Numerical Evaluation of a Novel Vertical Drop Airflow System to Mitigate Droplet Transmission in Trains
Round 1
Reviewer 1 Report
1. Title is looking a little bit longer if possible please reduce it
2. Explain Figure 9 in terms of improving its almost look similar to conventional and VDA system
3. Before conclusion add a table with references to sow improving your work to verify the contribution to the existing literature
4. Add mesh independence test your paper in reference to an important output parameter, explain mesh statistics and also optimal mesh decided for all simulations
Author Response
Correspondence to the Reviewer’s comments
Reviewer #1
1. Title is looking a little bit longer if possible please reduce it.
Reply) Thank you for the valuable comment. Based on the comment, the title was revised as follows: “Numerical Evaluation of a Novel Vertical Drop Airflow System to Mitigate Droplet Transmission in Trains”
2. Explain Figure 9 in terms of improving its almost look similar to conventional and VDA system.
Reply) Based on the reviewer’s comment, the following explanation was added: “There was no significant difference in deposition between the conventional and VDA systems.”
3. Before conclusion add a table with references to sow improving your work to verify the contribution to the existing literature.
Reply) Based on the reviewer’s comment, Table 1 was added to clarify the contributions of this study.
4. Add mesh independence test your paper in reference to an important output parameter, explain mesh statistics and also optimal mesh decided for all simulations.
Reply) Based on the reviewer’s comment, important parameters of the grid sizes and mesh statistics of the element quality were included in Figure 5. Further, the following sentence was added: “Moreover, from the grid number of 3.4 million, the average element quality was higher than 0.8.”
Please see the attachment for more information.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This paper proposes a novel vertical drop airflow system in railway vehicles to prevent the spread of infectious diseases. It would be a good idea to take this system in high-speed trains. But the conclusion in this paper needs a summary language, and it is best to summarize the previous analysis by dividing it into sections. And more detailed problems are raised as follows:
- 1. Line 77 to 83: there should be indication of the correlation among citations, and it is better to propose the correlation between citations and this paper.
- 2. Line 77: how about studies in 2021? It is better to cite some studies about preventing droplet transmission of the COVID-19 in 2021 here.
- 3. Line 89: “…this study was to evaluate the mitigating…” should be “…this study is to evaluate the mitigating…”
- 4. Line 109 to 114: how about the results if the infected person was seated at 2B, which was far way the windows? Did you consider this case?
- 5. Line 181: why do you emphasize that the mouth remained open after coughing? In fact, people would close his mouth soon after coughing, so it is better to present explanation here.
- 6. Line 212 to 213: “…in the x-y direction exhibits the shape…” should be “…in the x-y direction exhibited the shape…”
- 7. Figure 15 and Figure 16: it would be better if the color of bar chart was changed. And there should be data labels in the bar charts.
- Figure 17: keep the font of the figure and the text consistent.
- 9. Section 4 conclusions: the content of Section 4 is best described in paragraphs, especially the conclusions summarized in Section 3.
- 1 Section 4 conclusions: conclusion is a summary language, so the conclusions obtained from analysis need to be summarized.
Considering above-mentioned reasons, I suggest this paper needs a minor revision.
Author Response
Correspondence to the Reviewer’s comments
Reviewer #2
1. Line 77 to 83: there should be indication of the correlation among citations, and it is better to propose the correlation between citations and this paper.
Reply) Thank you for the valuable comment. Based on the comment, the following sentence was added to indicate the correlation between the citations and this paper: “These studies suggested masks, air purifiers, barriers, and ventilation controls to prevent droplet transmission but did not perform airflow design studies.” Moreover, Table 1 was added to verify the originality of this study.
2. Line 77: how about studies in 2021? It is better to cite some studies about preventing droplet transmission of the COVID-19 in 2021 here.
Reply) The studies corresponding to references [24-27] were performed in 2021. Therefore, the sentence was revised as follows: “Various studies to prevent droplet transmission of the COVID-19 infection were conducted in 2021.”
3. Line 89: “…this study was to evaluate the mitigating…” should be “…this study is to evaluate the mitigating…”
Reply) Based on the reviewer’s comment, the sentence was revised accordingly.
4. Line 109 to 114: how about the results if the infected person was seated at 2B, which was far way the windows? Did you consider this case?
Reply) If the infected person sat in 2B, droplet transmission would occur differently from that indicated in this paper because the 2B seat is far from the inlet and outlet of the ventilation system. We are currently conducting additional research on various cases according to the variation in the locations of the infected person.
