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New Research on Noise Reduction for Next Generation Aircraft Engines

The mystery of how futuristic aircraft embedded engines, featuring an energy-conserving arrangement, make noise has been solved by researchers at the University of Bristol.

The Boundary Layer Ingesting (BLI) ducted fan test rig inside the aeroacoustics wind tunnel facility at the University of Bristol. (Image: University of Bristol)

A study published in Journal of Fluid Mechanics, reveals for the first time how noise is generated and propagated from these engines, technically known as boundary layer ingesting (BLI) ducted fans. BLI ducted fans are similar to the large engines found in modern airplanes but are partially embedded into the plane’s main body instead of under the wings. As they ingest air from both the front and from the surface of the airframe, they don’t have to work as hard to move the plane, so it burns less fuel.

The research, led by Dr. Feroz Ahmed from Bristol’s School of Civil, Aerospace and Design Engineering under the supervision of Professor Mahdi Azarpeyvand, utilized the University National Aeroacoustic Wind Tunnel Facility. They were able to identify 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 how much thrust the fan is producing. When the fan is producing high thrust, they observed a noise pattern similar to what is seen in fans without ducts. But when the fan is producing less thrust, the noise pattern changes because the duct itself starts making more noise.

Dr. Ahmed explained: “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.

“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 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. They are becoming more popular due to advancements in powerful electric motors.

Dr Ahmed said: “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.

“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.

“So, in this study, we have closely examined the various factors that contribute to the noise produced by the embedded ducted fans installed on curved airframe surfaces.”

The researchers designed a BLI test rig featuring an electric ducted fan mounted next to a curved wall, replicating the setup of embedded engines seen in designs like the ONERA NOVA aircraft concept. They collected different types of data from the rig, including measurements of the fan’s thrust output and the amount of noise generated. By dissecting the complexities of noise interaction mechanisms among various sources, this framework helped uncover the underlying physics of where the noise originated and how it changed as the fan operated at different thrust levels.

Dr. Ahmed concluded: “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.

“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.”

This research was performed by Dr. Feroz Ahmed from Bristol’s School of Civil, Aerospace and Design Engineering under the supervision of Professor Mahdi Azarpeyvand. For more information, download the Technical Support Package (free white paper) below. ADTTSP-06243

Topics:
Aerospace Aircraft Aircraft Aircraft structures Airframes Aviation Engine components Engine cooling systems Engine/Powertrain Engines Fans Materials Noise Powertrains Propulsion Research and development Vehicles Wings
This Brief includes a Technical Support Package (TSP).
Document cover
Aeroacoustics of a Ducted Fan Ingesting an Adverse Pressure Gradient Boundary Layer

(reference ADTTSP-06243) is currently available for download from the TSP library.

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    Magazine cover
    Aerospace & Defense Technology Magazine

    This article first appeared in the June, 2024 issue of Aerospace & Defense Technology Magazine (Vol. 9 No. 4).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document presents a study on the aeroacoustics of a boundary layer ingesting (BLI) ducted fan, focusing on its noise generation mechanisms in the context of aerospace applications. Conducted by researchers from the University of Bristol, the investigation utilizes a test rig that simulates a ducted fan immersed in an adverse streamwise pressure gradient turbulent boundary layer over a curved wall.

    The experimental setup features a scaled-down version of the Bell X-22A ducted fan, designed with a straight duct of constant cross-sectional area. The fan, equipped with a three-bladed design based on the NACA-23012 profile, is driven by an electric motor and is supported by strut structures. The study employs advanced measurement techniques, including a far-field microphone array and pressure taps, to capture acoustic pressure data and assess the flow field around the fan.

    Key findings reveal that the noise produced by the BLI ducted fan results from intricate interactions among the fan, duct, and incoming boundary layer. The analysis categorizes the duct noise radiation into planar and non-planar waves, depending on the excitation frequencies relative to the duct's cutoff frequency. In low thrust regimes, the noise radiation field exhibits harmonically arranged fan noise, duct-induced noise from vortex shedding, and a haystack pattern, while in high thrust regimes, the noise displays a more organized structure.

    The study emphasizes the importance of understanding these noise radiation mechanisms to develop quieter airframe-integrated propulsion systems for future aircraft designs. The results contribute to the broader field of aeroacoustics by providing insights into how duct acoustics can alter noise characteristics, such as spectral broadening and the presence of humps in haystack noise signatures.

    In conclusion, this research not only enhances the understanding of noise generation in BLI ducted fans but also lays the groundwork for industrial guidelines aimed at reducing noise in aviation. The findings are expected to inform future designs and innovations in aircraft propulsion systems, aligning with the goals of the European Union’s Horizon 2020 research and innovation program.

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