Droplet Size Distribution Characteristics of Aerial Nozzles by Bell206L4 Helicopter under Medium and Low Airflow Velocity Wind Tunnel Conditions and Field Verification Test

Round 1

Reviewer 1 Report

In general, the introduction section is not well addressed. It contains a lot of piled information not adequately organized. The references used in the introduction are not the most relevant for introduce your research; the reference that you have used are suitable to support your discussion but are not adequate for the introduction. You need to introduce the research topic using reference to wind tunnel measurements, WSP deposition measurements, relationships between droplet size spectra and WSP including pro and cons of this methodology (there is a lot of papers related to the WSP and spot size measurements). The main aim of introduction is to provide to the readers a comprehensive framework in which your research is placed, underling the importance and  main aims of your research. The objectives of your experimental work are not completely clear (hidden between the lines) making the readers lost. Between the line 109 and 117 I can try to understand that is your intention to compare the droplet size spectra of different nozzles, obtained from laboratory trials using wind tunnel, with the dimension of stains/impact measured on the water sensitive papers –WSP- used in field trials to assess the coverage. Among researchers, it is well known that WSP are good collectors for canopy coverage measurement and estimation of deposition (even if for deposition estimation some authors have faced problems) but not for the measurements of droplet size. At least, you have to clearly state that you are trying to find a relationships between droplet size spectra and stains/impacts dimension. You are not measuring the droplets size on WSP but the stains/impact dimension that is well known to be always bigger than the droplets. Furthermore, as already demonstrated by other authors (Fox et al. 2003), any spot size measurements made on WSP with coverage greater than about 20% were not reliable due to the number of spots that were touching or overlapping. Furthermore, the most common and technical vocabulary is missed throughout the manuscript; e.g. in line 87 you mention “air-conducting nozzle” instead of air induction nozzles used by the author that you are citing (Guler et al., 2007).

In general the material and method section is not fully addressed. The methodology used for WSP spot size measurements and procedure adopted is completely missed. Also the section related to the statistical analyses used for data processing in which is clearly state the methodology used in missed. Some important experimental errors are noticeable. In line 148 you mention “All test nozzles had been used for more than three years.” and this aspects finally affects the results as you have stated between line 380-382 in the conclusion section. In your experimental work you need to test the technology/technique not the elements that are in use from a long time. Planning your experimental research aimed to test the technology you have two option: use new materials or check before the trials that the performance of materials used in the trials have acceptable performance similar to a new equipment. Furthermore, between lines 189-211 is never mentioned the numbers of replicates for each nozzle tested that have to be at least 3.

However, from the graph in line 307 I can understand that you have used the three collectors lines as replicates performing only three tests corresponding to the three nozzle types. It is strongly agreed that replicated means to repeat the experiment three times (3 replicates x 3 nozzles). The three lines can be used only to reduce the variability along the helicopter spraying path and cannot be considered as replicates. As it is, the experimental research has not replication of the three tested configurations.

The conclusions are deep affected by experimental errors due to the experiments design. Furthermore, as already mentioned, some conclusions are obvious (line 392-394) because you are not measuring droplet size but the spot dimension on the WSP.

Even if I’m not in light to judge the English language I suggest an extended language editing.

With regret, from my point of view, I recommend to reject the manuscript as it is at this stage.

Author Response

1. Response to comment: (The introduction section is not well addressed)

Response: Thanks very much for your comments, its helpful for us to improve the manuscript. We have modified the introduction according to the Reviewer’s suggestion.

2. Response to comment:(The objectives of your experimental work are not completely clear (hidden between the lines) making the readers lost)

Response: We have re-written this part according to the Reviewer’s suggestion (line 94-99).

The objective of this study was to draw on the respective advantages of wind tunnel test and field test, understand the droplet size distribution characteristics of the manned agricultural helicopters and their associated flat-fan nozzles commonly used in China, and try to find the relationship between droplet size spectra and stains/impacts dimension. Finally, it is expected to optimize the aviation application mode, provide data support for the nozzles selection of helicopter under medium-low velocity, and provide a reference for precise spray application.

