Acoustic absorption in aircraft
Honeycomb-like structures within nickel-based superalloys could reduce the noise emitted from aircraft engines.
A researcher in the USA is developing a microchannelled version of the material, which is well established in the aerospace sector for withstanding the extreme temperatures and corrosive conditions of the engine environment.
The technique uses a mechanism called viscous shear. ‘You create a boundary layer that is parallel to the direction of the incident sound wave,’ explains research engineer Jason Nadler at the Georgia Tech Research Institute in Atlanta. The honeycomb geometry traps the sound waves, which weaken and dissipate through viscous shear (caused by interaction with the porous solid medium and the impact of resulting friction).
‘If the pores are too small (less than hundreds of microns), they just reflect the sound as the waves cannot get in,’ explains Nadler. ‘You need to have the hole big enough to let the sound waves in, but also have enough surface area inside to shear against the wave. The result is the acoustic waves don’t resonate, they just dissipate.’
This contrasts to most other foams or cellular sound reduction materials. These allow the waves to penetrate the material and cause them to resonate at particular frequencies, depending on the volume of air in the cells. ‘Just like when you blow across a bottle you get a given frequency,’ says Nadler. His discovery, he says, can trap sound regardless of frequency.
Though Nadler is reluctant to discuss the synthesis technique, he believes the research could ease the noise-related concerns of communities living near airports or military bases. He explains, ‘I stuck with a material that is already used in aircraft. The selection means hot sections of the engine – the turbines – are the target area’.
However, the inherent thermal and chemical resistance of the nickel-based superalloy makes it difficult to work with. ‘The material is designed to be resistant to everything you do to it,’ says Nadler. ‘The biggest challenge is using high performance metals at these fine scales to incorporate features you can control and have 25% less material. In conventional metal foams, the pores are more random.’
The superalloy was considered light and strong enough to enable thin walls between the channels and an overall lower structural density, while maintaining its robustness to the aggressive environment of aircraft engines.
Further research will investigate the effect of the honeycomb structure on the properties of the material. ‘From a microstructural standpoint we have retained the characteristics of the bulk material,’ notes Nadler.
Impedance acoustic tests have demonstrated the material’s sound reduction capabilities, however, Nadler admits there is more work to be done.
‘There is a difference between the acoustic behaviour of the material itself and the overall contribution of the material when you put it in the engine. There are multiple contributions to noise in an aircraft and reducing sound in one part might have a small impact on the overall noise.’ Research now needs to focus on themagnitude of sound dissipation.
Nadler hopes to work with aerospace manufacturers to scale up the synthesis process and demonstrate the capabilities of his research.