Researchers at the University of Bristol have solved the mystery of how futuristic aircraft embedded engines, known as boundary layer ingesting (BLI) ducted fans, make noise. These engines are partially embedded into the plane’s main body and ingest air from both the front and the surface of the airframe, resulting in less fuel consumption. Using the University National Aeroacoustic Wind Tunnel Facility, the researchers identified distinct noise sources originating from the duct, the rotating fan, and the air flowing over the curved airframe surface. They found that the noise pattern changes depending on the amount of thrust the fan is producing.
The study, led by Feroz Ahmed and Professor Mahdi Azarpeyvand, found that when the fan is producing high thrust, the noise pattern is similar to fans without ducts. However, when the fan is producing less thrust, the noise pattern changes as the duct itself starts making more noise. The research aims to develop industrial guidelines for quieter airframe-integrated propulsion systems in future aircraft concepts, such as large-scale conventional aircraft and small-scale electric vertical take-off and landing (eVTOL) aircraft. These quieter aircraft designs are becoming more popular due to advancements in powerful electric motors.
Dr. Ahmed highlighted the importance of understanding the noise mechanisms in BLI ducted fans to address the major obstacle of noise in obtaining certifications for aircraft. He emphasized that the noise patterns vary with fan thrust levels, offering crucial insights for quieter next-generation aircraft design. The research focused on embedded engines ingesting airflow from around curved airframe surfaces, a previously understudied area in BLI configurations. By examining the factors contributing to noise production in embedded ducted fans, the researchers were able to uncover the physics behind the noise generated.
The researchers designed a BLI test rig featuring an electric ducted fan mounted next to a curved wall to replicate the setup of embedded engines seen in aircraft concepts like ONERA NOVA. Data collected from the rig, including fan thrust output and noise generated, helped analyze the complexities of noise interaction mechanisms among various sources. The research has potential applications in developing strategies to reduce noise emission in the aviation sector and has the potential to drive further research activity within the fluid mechanics community, particularly in aeroacoustics phenomena in ducted fans exposed to turbulent flows.
The study was conducted in collaboration with ONERA — the French Aerospace Lab, and several other partners, and was sponsored by the European Union’s Horizon 2020 research and innovation program under the ENODISE project. The research sheds light on how noise is generated by futuristic embedded ducted fans mounted on curved airframe surfaces and offers insights for quieter aircraft design. By uncovering the noise mechanisms in BLI ducted fans, the researchers hope to contribute to the development of quieter and more environmentally friendly next-generation aircraft.