In the realm of urban air mobility, the noise generated by electric vertical take-off and landing (eVTOL) vehicles is a critical factor influencing their acceptance and integration into densely populated cities. This article delves into the intricate world of propeller noise, specifically examining how different types of turbulence impact the sound produced by eVTOL propellers in forward flight.
The study focuses on two distinct turbulence scenarios: small-scale turbulence (SST) and large-scale turbulence (LST). By simulating the ingestion of these turbulent wakes by a two-bladed propeller, the research team aimed to understand the resulting aerodynamic loading and aeroacoustic radiation.
The findings reveal a stark contrast between the two turbulence types. With SST ingestion, the interaction is localized, affecting primarily the upper section of the propeller disk. This leads to the generation of narrow sidebands in the mid-frequency range, with modest increases in overall sound pressure levels (OASPL) and directivity. In contrast, LST ingestion produces a highly disorganized wake, disrupting the tip-vortex path and spreading axial-velocity fluctuations across the disk. This results in tone broadening, or haystacking, near the blade passing frequency (BPF) and its harmonics, along with an increase in broadband noise levels and a diffusion of directivity.
The study also highlights the importance of the integral length scale of the turbulence. For SST ingestion, the length scale is relatively small, leading to compact eddies that preserve carrier-phase organization. In LST ingestion, the larger length scale results in slowly convecting structures that broaden and redistribute carrier energy into humps around the harmonics.
The research team employed a sophisticated numerical framework, coupling an LBM-VLES flow solver with an impermeable FW-H acoustic analogy, to predict far-field noise and surface-source distributions. This allowed for a detailed analysis of the flow and acoustic characteristics, providing valuable insights into the mechanisms of noise generation for turbulence ingestion configurations of forward flight propellers.
The results of this study contribute to a deeper understanding of the interaction noise for eVTOL vehicles operating in urban environments, where turbulence scales can vary significantly. By establishing a mapping between inflow scale and coherence to the acoustic signature, the research provides a mechanistic basis for future predictive modeling, offering a crucial step towards quieter and more community-friendly eVTOL operations.