3. Response to comment:(The most common and technical vocabulary is missed throughout the manuscript; e.g. in line 87 you mention “air-conducting nozzle” instead of air induction nozzles used by the author that you are citing)

Response: Thank you for pointing out the mistakes in the professional vocabulary. The statements of “air-conducting nozzle flat fan nozzles” were corrected as “air induction nozzles”, and we have modified the manuscript accordingly.

4. Response tocomment: (The methodology used for WSP spot size measurements and procedure adopted is completely missed.)

Response: In fact, we have given the collection and processing methods of WSP in the materials and methods（line 192-195）.

“After each spray application test, the WSPs were immediately gathered and put into marked envelopes and placed in a cool place to be brought back to the laboratory later. Then WSPs were analyzed by using image processing software DepositScan (USDA. Wooster, USA) to obtain droplet data of each sampling point.”

The software DepositScan can be used to analyze and process the relevant data of droplets on the water-sensitive paper conveniently, its processing principle and method have been reported in detail by others, so this manuscript will not repeat them again.

5. Response to comment:(In line 148 you mention “All test nozzles had been used for more than three years.” and this aspects finally affects the results as you have stated between line 380-382 in the conclusion section. In your experimental work you need to test the technology/technique not the elements that are in use from a long time.)

Response: There are two reasons for using the old nozzle to test：

• Scholars are usually prefer to test new nozzles, but the nozzles will be used for a long time after production.During the entire life of the nozzle, it is only in the new nozzle state when it is first used, and most of the time it is in the worn out old nozzle state. The purpose of this article is to test the atomization characteristics of the actual operation used nozzles in China in order to understand the actual helicopter spraying effect. The test results may be more representative.
• A helicopter needs to install 51 CP nozzles, but in China CP nozzles need to be importedfrom abroad and the company's purchase is limited, which leads us to lack a sufficient number of new CP nozzles for testing. In order to ensure the consistency of the test nozzles, we used the same old nozzle in the wind tunnel test and field test. If conditions permit in the future, we will also consider comparing and testing new nozzles.

6. Response to comment:(The three lines can be used only to reduce the variability along the helicopter spraying path and cannot be considered as replicates.)

Response: Only three times field tests were conducted, which were limited by the test conditions. In China, field tests of manned aircraft required a lot of coordination and preparation. The demands for labor, material resources, and weather conditions is more stringent, and it is also regulated by relevant departments. Ultimately limited by the economic and time costs of helicopter operations, we can only conduct tests for 3 sorties. But in order to improve the accuracy of the test as much as possible, it can be seen from the sampling point arrangement (Figure 5) that we arranged three 40-meter-long sampling lines for each flight, and each line with 41 sampling points. A total of 123 sheets of WSP were required for each test, and it also took a long time.

In addition, we think that the three collection lines set up in the test can also be considered as the repeated of the test. Because the distance between each line was 40 m, which made the lines independent of each other, and their influence can be ignored. Some studies have also dealt with in this way（Lan et al, 2008; Chen et al, 2020）.

Lan Y, Hoffmann W C, Fritz B K, et al. Spray drift mitigation with spray mix adjuvants[J]. Applied Engineering in Agriculture, 2008, 24(1): 5-10.

Chen S, Lan Y, Zhou Z, et al. Effect of droplet size parameters on droplet deposition and drift of aerial spraying by using plant protection UAV[J]. Agronomy, 2020, 10, 195.

7. Response tocomment: (Suggest an extended language editing.)

Response:We have invited several native English speaker help to improve English expressions and correct grammatical errors in the text. We hope the revised paper will be more clear and accurate on expressions.

Special thanks to you for your good comments.

We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.

Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.pdf

Reviewer 2 Report

Dear Authors

you have done a big experiment with a lot of thesis compared. I have only few remarks.

Field test definition

You have define nozzles flow rate and operating pressure. Can you add also the volume application rate (l/ha) to better understand the spray situation?

