Experimental Parameters Influencing the Cavitation Noise of an Oscillating NACA0015 Hydrofoil
Round 1
Reviewer 1 Report
The manuscript addresses very relevant questions concerning an increase of the understanding of the relevant mechanisms associated with ship propeller cavitation noise, but there are a number of issues to be solved to make this manuscript suited for publication.
1) The authors link their study to the concern about the effects of shipping noise on marine fauna, but appear to be unaware of the relevant frequencies for such impact. The harmonics of the blade passing frequency that are addressed in this study cover just a small fraction or the ship noise spectrum. These frequencies are, for example, generally not very important for marine mammals. The first three references in the manuscript are quite old. There are many more recent articles that discuss the effects of shipping noise on marine life, such as for example [Erbe C, Marley SA, Schoeman RP, Smith JN, Trigg LE and Embling CB (2019) The Effects of Ship Noise on Marine Mammals—A Review. Front. Mar. Sci. 6:606. doi: 10.3389/fmars.2019.00606 ].
2) The authors calculate the cavitation volume V(t) from the measured sound pressure p(t), using eq.(3). This equation is only valid in free-field conditions and in the farfield from the sound source. It is unlikely that these conditions hold in their experimental setup. Hence, it is also unlikely that the calculated V(t) represents the actual cavitation volume. The authors should describe in more quantitative detail how they checked the plausibility of their assumptions (line 123-126). Was the acoustic response of the cavitation tunnel measured? How did the authors estimate 3D cavitation volume from 2D video observations?
3) Figure 3 presents an overview of the tested conditions. This needs further explanation. Are these numbers based on measurements of tunnel pressure p0, flow speed U and temperature T ? These must be described. Why are there variations in reduced frequency? It would help to provide the actual values of speed and pressure in addition to the non-dimensional numbers.
4) Figure 5 (upper) illustrates that the authors had difficulty controlling the AoA oscillations. Consequently, the three different mean cycles do not only differ in the speed at which the AoA is reduced, but also in the minimum AoA that is reached during each cycle. That makes it more difficult to understand what drives the difference in sound pressure (and in the derived cavitation volume). The authors suggest (l.193-194) that a fast decrease of AoA generates the highest sound pressure. This does not seem to be confirmed by Fig.10, which suggests that the sound pressure decreases with increasing oscillation frequency. May I suggest to analyse the sound pressure as function of the time derivative of AoA?
5) the authors argue (l.77-78) that the AoA oscillation frequencies of their setup are in the range of blade passing frequencies of ships. However, due to the applied scaling, this comparison should be made in terms of the reduced frequency.
6) the authors must provide upper and lower frequencies to describe pass and stop bands (l.120-123)
7) the authors should explain how the frequency spectra of sound pressure (figs 4, 7, 9, 11) were calculated. What averaging time and what time window was used?
8) The two spectra in figure 4 are for two different conditions. The authors need to explain why are these representative, or provide a broader overview of signal and noise for the various conditions.
9) The authors should explain how 'figure 7 shows that the frequency range of interest lies below 10fosc' (l.183). What determines the 'interest'?
10) In l.244 the authors introduce 'the amplitude of oscillating cavitation length'. This needs further explanation.
Author Response
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Reviewer 2 Report
1. Should the results analysis section include more detailed comparative analysis of cavitation noise levels for different courses of the angle of attack?
2. Besides the courses of the angle of attack and cavitation number, were any other parameters varied or controlled? Please provide specific information about the selection of these parameters and their potential influence on the cavitation noise results.
3. Did the paper provide a more in-depth discussion of the implications and applications of the research findings? How can these research results be applied to practical designs of quieter propellers? Are there any limitations or challenges in implementing these findings in real-world scenarios?
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Reviewer 3 Report
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Round 2
Reviewer 1 Report
The authors have not considered the major revision requested in my first review.
I do not agree with their statement in the response to my comment 'Point 2' that 'The acoustic monopole model is not limited to far-field'. This relation between sound pressure and sound particle acceleration does not hold at distances from the source that are much shorter than one wavelength. In the newly added paragraph (l.136-146) the authors suggest that the calculated quantity is qualitatively related to volume, which is a hypothesis. This limitation should be considered in the discussion section.
Moreover, the authors have not addressed my comment that they do not demonstrate if it is the steepness of the AoA variation or the minimum AoA that drives the cavitation behaviour. They ignore that the oscillation frequency affects the cycle length and therefore also the steepness.
Hence, I am not satisfied with the response and maintain the position that this manuscript needs a major revision.
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Reviewer 2 Report
none
Author Response
Thank you very much for reviewing our manuscript. Your comments have been appreciated.
Round 3
Reviewer 1 Report
Though I do not agree with the author's response to my comments, I do not have the time to go into further discussion. The proposed solutions remove my main concerns.
It would help if the final advise (l.299-304) could be described in terms of propeller design parameters instead of parameters describing the cavitation volume.
Please check the reference list. several references are incomplete.
The
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