The Hydrodynamic Noise Suppression of a Scaled Submarine Model by Leading-Edge Serrations
Round 1
Reviewer 1 Report
The manuscript reports an experimental and numerical investigation of a leading-edge serration applied to on the sail hull of a scaled submarine model. The style is clear and care of detail is devoted to the mathematical and physical arguments in support of the analysis. However, I can recommend a publication in archival journal form only after a major revision based on the following comments.
1) The introductive section is almost complete. I would have quoted one of the few available works focused on realistic usages of leading-edge serrations Casalino, et Al., “Aeroacoustic Study of a Wavy Stator Leading Edge in a Realistic Fan/OGV Stage”, Journal of Sound and Vibration, Vol. 442, 2018.
2) The introductive section reveals a fundamental misunderstanding about the aeroacoustic effects of LE serrations. Most of the quoted aeroacoustic studies are focused on reduction of interaction noise, when airfoils interact with convected turbulence or impinging wakes. This is the reason why the ratios between the integral scales of upstream turbulent disturbances and the serration amplitude/wavelength are important parameters. However, the way the authors link these observations with the generation of streamwise turbulent structures that affect the boundary layer development, or streamwise vortical structures, clearly reveal that they are fundamentally confused about the involved physical mechanisms. The statement at line 52 is what matters for interaction noise and should be better elaborated.
3) The paragraph describing the differences between air and water is not convincing and should be reformulated in a more correct way. Only Mach, Reynolds and Helmholtz numbers (or reduced frequency) are important. Some examples of relevant cases in air and water can be useful to support the authors’ arguments.
4) The comment about the materials and the difference between plastic (in air) and metal (in water) is not properly stated. In the field of aeroacoustics in air, the vibro-acoustic effects are typically neglected (I would say, by the definition), and nobody cares about the absorption properties of plastic parts. By the way, the usage of plastic is not as common as the authors’ believe.
5) The statement about vibration at line 140, reveals again a fundamental confusion about aeroacoustics and sound generation mechanisms. Moreover, the physical arguments behind the mathematical derivation of section 2.2 are wrong or not properly stated. In addition, quantities are erroneously defined or not defined at all. The authors state the problem as a purely vibro-acoustic problem, without any connection with theory of sound generated aerodynamically. The correctness of this assumption should be supported by arguments that are currently missing.
6) The benchmark case in section 2.3 should be explained in more detail. In this form it reduces to a plot showing two spectra. Curiously these spectra include some tonal noise component, which is not something the vibro-acoustic model of section 2.2 deal with. That model is in fact based on turbulent boundary layer over a flat plate (Corcos). I have serious concerns about the correctness of the validation process.
7) The connection between sections 2.1 and 3.2 is not clear. It looks like these are two separated entities: one looks like a generic description of an LES model, the other looks like a part of a software manual. Implications of usage of a wall function as well as of a PISO algorithm should be discussed.
8) Resolving boundary layer fluctuations with LES require a certain care in the way fluctuations/transition are triggered, and how these turbulent structures are convected. No information about these crucial aspects are provided, and this induces to suspect that the authors “just” made two runs and obtained some wall pressure fluctuations., without clearly understanding what they have got. If, on the other hand, the focus is only on how the serration affects the dynamic of the horseshoe vortex, then this would be clearly stated at the beginning of the work. However, this would invalidate many aspects of the work: the need of using LES (we are talking about a coherent large scale vortical structure), the usage of the vibro-acoustic model, the need of extending the serration along the whole span, etc.
9) The effect of the serration on the BL flow in proximity of the TE is not very significant, unless the authors’ show the aeroacoustics implications of this small difference. It would be more meaningful, for instance, to compare BL profile of rms velocity.
10) Units in Fig.11 should be detailed (SPL, which band, etc.). It is hard to understand if what we see is physical or not. Besides this, the impression is that the serrated LE case exhibit higher fluctuation levels at low and high frequency. Furthermore, I don’t see the reason why the contour levels are smoother for the serrated LE case, showing a sort of interference pattern on the submarine body.
Author Response
We will ask the MDPI service to check the manuscript's English level. The response to the questions can be found in the Word file.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Congratulations to the authors for interesting and well written paper.
In order to observe comparison of pressure distribution (Figure 6), please put same scale for both cases. Is better to increase the text for the legend. Also, please add some pressure differences in the text (in procents for example), in order to quantify the improvement when serration are added.
For Figure 7 and 8, is good to add in front of them a picture with the all numerical domain and a capture/detail were the analysis is made.
Discussion: the last part of the paper concentrated on the optimisation of the serrations. Taking into account that the submarine operates on wide range on speeds, also the velocity should be taking into account on the optimisation process. Do you perform this study ?
Author Response
We will ask the MDPI service to check the manuscript's English level. The response to the questions can be found in the Word file.
Author Response File: 
Author Response.doc
Round 2
Reviewer 1 Report
Some of may concerns have been properly addressed.
