Aerodynamic Noise Reduction with Porous Materials

Abstract

Public annoyances due to noise emission in aviation and wind energy sectors are expected to become more severe in the near future with increasing number of civilian flights and wind turbine installations. Such trend demands for deeper investigations into the noise generation mechanisms and the means to mitigate them. This dissertation addresses two cases of flow-induced sound: 1) turbulent wake-body interaction, which leads to the combined tonal and broadband noise produced by the fan stage of an aircraft engine, and 2) turbulent boundary-layer trailing-edge noise that is responsible for the swishing noise from a wind turbine rotor. Although both mechanisms are inherently different, they both describe situations where noise is produced when turbulence encounters a discontinuity in the flow field, such as when turbulence in a freestream impinges a sharp leading edge or when turbulence in a boundary layer is scattered as it flows past a sharp trailing edge. The usage of permeable/porous materials at the edge of an aerodynamic body has been proposed as a solution since their flow permeability characteristic realises an intermediary region, alleviating the aforementioned discontinuities. However, there are questions that still need to be addressed. How exactly do these porous treatments mitigate the noise generation mechanisms? How do they affect the flow field in their vicinity? How should they be optimally designed and integrated? To help answering these questions, a high-fidelity lattice-Boltzmann method has been employed in the present work, with which detailed flow and acoustic analyses have been performed.