Experimental investigation of geometric shape effect of coupled Helmholtz resonators on aeroacoustics damping performances in presence of low grazing flow

Abstract

Helmholtz resonators are widely used as acoustic dampers to mitigate noise in gas turbines and automotive exhaust systems. In order to increase the damping performance and broaden the damping frequency range, multiple Helmholtz resonators are typically implemented. In this work, two connected Helmholtz resonators by sharing the same flexible or rigid sidewall, i.e. parallel-coupled are designed and tested on a cold-flow pipe system with a mean flow present. Two different geometric shapes of the coupled Helmholtz resonators are considered. One is cylindrically shaped and the other is rectangular. Both experimental and 3D numerical investigations are conducted to study the effects of the geometric shape and the mean grazing flow on the noise damping performance of the resonators. The numerical model is built in frequency-domain and solving linearized Navier-Stokes equations. To characterize and quantify the resonators' noise damping performance, transmission loss in dB is determined. It is experimentally found that the cylindrical shaped resonators are associated with much larger transmission loss. Compared with the rectangular-shaped resonators, approximately 12 dB more sound pressure level reduction is achieved by the cylindrical resonators, which is independent on the rigidness of the shared sidewall. The rigidity of the shared sidewall is shown to shift the frequencies corresponding to the maximum damping peaks, depending on the sidewall's flexural rigidity. Numerical results confirm that the geometric shape does play an important role in determining the resonator's damping. Further investigation of the mean grazing flow effect is conducted. It is shown experimentally and numerically that when the Mach number of the mean grazing flow is lower than 0.01, the noise damping performance of the coupled Helmholtz resonators is little changed over the frequency range from 150 to 600 Hz. However, as the mean flow Mach number is increased, the damping performance becomes deteriorated as observed numerically. The present work reveals the critical roles played by the geometric shape and the mean grazing flow on the aeroacoustics damping performance of the coupled-resonators.