbulent Mixing in Channel Flow Past a Grid
Round 1
Reviewer 1 Report
Comments regarding the writing:
Please improve your writing and check spelling and grammar accordingly. A sentence is not a paragraph.
Comments regarding the conducting of the study:
Why wasn't a better method for creating the grid taken into account when you find out the poor quality of the printer in postprocessing? The influence of inaccurate manufacturing of the grid is briefly discussed but not considered in detail in the results section. The same applies to the use of two component PIV results. 3 component PIV was done in one plane and compared to the other results, but only very briefly described. The reader would wish that in a research topic like this, where a canonical test case in the laboratory is deliberately used, more emphasis would be placed on a clean execution of the experiment. This means to use a perfect grid and to perform stereo PIV in several places to eliminate most of the uncertainties described in this paper.
Comments regarding the overall paper:
The results are very varied and detailed, which makes it difficult for the reader to find and follow the red thread or storyline of the paper. Even the introduction is not good enough. It is only a short introduction to the topic. For a journal paper without a special page number limitation, you should take your space here! Also due to the fact that this is a research area that has been researched for over a hundred years, one should present the essential findings as the basis of your own research question and present your own research question in detail with all the necessary research results of other scientists accordingly. That means also citing more literature of others! Both the introduction and the results/discussion must be clearly revised and a red thread must be absolutely recognizable.
Comments regarding the presentation of the results:
Why do you explain your results in the captions of the figures? Many figures are not explained at all or only insufficiently. The chapter 3 Results is only a processing of the individual results without direct connection (red thread) the chapter 4 Discussion is quasi not available! This also applies to the summary!
General comment to the authors:
The topic of turbulence studies and in particular the dissipation of turbulence is an essential topic that urgently needs new insights. Therefore the significance of the topic is of high importance. I would like to encourage the authors to accept and implement the above comments!
Author Response
We thank for constructive comments, for more details see the attached file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors present a PIV study on grid turbulence. Beside the data analysis the authors also address the problem of geometrical deviations of the real model to the intended design.
The manuscript in its current state has a few shortcomings that should be improved:
- grid turbulence is of major interest in the turbulence community and there is plenty of literature dealing with it on either an analytic, numerical or experimental level. Considering this, the introduction section is very poor and contains only very little references.
- The section describing the experimental method does not state how many PIV frames have been used for the statistical calculations. Hence, a prove is missing about the convergence of the turbulent quantities. This could easily be checked by e.g. calculating the RMS values for an increasing number of PIV frames used
- the authors provide the Reynolds numbers but no absolut velocities upstream of the grid. Thus, it makes no sense to plot absolute velocities in Fig.5. They should rather be presented in relative scale with reference to the inlet velocity.
- the Symbols in Fig 15 cannot be identified, Figure should be improved
- although many people give the units in figure labels in square brackets this is not an international standard. Round brackets should be used instead. The actual usage of square brackets in the context of units is in the way of a mathematical operator meaning "unit of" like [v] = m/s
- the style of english writing can be improved significantly and there is a frequent misuse or absence of indefinite and definite articles. Please refer to the scanned pdf in the attachment where I marked these things and other typographical errors I noticed and some sentences that don't really make sense.
Comments for author File: Comments.pdf
Author Response
We thank for constructive comments, for more details see the attached file.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors make use of measurements on confined flows in order to characterize the turbulence using flow visualization techniques by stereo piv methods instead of more traditional techniques such as hot-wire x-probes. Their results and conclusions are important in the field not only in turbulence theory but also to better understand the three-dimensional phenomenon of the boundary layer.
The authors develop a turbulence generator based on a 3D printable design and with the possibility of being parameterized in future works. As 3D prints suffer from deformations intrinsic to the process used, which is not described, measurement techniques based on the scan were adopted to assess the deviations between the physical and theoretical models.
General Comments on the Manuscript
In the Abstract it says “Gird” when it should say “Grid”.
Figure 2 (a) shows a number of tables with numerical information that is very difficult to read. If it is relevant, it should be sought to expand it or otherwise it should be removed in favour of clarity.
