A recent study conducted by researchers at the University of Bristol has uncovered the process of how noise is produced and spread from advanced aircraft engines with an energy-efficient design. This study, which has been published in the Journal of Fluid Mechanics, provides the first insights into the generation and propagation of noise from these engines, known as boundary layer ingesting (BLI) ducted fans. BLI ducted fans have similarities to the large engines found on modern aircraft.The engines in modern airplanes are not only located under the wings but are also partially embedded into the plane’s main body. This design allows the engines to ingest air from both the front and from the surface of the airframe, making it easier for them to move the plane and therefore burn less fuel.
This research was conducted by Feroz Ahmed from Bristol’s School of Civil, Aerospace and Design Engineering, under the guidance of Professor Mahdi Azarpeyvand. They used the University National Aeroacoustic Wind Tunnel Facility to identify specific noise sources coming from the duct, the rotating fan, and the air flowing over the curved airframe surface.The noise pattern varies depending on the amount of thrust the fan generates. At high thrust, the noise pattern is similar to fans without ducts. However, at lower thrust levels, the duct itself starts to make more noise. Dr. Ahmed stated, “Our research tackles the pressing issue of noise, which is a significant barrier to obtaining certifications, by revealing the physics behind the noise produced by these setups.” Understanding the noise mechanisms in BLI ducted fans could lead to the development of industrial guidelines for quieter operation.The integration of airframe-integrated propulsion systems is being explored for future aircraft designs, ranging from traditional large-scale aircraft to small-scale electric vertical take-off and landing (eVTOL) aircraft.
Leading projects such as the Bell X-22A, Embraer X, Airbus E-fan, Lilium Jet, Green Jet, and Hybrid Air Vehicle are at the forefront of developing these systems for the next generation of aircraft. These innovations are gaining popularity thanks to advancements in powerful electric motors.
Dr. Ahmed noted that the noise levels of embedded ducted fans are still a mystery, especially when they are ingested into the aircraft’s structure. This is an important consideration for the future development of these propulsion systems.
ng 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 mouThe fan was placed next to a curved wall to mimic embedded engines seen in designs like the ONERA NOVA aircraft concept. Various types of data were collected from the setup, such as measurements of the fan’s thrust output and the level of noise produced. This framework allowed for a deeper understanding of the complex noise interaction mechanisms from different sources, revealing the underlying physics of noise generation and how it varied as the fan operated at different thrust levels.
Dr. Ahmed stated: “As there is an increasing demand for a more enjoyable flight experience with minimal environmental impact, quieter air travel is essential.”This study has the potential to be applied in developing methods to decrease noise produced in the aviation industry. “Moreover, our extensive examination of identifying the noise sources in BLI ducted fans could lead to substantial research efforts in the field of fluid mechanics. This could further enhance the understanding and exploration of aeroacoustic phenomena in ducted fans exposed to various turbulent flows.” Our research reveals insights into the generation of noise by advanced embedded ducted fans on curved airframe surfaces, uncovering the noise generated by these fans.The study found that patterns change depending on the level of fan thrust, providing important information for designing quieter next-generation aircraft. This research was supported by the European Union’s Horizon 2020 research and innovation program through the ENODISE (Enabling optimized disruptive airframe-propulsion integration concepts) project grant agreement number 860103, and was carried out in collaboration with ONERA — the French Aerospace Lab, and other partners.
Reference:
Feroz Ahmed, Ismaeel Zaman, Djamel Rezgui, Mahdi Azarpeyvand. Aeroacoustics of a ducted fan ingesting an adverse airway contraction.Essure gradient boundary layer was the focus of a study published in the Journal of Fluid Mechanics in 2024. The DOI for this article is 10.1017/jfm.2024.134.