Numerical Simulation on Primary Breakup Characteristics of Liquid Jet in Oscillation Crossflow

Round 1

Reviewer 1 Report

It is necessary to add a more detailed description of the software used

The text is written in quite understandable English

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Numerical simulation on primary breakup characteristics of 2 liquid jet in Oscillation Crossflow

Tao Zhang, Xinyu Song, Xingping Kai*, Yeguang He and Rundong LI

Computational research has been accomplished using the volume of fluid research and scale adapting mesh structure to determine if the incoming air flow oscillations would affect the break-up process of the fluid that is injected into the air flow.

Summary review:

The results are similar to the papers referred in the current paper. It is not clear that if one should use oscillatory incoming airflow or not during the liquid jet injection. The results are hindered/limited with the domain size and the number of cycles.

I am not sure that this paper adds more information to the flow physics either. Studying the formation of particles only two periods is not enough to get an overall convincing picture of the breakup phenomena.

A few questions, suggestions:

·         Page 2 line 46 : “ Bunce et al. [2] found that the uniformity of spray in oscillating crossflows is improved obviously”.. I am not sure if the word “obviously” was the word that was intended to be used there.

·         Page 10, line 267: change “first and two” to “first and second”

·         Figure 17: is the “y” given in the legend as same as the “y” described in equation 4?. Is that basically the oscillating incoming air velocity?

·         The introduction summarizes only three investigations with differing results. This might be the reason why the current investigation was conducted, but the introduction does not describe well the common consensus at the moment to what the previous results indicate.

Does the break-up increase with transverse oscillations?

·         It is not clear if a commercial code was used in this study. If that is the case please name the commercial code. If not please let us know what code was used. Was that a code written in house?

·         Page 3 Figure 1, caption: there are two “a)”.. I am sure one was intended to be b).

·         Figures 2 and 3 and associated values: the domain seems to be rather restricted for the study. Considering the main air flow is subsonic, the distances may have an effect on the solution itself. A typical flow field, say for a study over an airfoil may need 20 chord lengths upstream and so many downstream. Since there is no limit to the ending of the spray jet the downstream length could have been left longer, and this would also mean that the sideways distances are also rather short. I am guessing these distances were chosen based on a criterion, or with comparison to previous calculations. A few references would be useful at this point to make sure that the domain size is not going to affect the results.

In addition, the 4 ms time duration is also short to understand the breakup phenomena that can be observed. The incoming flow oscillations are for only 2 periods. This makes one to wonder if the results could change if one used 10 or 20 periods of oscillations. Please find references to support your choice of the periods.

The effects of the domain may be visible in the Figure 5. There there is no means to prove or disprove that the experimental curve fit is right or that the calculations are right. Please find some more references to make sure that your choice of the domain size is correct.

·         Figure 9: this figure definitely show the effects of the sidewalls on the flow field around the jet. Please clearly discuss why the sidewalls were placed so closely to the jet. Was that because of the limitations for a particular injector geometry?

The plots also show rapid variations of the contours looking like steps. Did the sidewalls have inflation layers, or were they considered as zero friction walls ?

Numerical simulation on primary breakup characteristics of 2 liquid jet in Oscillation Crossflow

Tao Zhang, Xinyu Song, Xingping Kai*, Yeguang He and Rundong LI

Computational research has been accomplished using the volume of fluid research and scale adapting mesh structure to determine if the incoming air flow oscillations would affect the break-up process of the fluid that is injected into the air flow.

Summary review:

The results are similar to the papers referred in the current paper. It is not clear that if one should use oscillatory incoming airflow or not during the liquid jet injection. The results are hindered/limited with the domain size and the number of cycles.

I am not sure that this paper adds more information to the flow physics either. Studying the formation of particles only two periods is not enough to get an overall convincing picture of the breakup phenomena.

A few questions, suggestions:

·         Page 2 line 46 : “ Bunce et al. [2] found that the uniformity of spray in oscillating crossflows is improved obviously”.. I am not sure if the word “obviously” was the word that was intended to be used there.

