Numerical Analysis of Tonal Noise Emissions from Open and Shrouded Contra-Rotating Propellers

Abstract

As market growth projections for Urban Air Mobility vehicles (UAMs) skyrocket, their presence in urban environments is likely to become increasingly prevalent, as will their noise. This creates a disturbance to both humans and wildlife, previously unaffected by aircraft noise. Further reinforcing the concern is the new multi-rotor designs, which introduce additional sources of noise.

Research on novel designs provides a limited understanding of the primary noise-generating mechanisms contributing to overall sound production. Among these innovative designs, complex geometries like open Contra-Rotating Propellers (CRP) and Shrouded Contra-Rotating Propellers (S-CRP) emerge. This study focuses on CRPs because they are anticipated to offer increased thrust with the same platform area, crucial for urban UAM operations. Additionally, shrouds are explored for their potential for increased thrust, acoustic shielding, and directivity manipulation, while also offering space for acoustic liners and increasing safety for ground operators.

The objective of this study is to isolate the aerodynamic and acoustic installation effects and identify the noise-generating mechanisms of the CRP and S-CRP configurations.

For this, the aerodynamics and acoustics of six propeller configurations are analyzed, including both the primary CRP and S-CRP geometries and sub-variants thereof. The exploration uses a hybrid numerical methodology, consisting of an aerodynamic flow solver based on the unsteady Reynolds-Averaged Navier-Stokes equations (uRANS) and a Finite Element Method (FEM) acoustic propagation solver. The latter generates acoustic sources from the aerodynamic solution using the source mode formalism.

This not only facilitates an aerodynamic understanding of the acoustic sources but can also detail near-field effects on acoustic propagation. While alternative methodologies achieving similar results necessitate high-fidelity simulations, the uRANS-FEM method can effectively capture aerodynamic and propagation effects within a moderate computational time. However, it also restricts analysis to tonal components of loading noise.

The CRP configuration showcased an improvement in efficiency metrics, marked by a 2.98% increase in FOM and a 57.53% increase in thrust per area, suggesting their higher efficiency and compactness compared to a single rotor. However, this came at the cost of increased noise levels, with amplifications ranging from 10 to 50 dB across various harmonics, attributed to the interaction of the contra-rotating blades. The study further noted a reduction in thrust for both the lead and rear propellers due to the interaction, particularly within the inner 75% radius of the rear propeller. A correlation between the azimuthal angle of the peak thrust and the angle of the highest noise generation was also observed.

When employing a shroud on a CRP, significant modifications in the performance of the shroud were not as anticipated in the literature. The shroud, while contributing to overall thrust, led to a considerable thrust reduction (over 54%) for both propellers due to separated flow. Acoustically, the shroud induced only a minor reduction in noise due to the aerodynamic effects, primarily due to decreased mean thrust. The dominant aerodynamic noise-generating mechanism in the S-CRP configuration is still the blade interaction...