Acoustic characterization of air-bubble films in a turbulent boundary layer

Human activities at sea generate impulsive and continuous noise sources that endanger the survival of marine species. Ongoing research and development focus on innovative mitigation devices and techniques. Among them, the bubble curtains represent air bubble barriers able to mitigate the level of crossing sound waves. This technique is currently adopted to reduce the impact of impulsive signals associated to off-shore construction or maintenance, thus helping to protect the surrounding ecosystems. Less attention has been given to the use of air bubble barriers to reduce continuous noise from sources such as propellers, which are responsible for 85\% of the noise emitted by ships. The numerical research presented in this thesis aims to examine the absorption of low-to-intermediate frequency underwater noise in air-water mixtures. The study focuses on a scenario where an immersed flat plate is covered by an air film, serving as a sound barrier. This setup is a representative model of a bubble injection system used to encase a part of a ship's hull, aiming to acoustically isolate a structure below the waterline. The study combines eddy-resolving fluid dynamics techniques to solve the complex inhomogeneous fluid flow and the solution of the wave equation in the time-space domain to characterize the properties of the air barriers when subjected to continuous noise sources. The first part of this thesis focuses on conducting numerical experiments to simulate the fluid dynamics of a channel flow with a uniform air jet entering the domain. Turbulent flows are reproduced using Large Eddy Simulation (LES) at both low Reynolds number (LRN) ($Re_\tau =180$) and high Reynolds number (HRN) ($Re_\tau =8000$) scenarios, the second one considering a wall-model approach. The two-phase flow solution is determined using the volume of fluid method. The primary goal of this initial part is to accurately calculate the distribution of density and speed of sound, which are essential for evaluating the properties of the acoustic environment and set the domain for the propagation of the acoustic source. In the second part of the thesis, the propagation of acoustic waves incident to the air-water mixture layer is analyzed, by solving the acoustic wave equation in the three-dimensional physical (space-time) domain. Once the air-water mixture layer is fully developed across the length of the channel, its sound absorbing capacity is investigated considering monochromatic monopole source. The noise signature is recorded at a probe positioned on the upper wall to assess the noise reduction property of the air layer covering the plane. The investigation focuses on the difference in the transmission of acoustic pressure between the pure water case and the air-water mixture layer case, both considering a mean distribution and an instantaneous distribution of the air layer. Thus, the effectiveness of the layer is analyzed with respect to the source's frequency and the air layer's distribution in space. The results show that the air film significantly reduces the acoustic signal incident to the plate, mostly for intermediate frequencies and when an average distribution of the mixture is considered. In particular, the averaged distribution of the air content within the layer largely increases the reduction of acoustic pressure incident to the plate compared to the instantaneous distribution of the flow field. The latter presenting a non-uniform distribution of the air along the upper wall. Additionally, as expected, the reduction in signal was notably enhanced, increasing the air layer's thickness. Air bubble barriers are also utilized to minimize the destruction caused by cavitation erosion to water-submerged structures. Hence, the final part of the thesis is devoted to a preliminary numerical investigation concerning the dynamics of single cavitation bubble collapse close to the rigid surface.