·         Page 10, line 267: change “first and two” to “first and second”

·         Figure 17: is the “y” given in the legend as same as the “y” described in equation 4?. Is that basically the oscillating incoming air velocity?

·         The introduction summarizes only three investigations with differing results. This might be the reason why the current investigation was conducted, but the introduction does not describe well the common consensus at the moment to what the previous results indicate.

Does the break-up increase with transverse oscillations?

·         It is not clear if a commercial code was used in this study. If that is the case please name the commercial code. If not please let us know what code was used. Was that a code written in house?

·         Page 3 Figure 1, caption: there are two “a)”.. I am sure one was intended to be b).

·         Figures 2 and 3 and associated values: the domain seems to be rather restricted for the study. Considering the main air flow is subsonic, the distances may have an effect on the solution itself. A typical flow field, say for a study over an airfoil may need 20 chord lengths upstream and so many downstream. Since there is no limit to the ending of the spray jet the downstream length could have been left longer, and this would also mean that the sideways distances are also rather short. I am guessing these distances were chosen based on a criterion, or with comparison to previous calculations. A few references would be useful at this point to make sure that the domain size is not going to affect the results.

In addition, the 4 ms time duration is also short to understand the breakup phenomena that can be observed. The incoming flow oscillations are for only 2 periods. This makes one to wonder if the results could change if one used 10 or 20 periods of oscillations. Please find references to support your choice of the periods.

The effects of the domain may be visible in the Figure 5. There there is no means to prove or disprove that the experimental curve fit is right or that the calculations are right. Please find some more references to make sure that your choice of the domain size is correct.

·         Figure 9: this figure definitely show the effects of the sidewalls on the flow field around the jet. Please clearly discuss why the sidewalls were placed so closely to the jet. Was that because of the limitations for a particular injector geometry?

The plots also show rapid variations of the contours looking like steps. Did the sidewalls have inflation layers, or were they considered as zero friction walls ?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors,

Thank you for sending me the manuscript "Numerical simulation on primary breakup characteristics of liquid jet in Oscillation Crossflow" for revision. The Reviewer has the following observations:

1. Minor editing of the English language is required.

2. The Introduction is short. The Authors could give more references on the subject. 

3. As the motivation, the Authors state that their research could be used to further study combustion chambers. However, the differences between the Authors' research and combustion chamber geometries and operation conditions are evident. 

4. Line 61. Mathematical physical model. The Authors could provide more justifications for model choices, including the references.

5. Line 98. The Authors could give more details about adaptive grid refinement criteria.

6. Line 137. Verification of calculation results. The Authors validated their research with a single and general (integral) formula (8). At the same time, the Authors study liquid breakup in a finite element approach. It would be helpful to validate the results locally as well.

7. Fig. 9. The wall boundary layer looks rough and non-uniform. Please explain this aspect. What does a red contour around the liquid jet mean? 

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

The authors investigate the penetration depth of a pulsating jet into crossflow using numerical methods. The instantaneous results are compared to empirical relations. Nonetheless, a critical validation is missing. - Physical mechanisms are not well described.

Not all the required specifications of the numerical approach are given.

Commonly the tube of the jet needs to be included in the numerical domain to get correct results at the root of the jet.

Please name the solver used.

How do the authors know that the surface waves are a Kelvin-Helmholtz instability?

In Figure 12, the authors write "under steady state condition". What does that mean?

The authors reference very few other works. I cannot find the references 6, 9, and 13. I don't even find a journal that is called "Propulsion Technology".

There are many other relevant works that the authors should include in the literature review, e.g. Nygard "Analysis of vortical structures in intermittent jets".

Moderate language editing is required. E.g. it is upwind and not on wind.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 4 Report

The manuscript improved over the revision. However, there are many details, which are not explained, for example regarding the numerical method. There is not statement, whether the homogeneous or inhomogeneous vof forumlation has been used. There is no statement on the interface compression or how it would affect the surface waves. Concluding, there is not enough informaion in the article that one can state that the conclusions are real.

There are many language error remaining.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 3

Reviewer 4 Report

The manuscript improved over the revision.

There are many language error remaining.