Numerical Study of Mixing Process by Point Source Pollution with Different Release Positions in a Sinuous Open Channel

Round 1

Reviewer 1 Report

The most serious problem with this manuscript is the conclusions. The authors claim three points, however neither of them contains real scientific novelty. In fact, these points are very general and had been given in the Introduction when discussing results of other studies. Consequently, a major revision is required to present novel and specific conclusions.

Another deficiency is related to the problem formulation. Below is a list to be addressed.

1. double eq. (1) .
2. Lowercase c in eq. (3)
3. Presence of time derivative in eq. (2) and (3) for in fact steady problem considered in the next sections was not explained. It can be unclear for some people that PIMPLE was used for relaxation; so, some comments should be given.
4. Body force fi is introduced in eq. (2) but is not used.
5. Point source term S in eq. (3) is not described properly. How is it implemented in this specific finite volume method? Is it of a size of a cell?
6. Eq. (1)-(3) are given dimensional, probably because OpenFOAM uses such form of equations. However, then the authors scale the fields and coordinates to make them dimensionless. It is recommended to present additionally the equations in dimensionless form to simplify further explanations and figure captions.
7. OpenFOAM contains many wall functions, why did you use nutUSpaldingWallFunction?
8. Page 4, Table 1: Re and Fr are undefined
9. Page 6, lines 194-196: periodic boundary conditions contradict to the presence of the point source only in a single section
10. Page 6, lines 201-206: the choice of the grid size is not properly justified. Is the solution grid independent? Despite the favorable comparison with the experiments, a grid independence check should be done.
11.  Page 7, lines 220-221: the information on CPU hours is irrelevant without computer specification.

The presentation of results also should be improved.

1. Page 8, eq. (10): what is n?
2. Page 12, eq. (11) and (12). What is ui without the bar? Comparing with eq. (1) and (2) and assuming that bar is missing, I conclude that n=3. But what is vi then?
3. Page 15, lines 384-385. Describe the interpolation procedure.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript describes the effect of meandering on pollutant dispersion in open channel flow using numerical model OpenFOAM. The paper is well written and would be very understandable even for the readers who is not familiar with numerical simulations. The topic is very traditional and important for material dispersion problems in meandering rivers. Overall, the numerical model used in this research predicts the experimental data very nicely and the results are clearly presented with beautiful figures. So this paper would be welcomed to the readers of this journal. However, there are some points to be improved before acceptance. Please see the following comments.

Firstly, for the numerical setup, the authors need to describe the reason to use a rigid-lid boundary for the free surface in this manuscript. You mention that previous researches used this approximation and obtained reasonable accuracy. But this cannot be the reason to use the rigid-lid approximation. If the authors can use the free-surface boundary, it would be far better to use the free surface boundary as it is real. In addition, there is no description for the driving force of the flow. Please clearly describe the details for this condition in the section of numerical setup.

Secondly, the figures to explain the numerical results are only for mean velocity, concentration and instantaneous velocity. In the case of mixing, the distributions of turbulent kinetic energy (TKE) or turbulent eddy viscosity/diffusivity give more information more than e.g. instantaneous velocity field. So please consider to add these figures in the section of mixing efficiency.

Lastly, the conclusions 1, 2, and 3 are so obvious. The most of these were already investigated by the previous experimental study. With these conclusions, readers may think that the authors repeated the same cases with the experiment and obtained the same conclusions with numerical model. Is this the purpose of this paper? Please modify the conclusion to include the results obtained from the present research research. I think if the authors can discuss about the effect of curvature of the channel (changing the curvature of the channel including straight channel) or different types of cross sections on mixing process using numerical model, this paper would be very original. Please think about this.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

I recommend accepting the manuscript after the authors do the major revisions included in the review report in the attachment file.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors took into account my recommendations. I recommend publishing the manuscript.

   

 

 

Reviewer 3 Report

The authors have adequately addressed the comments of the reviewers. Therefore, the revised paper is recommended for publication in Water Journal.