Aeroacoustic-source analysis of a structured porous coated cylinder

The aerodynamic noise generated by a circular cylinder in uniform flow represents a central topic in many engineering and environmental applications, such as high-speed train pantographs, landing gear systems, etc. In recent years, it has been proven that coating a circular cylinder with a porous medium represents an effective noise mitigation. However, the precise mechanism through which this suppression performance is achieved is not clear yet. In the most common view according to the Curle’s analogy, the aerodynamic noise generated by a cylinder in uniform flow can be modelled, in equivalent terms, by a surface distribution of dipolar sources associated with pressure fluctuations acting on the cylinder surface and by volumetric sources located in the wake, linked to the unsteady velocity fluctuations. At low Mach numbers, the latter contribution is typically overshadowed by the dipolar one. However, recent studies have shown that this assumption does not necessarily hold for a porous-coated cylinder. Indeed, by means of source-localization techniques, it has been found that the dominant acoustic sources lie in the wake rather than on the cylinder surface. As a result, the acoustic field emitted by a porous cylinder can be formulated as a problem of diffraction of flow sources (i.e., quadrupole) by the rigid body. The present work aims to provide a physical explanation for this phenomenon and clarify the link between flow-field alterations and related noise mitigation performance of a porous-coated circular cylinder. For this purpose, a structured porous medium, which means that pores are spread with regularity around the bare cylinder, and an existing data set coming from a high-fidelity simulation based on the Lattice Boltzmann method have been employed. Results of this simulation are post-processed to compute far-field data employing the Ffowcs Williams and Hawkings (FWH) formulation that predicts aerodynamic noise from unsteady flow data around moving bodies. In the low frequency regime, analysis of far-field data reveals that aerodynamic noise is dominated by quadrupolar sources located in the wake, contradicting the general view that considers the quadrupolar contribution negligible at low Mach numbers. The relevance of the quadrupolar sources is also confirmed through a coherence analysis between velocity fluctuations, near-field pressure, and far-field pressure, revealing that the former have a more pronounced effect on the far-field. The analysis of the flow field demonstrates that a downstream shift of the onset of vortex shedding occurs, increasing what is normally referred to as the “vortex formation length”. This effect has a relevant impact on the aeroacoustic results, since, according to the diffraction theory, it leads to a loss of efficiency of the scattering mechanism, which results in the dipolar source having a minor impact on the far-field noise. The existence of such a feedback mechanism is supported by the results of the spectral proper orthogonal decomposition (SPOD), which suggests that turbulent velocity fluctuations are responsible for imposing surface pressure fluctuations on the surface at the vortex shedding frequency. This hypothesis is also confirmed by a coherence analysis between near-field pressure and velocity fluctuations. The outcome of this thesis is expected to provide guidelines for a more effective design of noise-mitigation strategies applied to circular cylinders.