Numerical investigation of the aerodynamics and high-frequency noise production of a cylinder coated with a simplified porous medium.

The reduction of noise pollution has become a significant challenge in recent years. Noise exposure in areas close to railways and airports has been in fact closely associated with several discomforts and health conditions, prompting the World Health Organizations to implement regulations and restrictions on the noise emitted from transportation. This work focuses on the aerodynamic noise generated by a cylinder in a uniform flow, whose phenomenology replicates the one of elongated bodies with bluff body geometry, such as airplanes landing gears and high speed train pantographs. A method that has proven its efficacy in mitigating the shedding tone generated by bodies with such geometry involves the application of a porous coating on the surface of the body, which stabilizes the wake of the body by expanding the flow recirculation region downstream of the body and shifting downstream the formation of the vortex street. However, experimental investigations on the acoustic properties of cylinders coated with porous materials have revealed an increase in high-frequency noise with respect to the bare cases, whose mechanisms of generations have not been identified yet. This thesis aims to provide an understanding on the physical mechanisms of the high-frequency noise generation by numerically simulating the flow past a cylinder coated with a simplified porous medium, which allows for the study of the flow within the coating at a microscopic level. The software employed for this study is SIMULIA PowerFLOW 6-2021-R2, a software based on the Lattice Boltzmann Method, while the model utilized for the simulation consists of a cylinder covered with a simplified porous coating constituted by four arrays of 72 small-scale cylinders immersed in a uniform flow at a Reynolds number of Re = 8.2 · 104. The results of the analyses conducted on the model reveal the presence of a dominant component at St = 12.2 in the far field acoustic spectra related to a strong hydrodynamic field within the windward region of the coating, which is associated with the vortex shedding of the small-scale cylinders. Furthermore, the analysis conducted on the velocity field and the Turbulent Kinetic Energy (TKE) reveal that for angles greater than θ = 35◦ from the upstream direction the TKE reaches its maximum values, while the velocity direction transitions from being mainly radial to predominantly circumferential, leading to interactions along the layers between the small-scale cylinders and the turbulence generated within the coating. Finally, the analyses carried out on the hydrodynamic field within the coating pointed out that the presence of a dominant acoustic component in the far field region can be linked to resonant phenomena taking place due to the interactions between the turbulence and the small scale cylinders, which generates coupling between the latter in the windward region of the coating, leading to a coherent emission of acoustic waves.