This article is an extract from engineering.com
The study of the sound associated with our airplanes, also known as
aeroacoustics, is a complicated business. It not only covers the sound
you would hear in the plane, it studies the sounds heard from bystanders
on the ground. Noisy airplanes can make for unhappy customers, sleepy
pilots and a “not in my back yard” attitude. With the use of simulation,
though, we can all watch the skies without hearing them as well.
Noises travel to our ears as distortions of the air around us. It is
therefore no surprise that turbulent flow through the air, such as by
landing gear, can cause quite a racket. “Landing gear, being an
essential part of the plane,” remarks Fred Mendonça, a member of
CD-adapco’s technical organization, “sticks out like a bluff body and
violently disturbs the airflow creating noise and drag. This doesn’t
greatly affect fuel consumption, but it does account for 35-40% of the
noise associated with landings. Other landing configuration components
like the deployed slats and flaps of the wing will account for another
35-40%.”
Having contributed 70-80% of the aircraft’s noise, it is no wonder
that these areas of the plane have many aeroacoustics studies associated
with them.
Mendonça
adds that, “Simulation software, like STAR-CCM+, can act as a virtual
wind tunnel at a fraction of the cost. You don’t need to make a physical
model, book time at a wind tunnel, and borrow wind experts to run the
equipment. Just take your CAD model and perform the analysis in the
computer, you can even easily tweak and test the new design. This will
allow you to understand the flow around the area and the noise
generation mechanisms.”
The noise doesn’t stop at the wing; cooling racks and internal air
circulation ducts will also contribute to the racket. “A cockpit has a
lot of electronics and will therefore produce a lot of heat. Air
channels are needed for active cooling. Compressed air, bled into the
cabin compartment to maintain cabin pressure and temperature, create
noise in the delivery ducts and discharge nozzles,” explains Mendonça.
This air will pass through many passageways and ducts, each time it
turns or passes through an orifice turbulent flows can be created,
ultimately producing noise. “Once we simulate the airflow, we can
modify the design to limit noise,” said Mendonça.
He
adds that “even low noise exposure over a long time can cause fatigue.
This becomes a safety issue. There is the same issue in cars and trains.
It attributes to why commercial pilots and truck drivers have a limit
on how long they can operate the vehicle. If you have a manufacturer
which supplies low noise vehicles, then they can be operated more
comfortably, safer and for longer.”
The major difference between the acoustics simulations around the
plane and the simulations within the ventilation is a point of
reference. Instead of creating a wind tunnel for the plane to “fly
through,” you now are making a smaller wind tunnel to represent the
inside of the vent. Essentially, the CFD model will be solving the same
equations.
However, pilots are not the only thing associated with fatigue when
vibrations are involved. Portions of the plane can experience structural
fatigue due to aerodynamic flow. “Any vibrations will cause fatigue
stress,” said Mendonça. “But if resonance modes are excited then you
have more of an issue like possible component fatigue failure.
I’m not sure DaVinci knew how noisy his dream of flight might be –
but I’m sure he’d be proud of modern industry for using simulation to
ease the natural trial-and-error of the scientific process.
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