Turbulent-boundary-layer-propeller interaction noise with an exploration of noise mitigation strategies
Leandro Castelucci is a PhD student in the Department of Engineering Fluid Dynamics. (Co)Promotors are prof.dr.ir. C.H. Venner, dr.ir. L. Hirschberg and dr.ir. M.P.J. Sanders from the Faculty of Engineering Technology.
The aeronautical industry is changing its paradigms in terms of aircraft design concepts. The industry is shifting towards unorthodox aircraft designs for more flexible aviation, such as urban air mobility, or because of the increasing concern with efficient aviation for lower emissions. The inherent performance increase accompanies the noise problem caused by coupling propulsive systems with the airframe.
There is a reasonably good understanding of the mechanisms involved in noise sources, emissions, and mitigation for conventional aircraft. However, fundamental research is needed to understand the noise mechanisms involved in the new aircraft concepts. Especially to gather data for validation of numerical and optimization tools, fostering the current big-data need and strengthening the fundamental understanding of the mechanisms involved in noise production and mitigation.
This research presents an aeroacoustic experimental analysis of a propeller ingesting a wall-bounded, turbulent boundary layer. The setup is a simplified version of a boundary layer ingesting propulsor simulated by a propeller mounted on a flat plate. The turbulent boundary layer impinging on the propeller has its thickness manipulated by tripping devices positioned upstream of the propeller. The propeller is positioned close to the flat plate to be immersed in the incoming turbulent boundary layer. The effect of different boundary-layer-thickness to diameter immersion ratios is discussed in terms of performance and noise.
Details of the experimental model designed to run the propeller are shown in Chapter 2. Additionally, an effective cooling system was modeled (Appendix A) and tested (Appendix B) to ensure continuous and safe operation of the setup during experiments. A complete analysis of the boundary layers generated on the flat plate is presented in Chapter 3. Those boundary layers were manipulated with tripping devices of different heights to ensure different boundary layer thicknesses on the propeller plane. The self-similarity of the streamwise velocity was shown to be guaranteed at the propeller plane, and the thickness of the boundary layer increased at the desired level.
Chapter 4 presents a complete propeller performance analysis with uniform inflow, non-uniform inflow, and static-thrust conditions. The results were compared with a standard blade element model analysis. The results showed that the BEM model behaved as expected for the thrust coefficient but underestimated the power coefficient, resulting in a higher performance when compared to the experimental results. This is not usual, as the BEM model predicts propellers' efficiency well for propellers of similar pitch angles. That highlights the possibility of the setup vibrations, wind tunnel, and nacelle influencing the propeller's performance. The performance slightly dropped for the immersed condition.
Chapter 5 presents the experimental aeroacoustic characterization of that generic configuration, comparing a baseline condition with uniform flow and the immersed condition. The immersion resulted in considerable changes in noise directivity --- particularly in the upstream region of the propeller ---, with noise levels increasing by up to 9 dB for polar directivity and 5 dB for azimuthal directivity. Chapter 6 extends the analysis by investigating noise-mitigating materials, specifically band-stop quarter-wavelength resonators (QWRs) and a broadband metal-foam absorber placed beneath the immersed propeller. The results showed that the effectiveness of QWRs depends on their spatial configuration. Optimizing the mitigator placement by removing them from directly beneath the propeller and applying them elsewhere on the surface yielded the best noise abatement performance, significantly reducing tonal and broadband noise. Comparatively, the metal-foam absorber produced higher tonal peaks in the low-frequency range, indicating that the optimized QWR configuration offered the most significant noise mitigation under these experimental conditions.
In conclusion, this research highlights the complex interactions between boundary layer ingestion on propeller performance and noise generation. These insights contribute to the growing body of knowledge necessary to develop more efficient noise mitigation strategies in novel aircraft designs, especially those involving boundary layer ingestion. The results also offer valuable data for validating computational models and optimizing future propulsive systems to improve acoustic performance in advanced aviation applications.
