Development of thermoacoustic metastructures

The thermoacoustic phenomenon occurs due to an instability caused by acoustic waves propagating in a waveguide, inducing oscillations in a porous stack, the ends of which sustain a thermal gradient. This allows for the conversion of acoustic energy to heat or vice versa without the need for moving parts or expendables. Incorporating thermoacoustic elements into the structural framework of acoustic liners with innovative core geometries can create thermoacoustic metastructures (TAMS) enabling airborne noise mitigation, energy harvesting and environmental sensing. An experimental study on 3D-printed stacks using a benchtop rig was correlated with DeltaEC simulations. 3D-printed stacks generally performed comparably to conventional stacks offering affordability and enabling complex core geometries. Proof-of-concept demonstrations were achieved for energy harvesting and acoustic noise mitigation. An acousto-thermo-electric transduction scheme based on thermoacoustic and Seebeck effects was employed to harvest usable electrical power, achieving 33 mV peak voltage and 19.8 μW output power across a 1 Ω load for 119 dB acoustic excitation at resonance (117.5 Hz). A reduction of approximately 5.28 dB in sound pressure level was measured between the source and termination ends. Informed by these results, a metamaterials-inspired design approach was used to develop novel TAMS, such as spiral and 'pie-slice' resonators, to demonstrate enhanced broadband response and improved power-to-volume ratio in the low-frequency (<500 Hz) regime. Prototypical thermoacoustic liner test articles, including straight and folded-core configurations, were designed and fabricated for testing in NASA Langley’s normal-incidence and grazing flow impedance tubes. Rott’s approximation was used to optimize stack geometry for operational conditions, providing a 9.5 °C steady-state temperature gradient at 790 Hz for 140 dB tonal dwells. The temperature gradient correlated with the acoustic power at the leading stack-face. Higher excitation pressures (>140 dB) showed viscous heating contributions to deviations from simulations, while increased flow speeds (M>0.1) reduced the temperature gradient due to convective cooling of stack faces. Further, the relationship between acoustic absorption and thermoacoustic temperature gradient was examined. With current additive/ hybrid fabrication processes reaching commercial maturity, TAMS could offer enhanced solutions for aircraft noise mitigation, multifunctional structures for built environments, and energy harvesters/ sensors for remote or harsh locations.