Intrusive and Impact Modes of a Falling Drop Coalescence with a Target Fluid at Rest

Round 1

Reviewer 1 Report

This paper reports in details new experiments concerning the impact of drops on a free surface. The investigation is governed by energy leading consideration which is original and consists in a relevant point of view. The research team is worldwise known for the quality of performed laboratory experiments at low Reynoldsnumbers and stratified and multiphase flows. This new experiment are revisiting former ones by using modern very high resolution and high speed optical and data processing facilities with a carefully prepared setup. The first section (introduction) of the paper gives a comprehensive, engaging and extensive review of previous studies in the field which will be very usefull for next investigators.
Section 2 is particularly important because, due to the very small size and short time scales of observed secondary and even singular structures, generally admitted assumption of fluids as continuous medium reaches its limits and mechanical and thermodynamical quantities need to be defined and reconciliated.
Relevant non dimentional parameters and dimensional quantities are recalled to justify the scientific approach dealing chosen experiments and their analysis.
The experimental setup is explicitely presented in such a way experiments are made fully reproductible.
Based on energy balance, different experiments are reported and precisely described with very high quality photographs and quantitative synthetic figures. Secondary flows an structures are described in details and their occurence is clearly explained. The collection of new results presented here open several gates in fine complex flow analysis.

In conclusion, this paper gives a complete description of new experiments following a very comprehensive scientific pathway. I recommend this paper for publication.

A few misprints can be corrected, lines 214, 256 (mu_i), 283 (-g if z is upwards), l 340 (APSE need to be defined here instead of line l 354 where it becomes APES)...
 

Author Response

Dear Reviewer,

The authors are deeply grateful to you for your attention paid to reading and analyzing the content of the article, evaluation of the results of the work and useful comments aimed at correcting typographical errors and facilitating the perception of the text.

Corrections made to the text are highlighted.

With many thanks and gratitude,

Yuli D. Chashechkin and AndreyYu. Ilinykh

Reviewer 2 Report

This work presents an experimental study of the flow behavior when a drop falls into another fluid body at rest. The experiments tracked the evolution of the drop matter after it contacts the target fluid, as well as the free-surface motions. Two modes, slow intrusive and fast impact flow modes, are considered and compared. The paper gives detailed descriptions of the characteristic flow patterns and discusses the potential physical quantities controlling the flow patterns at different stages for both modes. I consider this work is of interest to the studies of interfacial flows. However, the manuscript has some weaknesses that need to be addressed before publication:

1. The introduction provides detailed review of the literature. However, it is not clear what the novelties of the present study are. I would recommend the authors to add a paragraph discussing specifically the goal that this study achieves over existing works.

2. The authors did not clearly explain the difference between the set-ups of the experiments shown in figures 3 and 4; the parameters given are the same except for the ink solution while the flow features seem to differ considerably. Why the flow features differ? If the difference is owing to the run-to-run variation. Why use different ink solution?

3. Line 551: figure 4 -> figure 5?

4. The authors use dimensional values extensively in the presented results. Can the authors discuss how the observed features can be normalized?

 

Author Response

Dear Reviewer,

The authors are deeply grateful for the attention to the article and valuable comments aimed at improving the quality of the publication.

Concerning remarks:

1. At the end of the Introduction, a paragraph was added explaining the novelty of the research setting, in which for the first time (up to the authors knowledge) the fine structure evolution of the drop substance in a transparent target fluid was experimentally studied in two flow modes.
2. Patterns of experiments in Fig. 3 and 4 are shown specifically to show the strong variability of flows at the same flow conditions in the intrusive regime. Text is supplemented by explanation.
3. Noticed by the reviewer and other typos corrected.
4. To discuss the normalization of data, a deeper study of the matter transfer processes in droplet flows is necessary on the basis of a system of fundamental equations of fluid mechanics.

With many thanks and gratitude,

Yuli D. Chashechkin and Andrey Yu. Ilinykh

Reviewer 3 Report

The authors present here mathematical modelling and experimental work related to drop impact flow. They claim to have an improved technique for observation of the flow. The paper content and presentation is very impressive. I have however, some comments which authors should address before its publication. 

1) The authors should should clearly mention the novelty of the present work in the introduction section of the paper. In addition, I am also unable to figure out scientific content confirming this claim. This means it would be nice (if possible) if the authors can compare this to any existing study and demonstrate that the presented technique is superior to any previous techniques. This comparison would add a lot to the content to the paper. 

2) Line number 160-172: The authors discuss about solution using Navier Stokes equation which basically means involving CFD codes. It would be nice to expand this paragraph a little more and include some more proper references for these CFD codes. for example couple of such code is REEF3D (https://doi.org/10.1016/j.oceaneng.2022.111211) and OpenFOAM. Recently, use of DNS is also being done to solve these problems. It would be good to include this in the paper. 

 

Author Response

The authors are deeply grateful for their attention to the work and valuable comments.

A paragraph was added in the Introduction explaining the novelty of the work related to tracing the systematic changes in the regular distribution pattern of the drop substance in the target fluid. The discontinuity in the distribution of the drop substance is associated with the influence of the fluid internal energy conversion and transfer processes. The impact of the internal energy conversion in drop impact flow, at the level of the authors' knowledge, has not been studied before.

A paragraph has been added between lines 160-172 discussing the results of numerical simulation of drop impact flows based on the Navier-Stokes equations in one- or two-fluid approximations. Accordingly, the list of references has been expanded.

 

With respect and gratitude,

Yu.D. Chashechkin and A.Yu. Ilinykh

Round 2

Reviewer 2 Report

The authors have addressed my concerns to my satisfaction.