High-Speed Digital Photography of Gaseous Cavitation in a Narrow Gap Flow

Round 1

Reviewer 1 Report

Summary of the Work

The aim of this work is to study the gaseous cavitation in narrow gap flow, such as the lubrication films in journal bearings or squeeze film dampers, by using a high-speed digital photography. The authors presented experimental results of cavitation based on high-speed photography of a lubricant-type flow in eccentric gap between a rotating cylinder and a cylindrical cavity. The film thickness was manipulated by means of a mechanism causing gaseous cavitation and pseudo-cavitation.

Main Results Obtained

- The time resolution of the high-speed imaging system provided digital photography of the entire bubble development from inception to the end of the shrinking process in detail. This allowed getting quantitative data such as displacement, pressure and temperature measurements and accruing data for the application in numerical simulations.

- The authors established the existence of three phases that divide the process of bubble growth which are sequentially showing:

1) a first phase characterised by a gaseous cavitation;

2) a second phase characterised by a co-existing of gaseous and pseudo-cavitation;

3) an ending phase characterised solely by pseudo-cavitation.

The computation of two-phase flows was based on the volume-of-fluid model. The authors’ approach utilises the Schneer-Sauer model, which depends on empirical factors for bubble increase and decrease.

General Considerations

- This paper belongs to a series of works in the field produced by the authors (one of them in particular).

- Please, make sure that all acronyms cited in the work are specified when they first appear in the text, even when the acronyms are well known in the literature (e.g., PMMA etc.).

- Please check the manuscript, there are some typos.

- A vulnerable aspect of this work is that it does not investigate modifications or improvements of the classical Schnerr-Sauer model able to take into account other effects of cavitation bubbles of practical interest (please, see the suggestions below).

- Even though the list of references associated with this work is rather extensive, the authors did not sufficiently mention other models of dedicated to the modelling of cavitation in thin liquid layer recently appeared in the literature (such as advances in analytical and numerical modelling, or thermodynamic aspects of cavitation etc.).

- The experimental apparatus is well described. However there some conceptual points that need to be clarified.

Suggestions

1) Please, specify the boundary conditions for P.D.E. (8) (on page 11) governing the dynamics of the volume fraction a and the fluid velocity u.

2) The concept of pseudo-cavitation is not defined in a clear way. Please, provide in more precise terms this type of cavitation.

3) At which cavitation number occurred the observed quasi-cavitation? Less than 1.5?

4) This question is related to the previous one. Attached cavitation can take several forms. When the attached cavity closes on the suction surface of the foil, the condition is referred to as partial cavitation. This is the attacked form of cavitation that is most commonly observed. For the sake of clarity, the authors are asked to briefly mention, perhaps by including the answer in the manuscript, the difference between the partial cavitation and the observed quasi-cavitation.

5) Please specify the types of the observed cavitation. Are they of inertial cavitation or of hydrodynamic cavitation? In their experiments, did the authors observe suction cavitation?

6) Have the authors investigated the effect of turbulent fluctuations? Please note that in case of turbulence the classical Schnerr-Sauer cavitation model for unsteady cavitating flow should be improved (or squarely replaced).

7) In many flows of practical interest, we observe the periodic formation and collapse of a "cloud'' of cavitation bubbles (the so-called cloud cavitation). The temporal periodicity may occur naturally as a result of the shedding of cavitating vortices or it may be the response to a periodic disturbance imposed on the flow. In principle, the authors' apparatus should have detected the coherent collapse of the cloud of bubbles which could have caused a more intense noise. In this case, the cavitation bubble cloud modelling can hardly be modelled using the model. Dispersed gas/vapor bubbles can be traced, for example, in a Lagrangian fashion and their compression and expansion can be described by a modified Rayleigh-Plesset equation. Authors are invited to comment on this.

8) The authors stated that one of the goals to be achieved in the future is to investigate the bubble diameter in relation to the film thickness in order to deeply understand of its effect. Generally, the cavitation model consists of a coupled problem between the compressible Reynolds partial differential equation that describes the flow and the Rayleigh–Plesset ordinary differential equation that describes micro-bubbles evolution. May the authors be more explicit and make this point clear. How do they intend to study this effect, by using the theoretical approach illustrated in section 3. of this manuscript?

