The influence of the grazing flow and sound incidence direction on the acoustic characteristics of double Helmholtz resonators

Restricted by the principle of resonance, Helmholtz resonators can effectively absorb noise only near the resonance frequency. It is an effective way that the coupling design of the multiple resonators to broaden the absorption band. In practical application, Helmholtz resonators are usually arranged in the flow duct, so that will be affected by the working condition. In order to investigate the influence of the grazing flow and the sound incident direction on the acoustic performance, both numerical simulations and experiments are conducted. A three-dimensional model of the double Helmholtz resonators is established. By solving the linearized Navier–Stokes (LNS) equations, the sound transmission loss of the system in two sound incident directions is predicted where the mean flow Mach number is 0 and 0.1, respectively. The absorption, reflection, and transmission coefficients are further obtained. Based on the comparison of the flow fields and sound fields, the mechanism of the variation of the acoustic characteristics is analyzed. The experimental results show that the maximum sound transmission loss decreases by 16.9 dB, and the resonance frequency increases by 59 Hz. Both numerical and experimental results indicate that the grazing flow weakens the ability of noise suppression and the low-frequency characteristics of the double Helmholtz resonators in this paper. The inherent asymmetric absorption/reflection phenomenon vanishes, and the acoustic characteristics of the coupled system are dominated by the background flow. While the exchange of the sound incidence direction can mitigate the penalty of the grazing flow on the coupled system.