Experimental Study of the Droplet Deposition Characteristics on an Unmanned Aerial Vehicle Platform under Wind Tunnel Conditions
Round 1
Reviewer 1 Report (Previous Reviewer 2)
The major issues as commented on the original submission have not been dealt with. The main problem is the inappropriate design and arrangement of the experiments. A fundamental improvement of the paper is necessary, this would require new experiments. Therefore, the publication is not recommended.
Author Response
Dear Editor,
Thank you for your valuable time and advices on my manuscript. On the basis of previous studies, this paper further explored the atomization characteristics of spraying water and water +PEG under different crosswind wind speeds and spray pressures, and obtained the deposition characteristics of the target area and off-target.
This study has so far met our expectations, and these findings can provide guidance for operators to determine the operating conditions for UAVs to increase the deposition of the target area and minimize drift during application.
More factors need to be considered for the deposition characteristics of UAVs. Your opinion is my next direction, and I will carry out a detailed exploration of it in the follow-up research. Thank you again for your valuable time and comments on my manuscript, which benefited me a lot.
Author Response File: 
Author Response.pdf
Reviewer 2 Report (Previous Reviewer 3)
Since authors have considered most of the comments, the revised version of the paper is now acceptable for publication.
Author Response
Dear Editor,
Thank you very much for your valuable time and comments on my manuscript, which benefited me a lot. I will make continuous efforts to make more achievements that contribute to my research direction!
Author Response File: Author Response.pdf
Reviewer 3 Report (New Reviewer)
This was a well-written manuscript and will help in the field of UAV spray applications.
Major comments/questions:
- L77 - what does wind tunnel are stable and adjusted within technical limits means?
- L235: Why does PEG increase droplet size, etc?
Table 2: There is a 10X and 20X increase in drift deposition when going from 1 -2 m/sec and 50X and 75X increase from 1-5 m/sec for water and water+PEG, respectively. Is this consistent with the literature? This should be discussed in the text.
- Add some discussion of why increase in pressure decreases droplet size.
Minor edits/corrections:
L12 - delete " and likely different..."
L20: replace "might result" with resulted
- Replace "clear water" with "tap water" or "water only". Also, define the sources of the water. Laboratory faucet or filtered water?
Table 2 and 3 - reduce numbers to 1 significant figure to clean up the tables.
Author Response
Dear Editors，
Thank you very much for your valuable time and comments on my manuscript. My answers to your questions are as follows：
1.The technical parameters of the wind tunnel are stable and can be continuously adjusted within the limited range.This means that parameters such as the wind speed and pressure in the wind tunnel can be adjusted continuously within the allowable range. For example, the wind speed can be adjusted from 0.5 to 0.6，0.7 and so on rather than 0.5 to1 and 1 to 1.5 and so on.
2.Why does PEG increase droplet size？This is because the addition of PEG leads to the increase of solution viscosity, which in turn leads to the increase of droplet size. I have added the explanation in the discussion.
3.Using PEG-20000 solution for spray is an innovative idea in this article. I compared the results with spray using other additives. Due to different additives, nozzle and spray parameters, the results are different, but the changes and trends are the same. I also explained this in the discussion.
4.The greater the atomization pressure, the greater the mass ratio of air to liquid. The larger mass ratio of air to liquid improves the efficiency of the nozzle to disperse liquid. The liquid is atomized more completely, so the droplets are smaller.This question was studied and answered 30 years ago or even earlier. I think it is a little inappropriate to discuss this question, which is more suitable to articles which thoroughly explore the mechanism of liquid atomization.
5.Minor edits/corrections have all be done.
Thank you very much for your advices again. I will keep learning and make better research contributions！
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report (Previous Reviewer 2)
I cannot see any substantial improvement of the manuscript as the main problem is the inappropriate design of the study. The study itself needs significant improvement. This is why I still recommend rejection of the paper.
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
Dear Authors,
the topic of the paper is of great interest and needs to be carefully addressed by the scientific community. However, in my opinion, your research is not adequately designed to answer the experimental question you set (L75-76) and thus the paper has to be rejected.
My main concerns are related to the experimental setting, with special regards to those reported in lines 172-173 “In this study, the airflow from the wind tunnel was regarded as the natural wind of 172 relative motion during the flight of the UAV.”. You are meaning that setting the wind tunnel speed at 1, 3 and 4 m/s you are simulating the UAV flying at the three mentioned forward speeds (this is also reported in the conclusion in line 379). In a wind-tunnel-based experiments, the droplets at the nozzle outlet are subjected to a constant airflow (forced convection) that can blow the droplets away from the spray source at different distances according to their dimension. This setup does not represent neither simulate at all the real conditions using UAV where the droplet is not subject to a constant airflow. Indeed, when the UAV is at certain distance from the point where the droplets has been sprayed by the nozzle, the droplets, which can be still in the air, are not subject to constant airflow. Therefore, the potential drift measured through lab measurements applying the experimental setup you proposed is not representative of real conditions and therefore cannot be measured as you did.
Based on my previous comments it derives that the conclusion are not supported by results achieved through experimental results.
Furthermore, my suggestion is also to deeply rewrite the introduction and material and methods as there is a mix between them. A lot of sentences belonging to introduction are reported in the M&M section. Also, the abstract is very confusing.
Even if I’m not a native English speaker I suggest an extended revision of the document.
