Resilient and Scalable Bio-Inspired Metamaterial for Passive Noise Control

Aerodynamic noise poses a significant challenge in urban environments, impacting the quality of life for residents. Interaction between an object's surface and air causes an adverse pressure gradient, resulting in flow separation and vortices around aircraft, contributing to aerodynamic noise. Experimental investigations were conducted using turbulent jet flow on cylindrical bodies, coated with periodically arranged, mushroom shaped micro-structures. The jet was tested at three different Mach numbers, ranging from the subsonic to transonic regions. The micro-structure coatings have different spacing-to-height ratios of 0.7, 1.0, and 1.8, along with an uncoated cylinder. The findings indicate a substantial reduction in overall noise, ranging from 2 to 6 dB (30-75%) when using the three coatings in comparison to the base cylinder. This reduction is especially noticeable at lower frequencies within the human hearing range of 20 Hz to 20,000 Hz. The surface modification with shark skin denticles facilitated the reduction in pressure fluctuations leading to remarkable attenuation of sound transmission. The 0.7 and 1.0 coatings show a larger reduction in pressure fluctuations compared to the 1.8 ratio. A suppression of vortex shedding intensity is believed to the cause of reduction, but there is no delay in flow separation. The study's results can significantly contribute to quieter and more efficient systems in various industries, offering noise reduction strategies based on biomimetic principles.