On the writing of captions of figures: they should be shortened and if it is believed that what is said in them is important in the communication process, look for a way to integrate said content within the main text. Statements such as those included in Figure 1 “We apologize for the lower…. “They should be avoided. Figure 3 has an extremely long caption. This should be simplified.
In Figure 1 the “Blue arrow” should be specified in a concrete way from the inclusion of a precise graphic indicator of the flow direction.
Questions for Authors
- Online 37 the author’s comment that the turbulence generating element was integrated into a low-speed wind tunnel. Details of the experimental facility and its layout should be given in terms of speed measurements and characteristics.
- How does the quality of the print surface affect the generation of turbulence?
- What printing technology is used in the grid pattern manufacturing process? Fused deposition modelling. Although the authors do not say so, it all suggests that the impression technique is based on FDM (Fused Deposition Modeling), do the authors believe that the use of another impression technique that allows better surface finishes could affect the results achieved?
- Inline 73 the authors only reference the generation of the particles but do not provide details of their sizes and fonts. Only references that the smaller the better but they do not quantify their size and it is important with regard to the signal/noise conditions in the measurement of the speed fields.
- Inline 80 it is mentioned that the sampling frequency is 7.4 Hz but it has not been possible to elucidate the time interval between light pulses. Inline 77 it is only vaguely mentioned that there are few microseconds between them. These values ​​should be detailed since the presentation of the information is not in accordance with that of a scientific report such as the one intended.
Author Response
We thank for constructive comments, for more details see the attached file.
Author Response File: Author Response.pdf
Reviewer 4 Report
This paper contains a description of experiments and analysis of turbulent mixing past a grid.
Overall comments:
1) A clear motivation of why the research is being done (and what is new) is missing and should be included in the Introduction. An outline of the paper should also be included here.
2) The English should be corrected, there is a good deal of confusing language. One example is the title of the paper, a better one would be "An experimental study of turbulent mixing in channel flow past a grid".
3) The section on the additive manufacturing of the grid, including the deviations, is a very useful contribution.
4) The layout has not been done carefully, e.g. Appendices A and B are from a template it seems.
Detailed comments:
1) Section 2: Add more description of the wind tunnel, maybe also a schematic drawing.
2) Section 2: Which material is used for particle seeding and what is the size of the particles?
3) Section 3: Lines 105-110 are confusing, I propose including the information in a table.
4) Section 5: Compare the decay exponent of -1.95 to literature findings.
Author Response
We thank for constructive comments, for more details see the attached file.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Dear authors, thank you for considering the reviewer comments. The paper has clearly gained in readability and comprehensibility. Only the conclusion is still a bit too superficial and too short for a journal article. It is more like a summary. Please present your main findings and how they further support the motivation and research questions of your research project. Also an outlook on current and future work that will further illuminate the topic is desirable.
Author Response
Dear reviewers,
Many thanks indeed for careful reevaluation of our paper.
We have done our best to integrate them into the text.
Conclusion has been extended.
Thank you again.
Author Response File: Author Response.pdf
Reviewer 2 Report
The authors have improved the paper after the first revision. However, there are still a few issues that need to be clarified.
In the reply to the comments they state
"The velocities in Fig.5 are given in multiplies of the incoming flow velocity U."
For my opinion the figure (now Fig 6) looks exactly the same as the (previously) Fig 5 in the first version.
Also they state
"The notation of units is modified.",
while in the revised pdf file all units are exactly the same as in the previous one.
Hence it is questionable if the authors uploaded the correct pdf file.
Author Response
Dear reviewers,
Many thanks indeed for careful reevaluation of our paper.
We have done our best to integrate them into the text.
The Figure was in dimensionless representation even in the original version of the paper, but not explicitly stated. So, we added only the statement (above).
We erased the square brackets. The current notation is number + space + unit in normal font. In figures, the unit is inside square brackets.
Thank you again.
Author Response File: Author Response.pdf
Reviewer 4 Report
Most of my points have been addressed. The English language could still be improved and a few more details on the particle seeding added to the paper.
Author Response
Dear reviewers,
Many thanks indeed for careful reevaluation of our paper.
We have done our best to integrate them into the text.
The text was revised and more info on seeding is added. According the English, we did the best we can.
Thank you again.
Author Response File: Author Response.pdf
