How Quiet Owl Wings Inspire Quieter Planes and Machines

  • Researchers are using biomimicry to design quieter turbomachinery, planes and drones.
  • The trailing edge of an owl’s wings changes airflow patterns, drowning out loud noise.
  • If animal biologists and engineers work together, it could lead to a more complete understanding of why owls are virtually silent fliers.

    If birds could be ninjas, owls would be the best. Silent and agile, the winged creatures leave their prey with no time to escape as they swoop in to kill it. This stealth capability has inspired scientists to create quieter motion for airfoil designs in everyday technology, including airplanes, vehicles, drones and wind turbines.

    Researchers at Xi’an Jiaotong University in Xi’an, China, specifically used owl wings as inspiration for quieter turbomachinery blades, which can be noisy as air sweeps across the edge of the blade. leaking from their curved surfaces. (Turbomachinery is a term that describes machines that transfer energy between a rotor and a fluid, including turbines and compressors).

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    Scientists have found that serrated (or serrated) trailing edges reduce noise from rotating machinery, especially when shaped like owl wings. The air flowing over these comb-like shapes breaks up instead of continuing in straight lines, drowning out the whistle of the passing owl. The downy feathers on the rest of the wing can also absorb sound. The researchers describe their work in an article published in the journal Fluid physics at the end of last year.

    “Night owls produce about 18 decibels less noise than other birds at similar flight speeds due to their unique wing configuration,” said Xiaomin Liu, one of the paper’s authors. Press release. “In addition, when the owl catches prey, the shape of the wings also constantly changes, so the study of the configuration of the edge of the wing during the flight of the owl is of great importance.”

    An engineering “spirit animal”

    Since the earliest days of human flight, inventors have drawn inspiration from birds. The Wright brothers, for example, recognized that the shape of a bird’s wings was crucial for taking off, so they copied those properties into their mechanical airplane design. Yet scientists are still perplexed when faced with the noisy problem of trailing edge dissonance, a phenomenon in which air moves noisily over the trailing edge of an airfoil; it is the last piece of an airplane wing that air touches as it passes over it back and forth.

    “I would say the owl is a long-standing spirit animal, if you will, for the air acoustics community,” Justin Jaworskimechanical engineer at Lehigh University in Bethlehem, Pennsylvania, says Popular mechanics. “[W]When you’ve exhausted a lot of innovative ideas, you look for other sources of inspiration. And the owl has been a source of inspiration for at least 80 years.

    Air behaves like a fluid as it flows over and under an airfoil, the surface that passes through air at high speed. For an airplane or a bird, the airfoil is the wing, and for wind turbines, it is the rotating blade. The shape of the airfoil is the key to noise generation here. As air flows over the rear of an airfoil, it becomes turbulent along the top and bottom surfaces of the airfoil. When this layer of air reaches the rear of a smooth-edged airfoil – the trailing edge – it disperses and generates noise. “Jagged edges can weaken this process in part by disrupting the local airflow where this noise is generated,” says Jaworski.

    Previous studies have found that serrated or serrated trailing edges reduce noise from rotating machinery, but the extent to which this depends on the end application. Some turbomachines already benefit from this knowledge. For example, Hamburg, Germany-based Siemens Gamesa has added serrated trailing edges to its wind turbines.

    “At present, the design of rotary turbine engine blades has gradually matured, but the noise reduction technology is still at a bottleneck,” Liu said in the press release. “The noise reduction capabilities of conventional sawtooth structures are limited, and some new non-smooth trailing edge structures need to be proposed and developed to further exploit the bionic noise reduction potential.”

    How Airfoils Work

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    Jaworski’s own work in fluid dynamics, the study of liquids and gases in motion, examines what features of owl wings are responsible for this nearly silent flight. Sound 2020 paper on the aeroacoustics of owl flight, published in the Annual review of fluid mechanics, reviews significant previous research in this area, including its own foray into how the physics of airflow could be applied to creating quieter technologies. Scientists typically conduct experiments to measure the sound of live owls in flight, test their own airfoil designs that mimic owl wings, and mathematically model new design possibilities.

    During this time, Liu and his team investigated seven different trailing edge designs using computer simulations; they created a virtual airfoil shape representative of part of a long-eared owl wing, but replaced the trailing edge shapes. They ran each theoretical model through a program that simulated airflow at speeds used for turbomachinery.

    Their tests showed that a smooth trailing edge creates the most noise. Having a pointed sawtooth shape on the trailing edge reduces noise somewhat. However, when the triangular teeth at the edge of the airfoil are curved and coupled in two layers, like the feathers at the tips of an owl’s wing, the airfoil pattern becomes noticeably quieter.

    Smooth trailing edges are why wind turbines and airplanes are so loud. In the case of airplanes, there is a fluid boundary layer, a thin layer of air that covers any surface. As the wing slices through the air, the layer thickens towards the trailing edge before merging again with the air around the wing. “It’s like a bunch of little speakers when they come towards the trailing edge,” says Jaworski. The edge amplifies this sound; the air flowing over the wing actually produces sound that we hear (in addition to the more obvious engine noise during flight).

    What makes Owl Flight so quiet

    owl gliding through the air

    photography by Linda LyonsGetty Images

    Owls are quiet so their prey, which has very sensitive hearing, won’t notice them coming. When owls fly past a highly sensitive microphone array to measure their sound levels, researchers have found that they are virtually silent, at least to human ears. To find out why, scientists converted owl-inspired airfoil designs into theoretical models. “Being able to expand on these simple computational models and learn from them is helpful in creating much more sophisticated models that can incorporate more realistic owl wing features,” says Jaworski.

    David Lentin from Stanford University, for example, study the flight of birds and translates it into technical design. Lentink investigated the wings of a swift, which stays in the air for a year, continuously, after leaving the nest. It eats flying insects, draws water from its wings and even mates in the air. Lentink’s team of undergraduate students then developed a drone whose design is based on the flight behavior of the swift. He also created the PigeonBot, the most advanced bird-like robot ever built due to its ability to mimic the movement of pigeon wings during flight. He published his work on understanding the biomechanics of pigeon flight in Scientific robotics in January 2020.

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    The fact is that we are far from solving all the mysteries of natural flight. Many theories abound about what makes owls so quiet, but no one knows exactly what is the combination of factors. “If we knew that, we would design silent objects all around you, using what we know,” says Jaworski. People have tried to mimic the scalloped geometry along the trailing edge of a wing, but no one has actually been able to replicate the noise reductions owls get on an owl-sized model. , he said.

    Like other birds of prey, owls’ wings have various structural parts and arrangements, and many different feather shapes and sizes. Trying to model these complex biological parts, whose specific functions aren’t even fully understood yet, is a challenge, says Jaworski. “Is it a part of the wing that makes them so quiet?” Or is it the combination of several parts of the wing? And how do they use it? This is what makes things difficult. There are only an infinite number of designs for edge geometries that you can examine. You know, it’s still an idealization. We are looking for ideas that may guide the next research program.

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