Turbulent inflow noise produced by a shrouded propeller

Abstract

A theoretical model is presented for predicting the noise produced by the interaction of turbulence with a shrouded propeller in an axisymmetric mean-flow. For cases where the free-stream velocity is low, the turbulent eddies well upstream of the propeller are significantly elongated as they pass through the induced streamtube contraction. These elongated turbulent eddies interact multiple times with the rotating propeller blades causing partially coherent loading on the blades. The resulting radiated acoustic pressure spectrum contains peaks which occur at harmonics of the blade passing frequency. The theory is developed in the frequency domain and incorporates rapid distortion theory to obtain the distorted turbulent field at the propeller face, accounting for the potential mean-flow effect of the propeller and the shroud. The unsteady loading on the rotating blades is calculated using isolated blade response functions. The noise radiated to the far-field is then evaluated using a tailored Green's function approach which accounts for the scattering effect of the shroud and is solved using a boundary integral approach. Results are presented for shrouded and unshrouded propellers operating in flows with a range of different free-stream velocities. It is shown that as the free-stream velocity increases, the peak in the noise spectrum at the blade passing frequency reduces in level and the ‘haystack’ surrounding this peak widens. Results showing the effect of the shroud on the radiated noise levels at different frequencies and observer locations are also presented.