Conclusions

Cavitation is been considered as a fundamental element to correctly describe the characteristics of lubricant films. In my opinion, the work deserves to be published as the experimental apparatus based on high-speed digital photography is really innovative. However, the work shows some vulnerable aspects as far as modelling is concerned and above all the fact that it does not dwell in depth on the different mathematical models of cavitation recently appeared in the literature. The suggestions expressed above are intended to draw the authors' attention to some aspects that in my opinion need to be better explained. I encourage the authors to answer the raised questions; this will increase, in my opinion, the soundness of their work.

Author Response

see file "Reviewer_1.pdf"

Author Response File: Author Response.pdf

Reviewer 2 Report

Cavitation Research in tiny gap flows, such as lubricating films in journal bearings or squeezing film dampers, is difficult work due to spatial constraints paired with a high temporal resolution. The typical lubricating layer thickness is a few microns, and the characteristic time for bubble creation and collapse is less than a few milliseconds. The authors created a Journal Bearing Model Experiment based on similarity principles, which provides entirely identical flow conditions to genuine journal flows while allowing optimal access to the flow using optical measuring equipment. In this paper, the authors show high-speed photography of bubble development and transit in a Stokes-type flow with shear and a large pressure gradient, both of which are common in lubricating films. The dynamic modulation (increase/decrease) of the minimum film thickness, which initiates cavitation in narrow gap flows, is a key component of the experiment. The study in progress includes time-resolved data on the gas release rate and the transient growth of gas bubbles. Both parameters are required for the construction of numerical models for the calculation of two-phase flows.

In my opinion, the results presented in this paper are new and original. However, I want to give some suggestions and point out some minor changes to improve the current version of the manuscript.

Line 26, "dampers is necessary for a robust" should be replaced with "dampers are necessary for a robust".

Line 28, "Wilson [1] particularly" should be replaced with "Wilson [1], particularly".

The linguistic quality of this paper is not good. The authors should enlist the help of a professional English language editor or colleague.

The authors did not present a strong motivation for the article and the acquired results in the introduction. After the motivation section, they should go over the primary contributions of their work in-depth, as well as the significance of the present work.

The reference format is not the same. Please make the format of references according to the journal’s requirements.

Author Response

see file "Reviewer_2"

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors answered some (not all) of the questions raised in my previous report by improving the quality of the work. In my opinion, this version of the manuscript deserves to be published, even if the work still shows some vulnerable aspects regarding modelling. Anyway, I appreciate the extra efforts made by the authors.
By the way, in response to my question 3): "At what number of cavitation did the observed quasi-cavitation occur? Less than 1.5?", The authors asked me to qualify my statement and/or provide a reference. Consequently, I provide below five relevant references on this concept. As far as I know, this terminology ("quasi-cavitation") was first introduced by D.W. Taylor in 1928 in his report presented to the "National Advisory Committee for Aeronautics" (see below ref.[1]).

[1] D.W. Taylor, "Some Aspects of the Comparison of Model and Full-Scale Tests", Report Nº 219, National Advisory Committee for Aeronautics (1926).

[2] J. Y. Zhao, I. W. Linnett, and L. J. McLean, "Unbalance Response of a Flexible Rotor Supported by a Squeeze Film Damper", Journal of Vibration and Acoustics, 120, pages 32-38 (1998).

[3] F. Ruiz and Lu He, "Turbulence under quasi-cavitating conditions: a new species?", 9, Issue 4, pp. 419-429 (1999).

[4] M. Dular and A. Osterman, "Pit clustering in Cavitation Erosion", Elsevier, Wear, 265, pages 811–820 (2008).

[5] K. Przystupa, B. Ambrożkiewicz, and G. Litak, "Diagnostics of Transient States in Hydraulic Pump System with Short Time Fourier Transform", Advances in Science and Technology Research Journal, 14, Issue 1, pages 178–183 (2020).

Author Response

attached you will find the minor revised files

Author Response File: Author Response.docx