Airfoil trailing edge noise is one of the major sources of broadband noise in many engineering applications such as aviation and renewable energy. In this thesis, various owl-inspired trailing edge noise reduction treatments are developed through anechoic wind tunnel experiments. The first type of treatment is velvety surface, which is inspired by the downy structures on the owls’ upper wing surface. In general, the velvety structures increase the low-frequency noise below a cross-over Strouhal number but reduce the spectral level at higher frequencies. The velvety surface also changes the boundary layer characteristics in terms of the boundary layer thickness, non-dimensional velocity distribution and turbulence distribution. Vortex shedding is suppressed by the velvety coating despite...[ Read more ]

Airfoil trailing edge noise is one of the major sources of broadband noise in many engineering applications such as aviation and renewable energy. In this thesis, various owl-inspired trailing edge noise reduction treatments are developed through anechoic wind tunnel experiments. The first type of treatment is velvety surface, which is inspired by the downy structures on the owls’ upper wing surface. In general, the velvety structures increase the low-frequency noise below a cross-over Strouhal number but reduce the spectral level at higher frequencies. The velvety surface also changes the boundary layer characteristics in terms of the boundary layer thickness, non-dimensional velocity distribution and turbulence distribution. Vortex shedding is suppressed by the velvety coating despite the blunt trailing edge. An analytic model is proposed to relate the measured flow characteristics and the trailing edge geometry to the far-field noise. The predictions, which require no empirical corrections, match well with the experiments for both the baseline and velvet-coated configurations. Further non-dimensional analysis reveals that the manipulation of boundary layer statistics by the velvet structures has a major impact on the trailing edge noise spectra. The second type of treatment is flexible trailing edge serrations, which is inspired by the soft trailing edge fringes of owls’ wings. It is observed that compared with rigid serrations, flexible serrations can achieve an additional broadband noise reduction up to 2-3 dB at high frequencies, and the effect also depends on the geometry of the serrations. Complementary deformation measurement and aerodynamic force measurement show that flexible serrations can align better with the flow and are expected to reduce the crossflow intensity near the serration roots, which has been related to the extraneous high-frequency noise generated by serrations in previous studies. The third type of treatment takes combinations of trailing edge serrations and porous membrane/velvet structures. It was observed that the noise reduction of the conventional serrations deteriorates significantly when the serrations are misaligned with the flow, while the performances of the combined structures are only slightly affected by misalignment. A novel configuration, in which the trailing edge serrations are surrounded by serrated porous velvet structure, is found to outperform the unmodified serrations and can achieve approximately 10 dB noise reduction in both flow-aligned and flow-misaligned conditions, within a wide frequency range corresponding to a boundary layer thickness-based Strouhal number between 0.3 to 1. In summary, this thesis proposes several new trailing edge noise reduction treatments, and detailed mechanism studies are provided to enable future engineering applications.