The University of Bristol researchers have uncovered the mystery behind the quiet operation of futuristic aircraft equipped with innovative integrated engines featuring an energy-efficient configuration.
The study uncover the process by which noise is produced and transmitted from these engines, referred to as boundary layer ingesting (BLI) ducted fans. These fans, which are partially integrated into the main body of the aircraft instead of being located under the wings, operate similarly to the large engines found in modern airplanes.
By drawing in air from both the front and the surface of the airframe, they exert less effort in propelling the aircraft, resulting in reduced fuel consumption compared to traditional engines.
Dr Feroz Ahmed, from the School of Civil, Aerospace and Design Engineering at Bristol, led a research project supervised by Professor Mahdi Azarpeyvand. The project made use of the University National Aeroacoustic Wind Tunnel Facility. They were able to distinguish specific noise sources from the duct, the rotating fan, and the air flowing over the curved airframe surface.
Researchers found that the noise pattern varies based on the fan’s thrust production. At high thrust levels, the noise pattern resembled that of fans without ducts. However, at lower thrust levels, the duct itself contributed more to the changing noise pattern.
“Our study addresses this urgent issue of noise, which poses a major obstacle in obtaining certifications, by uncovering the physics behind the noise these configurations produce,” Dr Ahmed explained. “By understanding the noise mechanisms in BLI ducted fans, it is hoped that industrial guidelines can be developed for quieter airframe-integrated propulsion systems in future aircraft concepts, from large-scale conventional aircraft to small-scale electric vertical take-off and landing, known as eVTOL, aircraft.”
Projects like the Bell X-22A, Embraer X, Airbus E-fan, Lilium Jet, Green Jet, and Hybrid Air Vehicle are leading the charge in developing these systems for next-generation aircraft due to advancements in powerful electric motors, which are making them increasingly popular.
“But, there is a catch for embedded ducted fans – how loud or quiet they are is still a mystery, especially when they are ingesting airflow from around the curved airframe surface,” Dr Ahmed said. “Previous research on BLI configurations mostly focused on fans without ducts, where the boundary layer forms over flat airframe surfaces. However, there is a knowledge gap when it comes to the ducted fans ingesting air around curved airframe surfaces, as seen in projects like ONERA NOVA, NASA/MIT Aurora D8, and Airbus Nautilus.”
A BLI test rig was designed by the researchers, featuring an electric ducted fan positioned next to a curved wall to replicate the setup of embedded engines found in designs such as the ONERA NOVA aircraft concept. Various types of data, including measurements of the fan’s thrust output and the level of noise produced, were gathered from the rig.
This framework has revealed the fundamental physics behind the origin of noise and its variations as the fan operated at different thrust levels by analyzing the intricate interactions of noise mechanisms among different sources.
“With the growing demand for a pleasant flight experience with minimum environmental impact, there is a need for quieter aircraft. This research has potential applications in developing strategies to reduce noise emission in the aviation sector,” Dr Ahmed concluded.
“Furthermore, our comprehensive investigation into unlocking the noise contributions in BLI ducted fans has the potential to steer significant research activity within the fluid mechanics community. This, in turn, could foster a deeper understanding and further exploration of aeroacoustics phenomena in ducted fans exposed to a broad spectrum of incoming turbulent flows.
“Our study sheds light on how noise is generated by futuristic embedded ducted fans mounted on curved airframe surfaces, revealing that noise patterns vary with fan thrust levels, offering crucial insights for quieter next-generation aircraft design.”
Journal reference:
- Feroz Ahmed, Ismaeel Zaman, Djamel Rezgui, Mahdi Azarpeyvand. Aeroacoustics of a ducted fan ingesting an adverse pressure gradient boundary layer. Journal of Fluid Mechanics, 2024; DOI: 10.1017/jfm.2024.134