Numerical Investigation on Influence of Number of Bubbles on Laser-Induced Microjet

Round 1

Reviewer 1 Report

 The article “Numerical Investigation on Influence of Number of Bubbles on Laser-induced Microjet” authored by Ishikawa, Nishida, and Tagawa, presents simulations of microjets generated in a channel from the pressure field associated with laser-induced microbubbles using an inviscid 5-equation multiphase model. This paper attempts to explain the mechanisms associated with experimental observations of microjets generated by laser-induced bubbles. While this is an interesting and relevant attempt, the following must be addressed before the paper can be considered for publication. 

1. While the authors provide a detailed account of the equations and numerical schemes, it is not clear if this was implemented in a previously documented code or if the authors have specifically developed the code for the present simulations. In either case, a section on the validation of the solver (not just the numerical schemes) is necessary. 
2. The choice of grid spacing and time-step needs explanation. Have the authors / previous workers carried out a detailed grid independence study? I would also like to see the grid.
3. A more detailed introduction should be provided with a broader, general fluid mechanics audience in mind. 
4. Please clarify what quantify is plotted in Figs. 4 and 5. I would like to see density and pressure contours and velocity vectors, which would more clearly show the bubble motion as well as the jet formation. 
5. How do the present simulations compare with previous experiments and simulations? Which findings are consistent with previous work, and what are the new results first observed in this paper? These must be clearly stated. 

 The writing must be improved throughout the paper. For example

Line 14

“ Next, we show that the propagation of pressure waves has two types: from a bubble to a meniscus and reciprocating around a bubble.”  Consider rewriting sentence.

 Line 30 :

 “ and the jet speed is only about one order of magnitude smaller”  -  remove ‘only’. 

 Line 61 :  The correct wording is “there has been a lot of research” 

 I believe subscript l stands for liquid and g for gas, but please state clearly.

 Eqn 1.   Does “i” refer the fact that the same equation is valid for both he liquid and gas phases, that is i can be either l or g ?  Please clarify. 

 Line 82 : The correct wording is “detailed structure”

 Line 110 :  Suggested wording : “due to its nonconservative nature”

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper used a numercial method to examine the microjet produced by a liquid-filled capillary. Some interesting results about the microjet from laser heating have been obtained, however, there are still some problems to be improved.

1. The last sentence in the par of 1. Introduction: "It can be achieved in the two-dimensional simulation because the pressure wave interaction with bubbles and/or walls in a two-dimensional microchannel is expected to have the same phenomena as in a microtube."

The reviewer could not agree with this statement, because your simualtion system of Fig.2 corresponds to a channel with infinite depth, other than a microchannal. Therefore, you are obliged to justify the difference between your 2D numerical system and the real 3D system.

2. The first line on Page 4 has an extra space.

3. How to determine the bubble initial conditions, including radius, reference pressure, temperature and so on?

4. Did you validate your numerical results with Lohse's experiments? Or other groups' data?`

5. About the wave reflection on the wall, have you consider the energy loss after the reflection?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript investigates laser-induced microjet dynamics. The topic is interesting and of engineering importance. The results are new and reasonable. I have several minor comments.

1, In the governing equations, how do you consider the effect of meniscus shape? for instance, if the meniscus change from concave to convex?

2, How to accomplish different modes (from single bubble to multiple bubbles) technically? and how to realize this in the simulation?

3, The pressure shown in Fig.7 an10 is dimensionless?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I am satisfied with the revision and the response to my comments.  The paper may be accepted in its present form.  The authors could consider showing velocity vectors, for example in Fig. 10.