5. Line 181: why do you emphasize that the mouth remained open after coughing? In fact, people would close his mouth soon after coughing, so it is better to present explanation here.
Reply) Implementing mouth movement in the simulation using the Eulerian-Lagrangian model was very difficult owing to the need for developing a moving mesh model additionally. Based on the reviewer’s comment, the sentence was revised as follows: “The mouth was maintained open after coughing because it is difficult to simulate the movement of the mouth.”
6. Line 212 to 213: “…in the x-y direction exhibits the shape…” should be “…in the x-y direction exhibited the shape…”
Reply) The sentence was revised according to the reviewer’s comment.
7. Figure 15 and Figure 16: it would be better if the color of bar chart was changed. And there should be data labels in the bar charts.
Reply) Based on the reviewer’s comment, Figures 15 and 16 were revised.
8. Figure 17: keep the font of the figure and the text consistent.
Reply) Based on the reviewer’s comment, the font of Figure 17 was revised.
9. Section 4 conclusions: the content of Section 4 is best described in paragraphs, especially the conclusions summarized in Section 3.
Reply) Based on the reviewer’s comment, the Conclusion was revised.
10. Section 4 conclusions: conclusion is a summary language, so the conclusions obtained from analysis need to be summarized.
Reply) Based on the reviewer’s comment, the Conclusion was revised.
Please see the attachment for more information.
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear Author(s),
Your article is interesting and can meet the requirements of scientific article. The overall quality of the paper is high and presented work will have positive influence on the discipline with just some minor corrections which include:
- In the introduction: Moreover, COVID-19 can survive in droplet forms for a long
time in indoor environmental conditions [5]. – please put a time frame, do not use the phrase “long time” by itself as it is a subjective term - In the literary review please also cite methods that use droplet detection methods to show that you have knowledge on this topic like: Szczepanik et al “ PIV measurement and CFD simulations of an air terminal device with a dynamically adapting geometry”, SN Applied Sciences, Vol. 1, Iss. 4, 2019
- Methods section – while the grid dependency test is good for validating the mesh quality, but did you validate in some way the simulations? Do you have empirical data to know e.g. the simulation input of the cough is comparable to a real one? Same for the ventilation system?
You only state that “the mass and number fractions of the droplets were applied by a previous study [38]” (line 181) but don’t compare the results from the grid dependency test to the results from the previous study. Please elaborate as this is important for the validation of the simulation.
The same should be done for the validation of the airflows in the ventilation systems
Author Response
Correspondence to the Reviewer’s comments
Reviewer #3
1. In the introduction: Moreover, COVID-19 can survive in droplet forms for a long time in indoor environmental conditions [5]. – please put a time frame, do not use the phrase “long time” by itself as it is a subjective term.
Reply) Thank you for the valuable comment. Based on the comment, the sentence and reference were revised as follows: “Moreover, COVID-19 can survive in droplet form for 3 hours in indoor environmental conditions [5].” and “5. Jayaweera, M.; Perera, H.; Gunawardana, B.; Manatunge, J. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environ. Res. 2020, 188, 109819. https://doi.org/10.1016/j.envres.2020.109819.”
2. In the literary review please also cite methods that use droplet detection methods to show that you have knowledge on this topic like: Szczepanik et al “PIV measurement and CFD simulations of an air terminal device with a dynamically adapting geometry”, SN Applied Sciences, Vol. 1, Iss. 4, 2019.
Reply) Based on the reviewer’s comment, a sentence and reference were included, as follows: “Through the experiment of PIV measurement, the validity of a CFD simulation study on air flow was verified [22].” and “22. Szczepanik-Scislo, N.; Antonowicz, A.; Scislo, L. PIV measurement and CFD simulations of an air terminal device with a dynamically adapting geometry. SN Appl. Sci. 2019, 1(4), 1-9. https://doi.org/10.1007/s42452-019-0389-4”
3. Methods section – while the grid dependency test is good for validating the mesh quality, but did you validate in some way the simulations? Do you have empirical data to know e.g. the simulation input of the cough is comparable to a real one? Same for the ventilation system? You only state that “the mass and number fractions of the droplets were applied by a previous study [38]” (line 181) but don’t compare the results from the grid dependency test to the results from the previous study. Please elaborate as this is important for the validation of the simulation.
