Metasurface zero-impedance matching mechanism for aerodynamic noise reduction

Abstract

In order to control the low-mid frequency aerodynamic noises, the zero-impedance matching mechanism of a stepped multi-neck Helmholtz resonator (SMNHR) metasurface on a 1/4 scale Ahmed upper surface is revealed through simulation and experiment. First, this stepped multi-layer neck widens the resonance peak bandwidth compared to the single neck of Helmholtz resonator (HR) due to the decreased acoustic mass and almost unchanged surface acoustic resistance under free inducement without fluid flow. Then, under incident fluid flow, the stepped neck improves the interaction between the cavity and main flow, and thus reduces the surface acoustic impedance. Moreover, the formation of larger pressure difference at the SMNHR interface results in greater shear force, which generates stronger vortexes and bigger ellipse vortex areas inside the neck causing rapidly increased flow velocity and further decreased impedance. Furthermore, increasing the absorption area ratio of SMNHR could make the impedance much smaller as well as less affected by the increasing main-flow velocity, i.e., it is easier to achieve zero impedance even under higher main-flow velocity. When the incident acoustic pressure at the interface are counteracted by the scattered acoustic pressure, the zero-impedance matching is achieved and the near-wall acoustic pressure could be minimized. Finally, a subwavelength SMNHR metasurface composed of four parallel cells with a larger broadband is designed and verified by the wind tunnel test, in which an average sound pressure level (SPL) reduction of 3.59 dB (A) within 1320 Hz−5350 Hz under a main-flow velocity of 50 m/s is realized. The zero-impedance matching mechanism with the metasurface design presented could have potential applications for controlling aerodynamic noises, especially in the low-frequency broadband aspects.