Brazilian researchers studied the morphology of owl wings in search of characteristics that enable these birds to fly silently (image: UNICAMP)

Published on 03/18/2021

By Elton Alisson  |  Agência FAPESP – The aircraft industry has been pressured by aviation regulators to reduce the noise produced by jetliners as they take off and land so that by 2030 they cannot be heard outside airports.

A potential solution to this problem has been found in the wings of owls (Strigiformes) by researchers at the University of Campinas (UNICAMP) in the state of São Paulo, Brazil. All owl species fly virtually silently, a quality that has long fascinated scientists.

A study of flight aerodynamics in owls by researchers at the Aeronautical Science Laboratory in UNICAMP’s School of Mechanical Engineering (FEM), in collaboration with colleagues at the Aeronautical Technology Institute (ITA) in São José dos Campos and Lehigh University in Pennsylvania (USA), identified owl wing features that could be emulated by designers of aircraft wings to eliminate noise.

“We developed a numerical mathematical model to simulate certain characteristics of owl wings in aircraft wings, and proved experimentally that noise was significantly reduced by these design features,” said William Wolf, a professor at FEM-UNICAMP and one of the researchers responsible for the project on the Brazilian side.

Wolf is a principal investigator in the Center for Computational Engineering and Sciences (CCES), hosted by UNICAMP’s Chemistry Institute and associated with the Center for Mathematical Sciences Applied to Industry (CeMEAI). 

CCES and CeMEAI are Research, Innovation and Dissemination Centers (RIDCs) funded by FAPESP.

According to Wolf, aerodynamic noise is due to turbulent airflow over the aircraft’s fuselage and wings, among other parts. The resulting friction converts airspeed energy into sound waves. 

During takeoff, the engines are at full throttle and produce most of the noise, but while an airplane is preparing to land engine power is reduced and the main sources of noise are aerodynamic, including the flow of air over the landing gear and wings, especially the flaps and slats (trailing- and leading-edge wing sections used to control lift). 

“Aerodynamic noise is caused by turbulence in the airflow around these parts of the aircraft,” Wolf said.

In recent years, newly designed, more efficient jet engines have become larger, and have had to be placed nearer to the wings in order to be further from the ground. This proximity increases the interaction between engine exhaust and the trailing edges of the wings, causing acoustic scattering and making the aircraft noisier, he noted.

To find a solution to this problem, the researchers studied owl wing morphology in search of characteristics that help explain why these birds fly silently. They observed that owls’ wing feathers have a velvet-like texture with porous elastic fringes on both the leading and trailing edges, breaking up the turbulence and reducing noise. Trailing-edge serrations also act as acoustic dampeners.

“All these wing elements combine to enable owls to fly silently,” Wolf said.

Based on these observations, the researchers designed a wing system with a forward-swept trailing edge. This change attenuated engine noise scattering, modifying acoustic diffraction and reducing total noise.

The study, which was supported by FAPESP, resulted in patent applications in Europe and the US for the new silent wing design. The research was conducted in partnership with researchers at ITA, Poitiers University in France, and Airbus.

Landing gear noise

The UNICAMP researchers have also worked with Boeing on projects in this research area, known as aeroacoustics, using computer simulations and statistical techniques to assess the effects of landing gear turbulence in a Boeing 777. 

The analysis showed that the main sources of landing gear noise were wheel cavities used for maintenance and brake cooling, and the wheel wells in the undercarriage into which the landing gear is retracted immediately after takeoff (read more at: agencia.fapesp.br/25559).

“We discovered that at certain frequencies excited by turbulent airflow some of these cavities displayed resonance effects that generate intense noise and can be extremely disturbing to the human ear,” Wolf said.

The simulations required 7.5 million hours of computation on a supercomputer in the US with 3,200 parallel processing cores, running non-stop for six months. 

“It was one of the biggest computer simulations Boeing had ever performed. A single run of the simulation generated 50 terabytes of data,” Wolf said.

Applications in other areas

The discoveries made in aeroacoustic studies have been applied elsewhere: in the automotive industry, in industrial ventilation, and in wind turbine designs, for example.

Like aircraft wings, wind turbine blades and industrial fans are also directly affected by turbulence, generating noise at both the leading and trailing edge, as the researchers observed in projects conducted in partnership with General Electric (GE), in the case of wind turbines, and FanTR, a manufacturer of industrial fans.

In automotive vehicles turbulence increases both fuel consumption, owing to the rise in drag, and noise, distressing the driver and passengers. The latter problem will be even more evident in the years ahead as electric, self-driving and flying vehicles become more common. As a result, it is receiving more attention from the automotive industry – and, in the case of flying cars, the aerospace industry, Wolf noted.

With the advance of electric vehicles, which produce almost no noise, the automotive industry will have to make an effort to reduce noise from other types of vehicles. In the case of self-driving or autonomous cars, passengers will probably pay less attention to the road and occupy themselves with other activities. “They’ll become more sensitive to external noise generated by turbulence,” Wolf said.

In the case of flying vehicles, vertical landing and takeoff will be noisy operations. “If small drones currently make a lot of noise, imagine numbers of flying cars transporting people in a city like São Paulo,” he said.

 

Source: https://agencia.fapesp.br/34298