Low-order acoustic prediction tool for estimating noise emissions from distributed propeller configurations

A low-order numerical tool for estimating noise emissions from distributed propeller configurations is presented. The paper describes the tool's computational framework, which uses Hanson's near-field theory to calculate the thickness and loading noise components. The formulation assumes steady blade loading, but an unsteady case can be handled numerically by redefining the pressure distribution over the blade at each new time step. Two representative cases are analyzed to validate the tool: an isolated propeller operating in uniform flow and an array of three propellers in a side-by-side configuration under aerodynamic interference caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed propeller system. The low-fidelity tool provides accurate results for both cases, with less than a 1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise predictions at an observer located 10 diameters away and at the propeller plane. When analyzing the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the propeller plane. However, this difference increases to 6 dB as the observer moves toward the inflow direction. This difference is due to the dominance of broadband noise near the propeller axis. The paper concludes with a noise analysis of the distributed propeller system, examining the relative importance of aerodynamic interference in the noise emitted by a propeller. In this case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the propeller axis, while it had an insignificant effect at the rotor plane.