Direct Numerical Simulation of Bubble Cluster Collapse: Shape Evolution and Energy Transfer Mechanisms
Round 1
Reviewer 1 Report
The manuscript presents a study on the collapse progress of near-wall bubble clusters using the Volume of Fluid (VOF) method for direct numerical simulation. The inclusion of viscosity, compressibility, and surface tension allows for the analysis of the energy evolution within the flow field. The research focuses on the collapse of cubic bubble clusters and columnar bubble clusters, resembling cloud cavitation. However, a few improvements can enhance the clarity and impact of the study:
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Provide a clear motivation for the research, explaining the practical relevance and engineering applications of understanding the collapse mechanism of bubble clusters.
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Discuss the specific methodology and numerical techniques employed in the VOF method for direct numerical simulation, including any assumptions or limitations associated with the approach.
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Clarify the significance of the findings in the context of existing literature on bubble cluster collapse. Highlight any novel contributions or unique aspects of the study.
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Expand the discussion on the implications of the research findings for practical applications, such as cavitation erosion prevention and prediction, as well as potential applications in ultrasound-based therapeutic diagnosis.
Minor editing of English language required.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
(1) The total energy could be defined together with the bubble potential energy, flow kinetic energy, and pressure wave energy. A confusion may arise when examining Figure 4, since the three defined energies do not give a constant sum, meaning that the total energy has other components which had not been defined. It would be relevant to know this since the focus of the paper is on energy transfer mechanisms.
(2) The deviation of the numerical results from the analytical solution in Figure 1 appears to be on the order of several percent for T* > 0.5. This appears small but could it propagate signifcantly when calculating more complex problems or even in this study?
(3) Small 'p' could be used in the 'delta P' term of Equation 10 just to be consistent. Also, a minor issue, having brackets as in (delta p)^2 would prevent the unlikely misunderstanding of the term being 'delta(P^2)'
(4) The inter-bubble spacing (D) in Table 1 should be 2.5, rather than 2.0, as stated in the text?
(5) Lines 190-194 talk about symmetry of the cubic 64-bubble cluster causing the wave energy peak in the center of the cluster to be unobservable. Maybe one could use the plane a half-unit-cell from the centre (i.e. plane of bubbles rather than fluid) in this 64-bubble case, and compare it to the central plane of the 27-bubble case (also a plane of bubbles) for direct comparison of asymmetric versus symmetric cases. The same could be done for the planes of fluid in both cases (i.e. central plane of 64-bubble case, and off-centre plane of 27-bubble case).
(6) Spelling error in Line 219. 'analyse' instead of 'analysis'.
(7) The conclusion or end of the results section could include some context relating to cavitation erosion mentioned in the introduction, which is also a great concern for researchers of cavitation phenomena.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
