<p>The aerodynamic excitation of the diametral acoustic modes of an axisymmetric cavity-duct system is investigated experimentally. The change experienced by the acoustic diametral modes with the increase of the mean flow Mach number is investigated numerically. The first objective of this research is to examine the ability of the axisymmetric free shear layer forming along the cavity mouth to excite the asymmetric diametral modes which do not have preferred azimuthal orientations. The dependency of the system aeroacoustic response on both the cavity length and its depth is investigated to determine the limitations imposed by the relative dimensions of the cavity on the excitation process. The azimuthal behaviour of the self-excited diametral modes is also characterized.</p> <p> An experimental set-up is designed to ensure the coincidence of the frequencies of the shear layer oscillation with the acoustic resonance frequencies. The selection of the test section dimensions is based on finite element simulation of the acoustic diametral modes for several geometries. To simulate the diametral modes at different flow Mach numbers, a finite difference code is developed based on a two-step computational aeroacoustic approach. This approach allows the simulation of the acoustic field, taking into account the convection effect of the mean flow.</p> <p>The experimental results show that the diametral modes are very liable to be self-excited when the mean flow Mach number is higher than 0.1. The level of acoustic pressure during the diametral mode resonance increases rapidly with the increase of the ratio of the cavity depth, d, to the pipe diameter, D. However, the maximum acoustic pressure during each resonance decreases with the increase of the ratio of the cavity length, L, to the pipe diameter, D. The selfexcitation of the diametral modes is sustainable with d/D as small as 1/12. Further reduction in this ratio may result in complete suppression of the resonance. For deep cavities, d/D>3/12, the first and second diametral modes are more liable to excitation than the higher order modes. This is attributed to the fact that the low order modes have relatively higher radial acoustic particle velocity amplitude at the cavity mouth compared to the higher order ones. For d/D=l/12, the higher order modes have relatively higher radial acoustic particle velocity amplitude and consequently their tendency to be self-excited increases. For long cavities, L/D>2/3, the duct longitudinal acoustic modes start to be excited and become more dominant as the cavity length is further increased. The excitation mechanism of these longitudinal modes has not been investigated in this work since sufficient details already exist in the literature.</p> <p>The azimuthal behaviour of the diametral modes is characterized for all the tested cases. For short cavities, the diametral modes are classified as spinning modes; while for long cavities, L/D> 1/2, the orientation of the mode changes randomly over time. Small imperfections in the axisymmetric geometry result in what is described as partially spinning modes. An analytical model is developed to describe quantitatively the spinning behaviour of the diametral modes. The free shear layer and the diametral modes are found to be fully coupled in the azimuthal direction. The random behaviour of the diametral modes in the case of long cavities is attributed to the increase of randomness in the turbulent shear layer </p> <p>The numerical simulations show that the diametral modes experience considerable changes with the increase of the mean flow Mach number. At the cavity mouth, both the amplitude and phase distributions of the acoustic particle velocity are altered with the increase of the Mach number. This demonstrates the importance of considering the effect of the mean flow on the acoustic power production process. Moreover, the resonance frequency of the diametral modes decreases with the increase of the Mach number.</p> / Thesis / Doctor of Philosophy (PhD)