Reply) The simulation input of the cough and ventilation system used real data. Based on the reviewer’s comment, the following sentences were included: “The experimental data [38] of the cough flow rate were applied to the boundary condition as a user-defined function” and “The conventional system was designed based on the Korea train express (KTX).”
In addition, the input data, namely the mass and number fractions of the droplets, were measured through an actual experiment. The sentence was revised as follows: “The initial conditions of the mass and number fractions of the droplets were applied based on the experimental data [39], as depicted in Figure 3(b).”
Based on the reviewer’s comment, Figure 5 was revised to provide more information on the grid dependency tests and validate the simulation. Further, the following sentence was added: “Moreover, from the grid number of 3.4 million, the average element quality was higher than 0.8.”
Please see the attachment for more information.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments for Authors
The paper is now improved from previous version. However still some of the points are not properly addressed. The authors need to fix the following two concerns before possible publication of this paper in this journal.
- There wasno significant difference in deposition between the conventional and VDA systems. Then why two different approaches or you applied VDA and not conventional one any solid reason?
- Mesh independence explanation is good, However you need to make mesh independence line plot between number of elements in the mesh versus and important variable of your study. You can get guideline from the papers below for mesh independence test. It is suggested to add the mesh independence test line plot and also reference these articles in your current paper as evidence it will make your case stronger.
- Experimental and numerical analysis of Y-shaped Split and recombination micro-mixer with different mixing units
- Numerical study of 3-D helical passive micromixer having both inlets at offset with blood as fluid
- Experimental and numerical analysis of three Y-shaped split and recombination micromixers based on cantor fractal structures
Author Response
Correspondence to the Reviewer’s comments
Reviewer #1
1. There was no significant difference in deposition between the conventional and VDA systems. Then why two different approaches or you applied VDA and not conventional one any solid reason?
Reply) Thank you for the valuable comment. Most droplets were deposited on the back surface of the front seat within 10 s, as shown in Figures 8 and 9. However, droplets smaller than 10 µm spread through the airflow in the cabin, as shown in Figure 11. This study proposed the VDA system to mitigate droplet transmission through the new airflow design. The VDA system significantly decreased minimum and maximum droplet travel distances compared to the conventional system, as shown in Figure 16. Based on the comment, the sentence was included as follows: “However, the remaining droplets under 10 µm spread through various routes in the cabin, as shown in Figure 8.”
2. Mesh independence explanation is good, However you need to make mesh independence line plot between number of elements in the mesh versus and important variable of your study. You can get guideline from the papers below for mesh independence test. It is suggested to add the mesh independence test line plot and also reference these articles in your current paper as evidence it will make your case stronger.
“Experimental and numerical analysis of Y-shaped Split and recombination micro-mixer with different mixing units”, “Numerical study of 3-D helical passive micromixer having both inlets at offset with blood as fluid”, “Experimental and numerical analysis of three Y-shaped split and recombination micromixers based on cantor fractal structures”
Reply) The mesh independence line plot between the number of grids versus the droplet mass, which was the important variable, had already been included in the paper, as shown in Figure 4. Based on the reviewer’s comment, Figure 4 was revised to provide more information on the grid dependency test of the velocity. Moreover, the sentences were revised as follows: “During the 3 s, a significant difference was observed in the droplet mass depending on the number of grids because a turbulent cloud with numerous droplets was spread out of the mouth during coughing, as shown in Figure 4(a) [43].” And “A grid number of 3.4 million was selected owing to its trend of the mass droplet and velocity similar to that of 5.2 million grids.”
In addition, Based on the reviewer’s comment, the references were included as follows:
“41. Shah, I.; Kim, S. W.; Kim, K.; Doh, Y. H.; Choi, K. H. Experimental and numerical analysis of Y-shaped split and recombination micro-mixer with different mixing units. Chem. Eng. J. 2019, 358, 691-706. https://doi.org/10.1016/j.cej.2018.09.045”
“42. Kim, K.; Shah, I.; Ali, M.; Aziz, S.; Khalid, M. A. U.; Kim, Y. S.; Choi, K. H. Experimental and numerical analysis of three Y-shaped split and recombination micromixers based on cantor fractal structures. Microsyst. Technol. 2020, 26(6), 1783-1796. https://doi.org/10.1007/s00542-019-04724-5”
Author Response File: Author Response.pdf
Reviewer 3 Report
Dear authors, thnk you for the changes in the amnuscrips.
All the best
Author Response
Thank you for the valuable comment. Based on the comment, the paper was improved.
