Aerospace & Defense Technology - June 2024 - 34

Tech Briefs
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.
University of Bristol, Bristol, UK
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
The Boundary Layer Ingesting (BLI) ducted fan test rig inside the aeroacoustics wind tunnel facility at the
University of Bristol. (Image: University of Bristol)
34
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potential to steer significant research
activity within the fluid mechanics community.
This, in turn, could foster a deeper
understanding and further exploration
Aerospace & Defense Technology, June 2024

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Aerospace & Defense Technology - June 2024

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