Line 147

you have tested nozzles in use. Why you make this choiche instead of use new nozzles? Please explain the reason.

Author Response

1. Response tocomment: (You have define nozzles flow rate and operating pressure. Can you add also the volume application rate (l/ha) to better understand the spray situation?)

Response: Thanks very much for your comments, which are very helpful for us to improve the manuscript. We have added spray volume application rate data for each flight test at line 297-299 (Table 5).

The spray volume application rates were 12.83 L/ha (1#), 18.49 L/ha (2#), and 28.58 L/ha (3#), respectively. Corresponding supplements were also made in the text.

2. Response to comment:(Why you make this choiche instead of use new nozzles?)

Response: The main reasons are as follows:

• Scholars are usually prefer to test new nozzles, but the nozzles will be used for a long time after production. During the entire life of the nozzle, it is only in the new nozzle state when it is first used, and most of the time it is in the worn out old nozzle state. The purpose of this article is to test the atomization characteristics of the actual operation used nozzles in in China in order to understand the actual helicopter spraying effect. The test results may be more representative.
• A helicopter needs to install 51 CP nozzles, but in China CP nozzles need to be imported from abroad and the company's purchase is limited, which leads us to lack a sufficient number of new CP nozzles for testing. In order to ensure the consistency of the test nozzles, we used the same old nozzle in the wind tunnel test and field test. If conditions permit in the future, we will also consider comparing and testing new nozzles.

Special thanks to you for your good comments.

We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.

Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper, different aerial nozzles were used, and droplet size measurements were carried out in a wind tunnel and in a field using an agricultural helicopter. In general, this paper contains some very interesting results. The manuscript is well-written and explains the research clearly. However, the authors haven’t used any nondimensional numbers in their work. From fluid mechanics point of view, spray dynamics and liquid atomization as well as droplet size distribution are functions of nondimensional numbers. The main nondimensional numbers are gaseous Weber number, gaseous Reynolds number, liquid Reynolds number, liquid Weber number (or liquid-to-gas momentum ratio), and liquid-to-gas density ratio. To make this manuscript suitable for publication the authors need to add/calculate the nondimensional numbers and discuss their effects on the size distribution.

Author Response

Response to comment: (The authors need to add/calculate the nondimensional numbers and discuss their effects on the size distribution)

Response: Thanks very much for your comments, which are very helpful for us to improve the manuscript. We have re-written this part according to the Reviewer’s suggestion (line 251-265).

In essence, the generation of fine droplets was caused by the aerodynamic fragmentation of the relative movement between the two phases (liquid phase and surrounding gas medium). And the most important parameter affecting the droplet stability was the nondimensional Weber number. As the airflow velocity increased, the flat liquid film was gradually increased by the shearing force of the air, the droplets deformation rate accelerated, and the Weber number also increased. When a certain critical state was reached, the surface tension was not enough to resist the liquid flow, the droplets were continuously stretched, so that the droplets were more likely to be broken and secondary broken, and the droplet size changed. Some studies also showed that for different types of nozzles, when the Weber number was the same, the larger the droplet size, the lower the relative gas/liquid velocity. As a result, the deformation rate of the larger size droplets was slightly lower than that of the smaller size droplets. Moreover, in the above process, there were accompanied by the effect of droplet dispersion and droplet aggregation. The interaction of the two effects on the droplets also caused a series of uncontrollable changes in droplets size.

There are many factors that affect the droplet size distribution. Any factor that affects the droplet size will affect the droplet size distribution. I have to say that at present our analysis and discussion of nondimensional numbers is still relatively limited. But this also points us to a good research direction. We will further study the atomization characteristics of the nozzle from the perspective of fluid mechanics.

Special thanks to you for your good comments.

We tried our best to improve the manuscript and made some changes in the manuscript. These changes will not influence the content and framework of the paper.

We appreciate for Editors/Reviewers’ warm work earnestly, and hope that the correction will meet with approval.

Once again, thank you very much for your comments and suggestions.

Author Response File: Author Response.pdf