Reviewer 2 Report
The paper describes a study on spray deposition from a simulated drone in a wind tunnel. It is also claimed to study the spay drift characteristics using this arrangement.
Although the concept is interesting, the performance of the study is not sufficient for a scientific publication. Also the writing style and some terminology are not appropriate. The language needs significant improvement.
Main points of criticism are:
1. Although the use of a wind tunnel has significant advantages, there are some problems that would limit the scientific significance of the study. In practical conditions, the downwash characteristics and the downwind airflow turbulences caused by the moving UAV are different from those in the wind tunnel due to the static arrangement of the simulated UAV.
2. The limited size of the wind tunnel section required the arrangement of the rotors above the wind tunnel outlet and the limitation in nozzle arrangement. This results in significant deviations from practical conditions.
3. It is assumed that the addition of a surfactant to water would give “physical and chemical” properties similar to PPPs. These properties are not specified and similarity is not proved.
4. The arrangement of the “drift” collectors seem to be not very suitable for the measurements. The distance from the nozzle to the upper horizontal collector and to the farthest vertical collector seem too big and probably a part of the spay plume was missed especially at higher wind speeds.
In conclusion, the significance and the novelty of the results seem very limited.
Reviewer 3 Report
General
This paper is interesting because the methodology aims at providing drift potential data based on deposition in controlled conditions that is quite innovative for UAVs. Indeed, the definition of drift data from field tests reveals several issues related to the comparison of flying conditions, spray application settings and the definition of the intended vs non-intended sprayed area. Comparing several nozzles and wind conditions is of interest for the scientific community.
However, this paper also reveals some weaknesses
Downwash airflow produced by UAV rotors are typically linked to the rotor speed that is under control of an inertial station that takes into account the compensation of the weight and the atmospheric conditions (lateral wind for example). For example, a 25kg UAV carrying 10 kg payload will loose 40% of its weight during the spray application with a resulting difference in terms downwash airflow and sensitivity to lateral wind. In this paper, the rotors rev speed is fixed at 3000 rpm. It would have been interesting to measure the airspeed produced by rotors.
The data tables (2-3-4) show inconsistencies that are to be corrected.
The different modalities (pressure/rotor) are compared to a reference obtained at a single wind speed. Only data obtained at the same wind speed are then comparable…
Detailed comments
Page 1 line 22 : “…while inhibiting…” this part of the sentence is a bit confusing. Please keep the same wording along the text. do flying airflow means downwash airflow?
Page 1 line 24 : “when the rotor was closed…” is this considered as the reference situation to which drift reducing ratio are calculated. It shall be more clear in the text.
Page 1 line 25-26. There is a potential risk of confusion here since a reduction % is estimated from reduction percentages. Would it be more simple to say that DPRP at 1 m/s was X and the one at 5m/s was increased to 0.2X?
Page 3 line 119 : what is the spatial deviation of a droplet size? Is it related to the heterogeneity of droplet size distribution depending on the sampling position into the spray? I understand this criteria is used in order to make only one droplet size measurements at one position into the spray. Is this correct?
Page 4 line 147-148 : the nozzle setup is not crystal clear since Fig 4 is a bit confusing. It looks like some rotors are placed outside the wind tunnel cross section? Does the blue painted part correspond to the wind tunnel section ?
Page 6 equation 2 and 4 : from a mathematical point of vue, the numerical integration along the height or the distance downwind are similar to averages because the sampling distance is constant.
Page 6 lien 218-219 ; what is the reference spray exactly in this study ?
Page 7 line 247. Due to the large amount of data, it would be better to separate the analysis of table 2 and Table3… The title of this 3.2 part is a bit confusing because deposition is used to qualify measurements inside the target area and the drift area… Many other comments here.
Table 3:
- Is total drift deposition correspond to DP described in eq 2 and 4 ?? the way this total drift deposition is calculated is not explicit… since not consistent with Table2…
- The reference spray was a no-rotor situation with a wind of 3 m/s ??? I don’t see the point comparing the different spray settings (rotor + pressure) at other wind speeds?
- Is there a way to translate target deposition into a recovery rate (estimated from the total outflow)?
- Why do target deposition values increase as the wind increases, this does not sound logical
- Comparing with Table 2, it seems there is a problem with 2 last rows values that are mixed up?
Table2 :
- In this case the total drift deposition corresponds to the sum of individual deposition
- The reference spray was a no-rotor situation with a wind of 3 m/s ??? I don’t see the point comparing the different spray settings (rotor + pressure) at other wind speeds?
Page 7 line 247-250. “It can be clearly seen…” This sentence is not as clear as it indicated. Where are the 0-2m and 2-7m values in the table 3 exactly ?
Page 7 line 254 : 3.61% / 1.77% how these percentages were calculated exactly?
Page 8 line 273 : It is confusing to express deposition ratio between areas (199%/142%....) a single ratio value (1.99/1.42) is easier to understand.
Page 8 line 287: the reference spray is still yet (well) defined…
Page 11 Table 4 : I don’t understand why data for the reference are different from those in Table2 and Table3… What is Shimizu? Water? This table is also inconsistent since spray modalities are compared with different wind speeds than the reference. If the reference was tested at 3 m/s then the effect of pressure and rotor is be compared at the same speed. Otherwise authors have to consider to test the reference at 1 and 5 m/s as well.
Figures page 12 and 13 are worth to be redrawn according to previous comment.
