This paper experimentally characterises the far-field noise emissions of a rotor operating in reverse non-axial inflow conditions. Specifically, experiments were undertaken at a range of rotor tilting angles and inflow velocities to investigate the effects of negative tilting on rotor acoustics and their correlation with aerodynamic performance. The results show that the forces and moments experienced by the rotor blades change significantly with increasing inflow velocity and increasing negative tilting angle. Correspondingly, distinct modifications to the far-field acoustic spectra are observed for the negatively tilted rotor when compared to the edgewise condition, with the broadband noise content notably increasing. Moreover, for a given tilting angle, the broadband noise component is accentuated with increasing inflow velocity, similar to when the negative tilting angle is increased. With reference to the flow-field surveys conducted in the literature and a preliminary in-house flow measurement, the increase in broadband content can possibly be attributed to the heightened level of ingestion of blade self-turbulence, i.e., the ingestion of turbulent wake generated by the upstream portion of the rotor by the downstream portion. At lower inflow velocities, the magnitude of the blade passing frequency at each of the observer angles is found to change minimally with negative tilt. In contrast, at higher inflow velocities, the directivity pattern and intensity of both the blade passing frequency and the overall sound pressure level are shown to change with increases in magnitude, particularly at downstream observer locations with negative tilt. These findings have important ramifications for the design and suitable operational profile of aerial vehicles for future urban air mobility applications.

The growing demand for urban air mobility necessitates the development of quieter and more efficient electric vertical takeoff and landing aircraft. This study presents numerical simulations of large-scale turbulence in the wake of a cylinder being ingested by a propeller in forward flight. The computational fluid dynamics solver SIMULIA PowerFLOW is employed to investigate both the flow field dynamics and aeroacoustics of the configuration, and the numerical setup mirrors an earlier experimental campaign, providing validation to the current numerical framework. The results demonstrate that turbulence ingestion leads to a significant increase in the far-field broadband noise components from mid to high frequencies. Interestingly, multiple tonal side-peaks emerge around the fundamental blade passing frequency as a result of the interaction between the propeller and shed vortices from the cylinder, partly attributed to the non-linear turbulent mixing. Flow field results reveal that the significant blade-turbulence interaction modifies the flow downstream of the propeller, particularly a rapid turbulent mixing between the turbulent inflow and the tip vortices, intensifying the asymmetric unsteady loading on the blade. Noise source decomposition identifies the leading edge as the dominant contributor to tonal noise and the broadband component in low-to-mid frequencies, while sources from the trailing edge become significant at higher frequencies.

Advanced air mobility represents efficient and sustainable transportation through the use of electric vertical takeoff and landing (eVTOL) aircraft, which will see their propellers operating in edgewise flow for a significant portion of their mission profile, giving rise to challenging aerodynamic and aeroacoustic features, different from the conventional helicopter rotors. The present experimental study examines the effect of the blade tip Mach number on the acoustic signature emitted by a propeller operating in edgewise flight conditions. Both aeroacoustic and aerodynamic measurements were performed for a 12-in., two bladed propeller over a wide range of tip Mach numbers (MT) from 0.12 to 0.56, and inflow velocities from 0 to 24 m/s. The results confirm the highly sensitive nature of the propeller noise to the tip Mach number. The overall sound pressure level (OASPL) in the plane-parallel to the propeller's plane of rotation, scales with MT3 to MT4 at low tip Mach numbers (MT0.3, for the no inflow velocity case. Similar trends are observed for the plane perpendicular to the plane of rotation, but with noise scaling up to MT8. At the highest inflow velocity case, the OASPL scales with MT3 to MT7, depending on the observer location and MT, highlighting the increased complexity and sensitivity of noise directivity at higher inflow velocities. Tonal noise is the dominant source of noise at lower inflow velocities, with an increased contribution of broadband noise at higher inflow, possibly leading to the larger deviations of the tip Mach number scaling observed. These findings offer valuable insights into the acoustic behaviour of propellers during edgewise flight conditions, which can be leveraged to design more efficient, low-noise propellers in an eVTOL application.