| Author: | Xue, Rong |
| Title: | A study of low-frequency duct noise control based on Helmholtz resonators in the presence of grazing flow |
| Advisors: | Mak, Cheuk Ming (BEEE) |
| Degree: | Ph.D. |
| Year: | 2025 |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xvii, 125 pages : color illustrations |
| Language: | English |
| Abstract: | The ventilation ductwork system has become an important part of modern buildings that maintain good indoor air quality. However, transmission noise from a ventilation fan and flow-generated noise from the interaction of moving air and duct flow discontinuities are unavoidable. While the commonly used duct noise silencer can effectively reduce medium to high-frequency noise over a broadband frequency range, low-frequency noise control still remains a challenge to most engineers. The Helmholtz resonator (HR), with simple geometric structures and easy to manufacture and maintain, has become an effective strategy for low-frequency noise attenuation. In the present study, the low-frequency noise attenuation performance of silencers based on HRs in the presence of grazing flow is investigated. In the first part of this thesis, a dual HR system is designed by connecting a pair of HRs in series (neck-cavity-neck-cavity). The influence of neck length, cavity volume, and flow Mach number on the noise attenuation performance of a dual HR system is investigated. A three-dimensional numerical simulation is implemented to calculate the transmission loss results. The transmission loss (TL) results show the second neck length can influence the second resonance frequency and the TL peak. Changing the cavity volume will significantly influence the noise attenuation ability under lower flow rate conditions than the higher flow rate conditions. The flow field results show that air only enters the first neck and cavity. The flow Mach number has a more significant impact on the first TL peak than on the second TL peak. In the second part of this thesis, the acoustic behavior of the HR model in the presence of grazing flow is investigated experimentally. The four-microphone method is performed to measure the transmission loss performance of the resonator model under flow speeds of 0-20 m/s, and the particle image velocimetry (PIV) measurement is adopted to evaluate the fluid characteristics of the resonator neck. The flow effect on the acoustic performance of the resonator model is derived by using the curve fit technique. An empirical model is established to predict the transmission loss of the Helmholtz resonator in the presence of the grazing flow. The empirical model is added to the transfer matrix method to predict the transmission loss of the period Helmholtz resonator model under different grazing flow speeds. The PIV experiment results reveal the fluid dynamics in the neck region of the resonator model. In the third part of this thesis, a lightweight and compact sound-absorbing metamaterial-based Helmholtz Resonator Array (HRA) that exhibits broadband low-frequency noise attenuation performance even under low-speed flow conditions is presented. A semi-empirical theoretical model is developed by integrating Guess's empirical model with a flow correction term to predict the transmission loss performance of the designed HRA under various grazing flow velocities. The designed HRA is composed of 8 single Helmholtz resonator models and is optimized to increase the noise attenuation band. Results show that the designed HRA can achieve around 20 dB/m in the prescribed frequency range of 200 to 800 Hz under grazing flow speeds of 0-20 m/s, and the proposed theoretical model can accurately predict transmission loss performance under different grazing flow speeds. The design strategy of the HRA model offers an effective solution for noise reduction in ventilation duct systems. |
| Rights: | All rights reserved |
| Access: | open access |
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