Central to this study is the concept of “haystacking,” a term in the field of acoustics that describes the scattering of tonal sound fields due to turbulent airflow. This phenomenon leads to the dispersion of specific tonal sounds over a broader spectrum of frequencies, resulting in a sound that is not only loud but also unpleasant. The researchers conducted a fluid-mechanics-based assessment that informs the understanding of how two distinct types of broadband noise patterns—namely duct haystacking and fan haystacking—emerges when an aircraft operates under different thrust conditions.
During cruise conditions, characterized by low thrust, the fan’s suction is weaker, allowing the boundary layer flow around the aircraft to remain relatively undisturbed. This results in a lasered focus on the interaction between the turbulent flow and the acoustic properties of the duct, where duct haystacking becomes the primary contributor to perceived noise. In contrast, during take-off, high thrust results in significant changes to the airflow due to the strong suction from the fan. This phenomenon introduces high-momentum turbulent structures into the airflow, leading to fan haystacking that is particularly pronounced as the rotating blades slice through this chaotic flow.
Lead researcher, Dr. Feroz Ahmed, emphasizes the study’s implications beyond mere noise measurement. He notes that the two forms of haystacking can make future aircraft not only loud but also subjectively irritating. This raises pressing questions about the acoustic design of future aircraft and how engineers can optimize performance to create aircraft that fulfill both speed and noise reduction criteria. The findings provide a valuable framework for engineers aiming to develop quieter aircraft, ultimately enhancing the passenger experience and supporting urban mobility initiatives.
The research was conducted using a high-fidelity wind tunnel setup that closely mirrors real-world flight conditions. By employing cutting-edge instrumentation, including hot-wire anemometry, pressure sensors, and advanced microphones, the researchers gathered unprecedented levels of data on both airflow and acoustic signatures across various flight regimes. This exhaustive analysis enabled them to establish a direct link between the aerodynamic mechanisms at play and the actual sounds that passengers and communities nearby would experience.
Moving forward, the research team plans to refine and develop aerodynamic and acoustic controls that would minimize the effects of both fan and duct haystacking. Furthermore, they aim to broaden their analysis to include other propulsion concepts that involve turbulent flow ingestion. This will allow them to create a comprehensive understanding of the acoustic landscape associated with future aircraft designs.
In summary, this revolutionary work challenges conventional practices in aircraft noise management and provides a fresh perspective on how engineers can harness knowledge of airflow and acoustics to create future aircraft that not only maintain high-performance standards but also integrate seamlessly into urban environments. Understanding the noise produced by BLI engines could therefore set the stage for innovations that reinvigorate public trust and acceptance of electric flight technology.
Subject of Research:
Article Title: Aeroacoustics and psychoacoustics characterization of a boundary layer ingesting ducted fan
News Publication Date: 15-May-2025
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Image Credits: Credit: Dr Feroz Ahmed
Keywords
Engineering
Tags: acoustic interactions in aircraftaerodynamic sound generation researchaviation industry sustainabilityboundary layer ingesting engineselectric aircraft engine noiseelectric and hybrid flight technologiesflight safety and passenger comfortnoise perception in aviationpublic acceptance of electric aviationturbulence and engine mechanicsUniversity of Bristol research studyurban air mobility noise challenges
