Abstract:
Unmanned aerial vehicles (UAVs) can pose problems concerning safety and noise. A possible solution to improve the safety of UAVs is to install an axisymmetric shroud around the propellers. However, the presence of the shroud will modify the noise produced by the propellers. This thesis describes an experimental, numerical, and theoretical investigation into the effect of a shroud on two significant propeller tonal/quasi-tonal noise mechanisms: (1) steady loading and thickness noise, and (2) the turbulent inflow noise. A probe microphone was used to measure the time-average rotating pressure field on the inner surface of an acoustically rigid shroud which is compared with the results of a computational fluid dynamics (CFD) simulation. The measured and predicted pressure was then used to calculate the steady tonal noise radiated from the loading sources on the inner surface of the shroud. High-resolution near-field measurements were also acquired. This permitted separation of the noise produced by the shroud and propeller sources and therefore the relative contribution to the total noise could be predicted. For the shrouded propeller configuration tested, the effect of the shroud was shown to decrease the steady loading and thickness tones for observers below the rotational plane of the propeller. A model is also presented which can be used to predict the noise produced by the interaction of turbulence with a shrouded propeller. The turbulent inflow noise model incorporates the aerodynamic and acoustic effect of a shroud with arbitrary cross-sectional profile. Experiments were also undertaken in an anechoic wind tunnel to assess the noise of a shrouded propeller operating in grid-generated turbulence. The acoustic measurements are compared to predictions using the proposed model and show reasonable agreement. The approach used to calculate the acoustic scattering effect of the shroud was able to capture the shape of the directivity pattern of the turbulent inflow noise produced by the shrouded propeller reasonably well. The theoretical models and results presented in this thesis could assist in the design of safer and quieter UAVs.