Researchers at the University of Bristol have experimentally revealed why future aero engines will be able to produce less noise. The study, published in the Journal of Fluid Mechanics, reveals for the first time how these engines, technically known as boundary layer intake (BLI) duct fans, generate and propagate noise. BLI fans resemble the large engines found on modern aircraft, but are partially embedded in the main body of the aircraft, rather than under the wings. Since the fan draws air from the front of the fuselage and from the surface, the fan does not have to work as hard to propel the aircraft into flight, reducing fuel consumption.
Boundary Layer Intake (BLI) duct fan test rig at the University of Bristol's aeroacoustic wind tunnel facility. Source: Feroz Ahmed
Illustration of complex noise sources in an embedded engine or boundary layer intake (BLI) duct fan. Source: Feroz Ahmed
The study was led by Feroz Ahmed from the University of Bristol's School of Civil, Aerospace and Design Engineering and directed by Professor Mahdi Azarpeyvand, utilising the university's national aeroacoustic wind tunnel facility. They were able to identify different sources of noise originating from ducts, rotating fans, and air flowing over the surface of the curved fuselage.
They found that the change in noise pattern depended on the amount of thrust generated by the fan. When the fan generates a large thrust, they observe a noise pattern similar to that of a fan without air ducts. But when the fan produces less thrust, the noise pattern changes, as the air duct itself produces more noise.
Aviation noise
"Our study addresses the pressing issue of noise, which is a major barrier to certification, by revealing the physics behind the noise produced by these configurations," said Dr. Ahmed. By understanding the noise mechanism of BLI ducted fans, we hope to develop industry guidelines for quieter airframe integrated propulsion systems in future aircraft concepts, from large conventional aircraft to small electric vertical take-off and landing aircraft, known as eVTOLs. "
Projects such as the Bell X-22A, Embraer X, Airbus E-fan, Lilium Jet, Green Jet, and Hybrid Air Vehicle are leading the way in developing these systems for next-generation aircraft. These systems are becoming increasingly popular due to advancements in high-power motors.
"However, there is a flaw to recessed ducted fans – their noise and quietness remain a mystery, especially when they absorb airflow from around the curved surface of the fuselage," Dr. Ahmed said. "
An electric duct fan mounted next to a curved surface. Image courtesy of Feroz Ahmed
"Previous research on BLI configurations has focused on fans without air ducts, with boundary layers forming on a flat fuselage surface. However, there are knowledge gaps for ducted fans that draw air around the surface of the curved fuselage, such as the ONERA NOVA, NASA/MITAurora D8, and Airbus Nautilus. Therefore, in this study, we took a closer look at the various factors that contribute to the noise generated by recessed duct fans mounted on the curved surface of the fuselage."
The researchers designed a BLI test rig featuring an electric ducted fan installed next to the curved wall, replicating the setup of the embedded engine seen in designs such as the ONERA NOVA aircraft concept. They collect different types of data from the test bench, including measuring the thrust output of the fan and the amount of noise generated. By dissecting the complex interaction mechanisms between various noise sources, this framework helps uncover the underlying physics of noise sources and how noise changes when fans operate at different thrust levels.
Conclusions and potential impacts
Dr. Ahmed concluded: "With the growing demand for a pleasant flying experience and minimal environmental impact, there is a need for quieter aircraft. This research has potential applications in developing strategies to reduce noise emissions in aviation. In addition, our comprehensive investigation revealing the noise contribution of BLI duct fans has the potential to lead to important research activities in the fluid mechanics community. This, in turn, facilitates the in-depth understanding and further exploration of aeroacoustic phenomena in ducted fans exposed to various turbulences. Our research reveals how futuristic recessed ducted fans mounted on fuselage curves generate noise, revealing that noise patterns vary with fan thrust levels, providing important implications for quieter next-generation aircraft designs."
编译来源：ScitechDaily