Abstract
Aircraft noise has been an increasing urgent environment issue especially for people living near airports. In recent years, airframe noise has been comparable to engine noise particularly for landing aircraft. This thesis addresses a previously neglected source, surface roughness generated boundary-layer noise, with the aim of developing a prediction model to assess the potential contribution of surface roughness to airframe noise. The thesis is focused on the sound scattering mechanism. The generation of sound by turbulent boundary-layer flow over a rough wall is investigated by applying a theoretical model which describes the scattering of turbulence near field into sound by roughness elements. Models for the source statistics are obtained by scaling smooth-wall data by the increased friction velocity and boundary-layer thickness for a rough surface. Attention is focused on a numerical method to predict the absolute level of far-field radiated roughness noise. Direct numerical integration is used to obtain the prediction model which is able to reproduce the spectral characteristics of the available empirical formula and experimental data. Acoustic experiments are conducted for two rough plates in an open jet. The reasonable agreement between measured and predicted noise spectra is observed, and beamforming source maps by phased microphone arrays exhibit satisfactory similarities between measurement and simulation in source pattern and source strength. The dipole directivity features are demonstrated. However, the prediction model underestimates the streamwise gradient of source strength and overestimates the sound radiation in the high-frequency region. Hot-wire measurement is performed and it determines the applicable wake strength and skin friction coefficient that account for the roughness effects of turbulence enhancement. Numerical estimates for a current aircraft wing and a conceptual ``silent aircraft'' design with idealized roughness levels show that in the high-frequency region the sound radiated from surface roughness may exceed that from trailing edge. A parametric study indicates that roughness height and roughness density significantly affect the noise radiation with roughness height having the dominant effect. The noise directivity pattern varies with different levels of surface roughness. The maximum allowable roughness levels on the surface of the silent aircraft are studied to achieve an aggressive noise target. An alternative method to the scattering of near-field turbulence is that large roughness elements shed vorticity resulting in unsteady drag. This drag dipole mechanism is extended to consider very large roughness elements by modifying a previous model for spheres to determine the unsteady drag on the hemispherical elements and then the radiated sound. The preliminary prediction shows that this noise source can be comparable to the scattering roughness noise. Finally, a beamforming correction for dipole measurement using phased microphone arrays is presented. A new beamforming algorithm for identifying dipoles is developed and validated by numerical and experimental implementation. In conclusion, the validated theoretical model shows that it is capable of approximately predicting the far-field radiated roughness noise. The assessment of the contribution of surface roughness to airframe noise suggests that surface roughness noise was underestimated and needs to be carefully considered in the design of a low-noise airframe.