Abstract
To broaden its’ effective frequency range and to improve its transmission loss performance, a modified design of a Helmholtz resonator is proposed and evaluated by implementing a rigid baffle in its cavity. Comparison is then made between the proposed design and the conventional one by considering a rectangular duct with the resonator implemented in the presence of a mean grazing flow. For this, a linearized 2D Navier-Stokes model in frequency domain is developed. After validated by benchmarking with the available experimental data and our experimental measurements, the model is used to evaluate the effects of (1) the width Lp of the rigid baffle, (2) its implementation location/height Hg, (3) its implementation configurations (i.e., attached to the left sidewall or right sidewall), (4) the grazing mean flow Mu (Mach number), and (5) the neck shape on a noise damping effect. It is shown that as the rigid baffle is attached in the 2 different configurations, the resonant frequencies and the maximum transmission losses cannot be predicted by using the classical theoretical formulation ω2=c2S/VLeff, especially as the grazing Mach number Mu is greater than 0.07, i.e., Mu>0.07. In addition, there is an optimum grazing flow Mach number corresponding to the maximum transmission loss peak, as the width Lp is less than half of the cavity width Dr, i.e., Lp/Dr≤0.5. As the rigid plate width is increased to Lp/Dr=0.75, one additional transmission loss peak at approximately 400 Hz is produced. The generation of the 12 dB transmission loss peak at 400 Hz is shown to attribute to the sound and structure interaction. Finally, varying the neck shape from the conventional one to an arc one leads to the dominant resonant frequency being increased by approximately 20% and so the secondary transmission loss peak by 2-5 dB. The present work proposes and systematically studies an improved design of a Helmholtz resonator with an additional transmission loss peak at a high frequency, besides the dominant peak at a low frequency.
Highlights
Helmholtz resonators are widely applied in automobile [1] and aerospace industries as an effective acoustics noise damper [2, 3]
With the model being validated, the Helmholtz resonator with a rigid baffle attached to the right sidewall is numerically studied, as the grazing flow Mach number Mu is set to 4 different values
The local maximum transmission loss at the secondary peak is increased by approximately 25 dB. This means that modifying the structure of the Helmholtz resonator, such as the neck shape, or implementing a rigid baffle leads to the shift of the resonant frequency and the improvement on the secondary transmission loss peaks
Summary
Helmholtz resonators are widely applied in automobile [1] and aerospace industries as an effective acoustics noise damper [2, 3]. To achieve optimum noise damping, acoustic resonance is expected, at which a large volume of the working fluid in the cavity periodically expands and compresses This means that if the resonator cavity is structurally modified, the resonant frequency and the effective bandwidth are varied. It affects the acoustic damping performances of Helmholtz resonators. Ćosić et al [25] experimentally study the acoustic noise damping performance of Helmholtz resonators, when the grazing and cooling flows have a temperature difference between them. Foregoing studies are conducted to improve the acoustic noise damping performance and/or broaden the effective frequency ranges of Helmholtz resonators.
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