|Acoustic behavior of micro-perforated panels in grazing flow
|Cheng, Li (ME)
|Absorption of sound
Hong Kong Polytechnic University -- Dissertations
|Department of Mechanical Engineering
|xxii, 202 pages : color illustrations
|Micro-perforated panels (MPPs) are widely used for broadband sound absorptions. A MPP exposed to a grazing flow is an important problem in acoustics and has many practical engineering applications. Despite the considerable efforts and the progress made during the last decades, many fundamental issues still remain to be addressed. To mention a few, explanations on the sound energy dissipation mechanism in the presence of flow are not consistent and convincing in the open literature; existing acoustic impedance formulae based on different flow parameters give inconsistent results etc. This calls for a systematic investigation of these important issues and eventually find more intrinsic flow parameters allowing for a reliable acoustic impedance prediction. In this thesis, 3D CFD simulations are conducted on a MPP with a backing space in a flow duct. Numerical analyses allow scrutinizing the flow field near the perforation hole and its interaction with the incoming acoustic waves, identifying viscous dissipation in the shear layer near the orifice as the dominant sound energy dissipation mechanism in a linearly low acoustic excitation regime, identifying the flow velocity gradient in the viscous sublayer as the intrinsic flow parameter and showing its linear relationship with a flow-related term in the acoustic resistance formula. Through a linear regression analysis, a new set of acoustic impedance formula is proposed, applicable within a certain flow range under the linear acoustic regime. The proposed impedance formulae are validated through comparisons with existing impedance data reported in the open literature as well as with experimentally measured results using an inverse derivation method. Results show a good agreement with these data and the superiority of the proposed impedance formulae over the existing ones in terms of prediction accuracy. Capitalizing on the established acoustic impedance prediction formulae, the noise attenuation performance of MPPs in flow ducts with grazing flow is investigated for various configurations. Incorporating the acoustic impedance formulae into the general Patch Transfer Function (PTF) framework, numerical analyses are conducted to analyze the effects of various system parameters and to shed light on the underlying sound attenuation mechanism of MPP silencers in flow ducts. Effects of various system parameters, such as grazing flow velocities, solid partitions inside the backing cavity of the MPPs, their dimension and other panel parameters, are examined in vies of providing guidelines for the practical design of MPP-based silencers. The numerically predicted noise attenuation curves are then validated through comparisons with measurements under various grazing flow conditions.
Finally, the feasibility of integrating MPPs in a simplified home appliance model having a more complex geometry and being subjected to flow is explored. Two methods, the hybrid theoretical-numerical technique based on PTF approach and the other one using coordinate transformation technique, are presented to tackle the numerical challenges in coping with the increasing system complex. The possibility of implementing MPP absorbers in practical industrial devices for acoustic noise mitigation is demonstrated. Results reveal a hybrid noise reduction mechanism and point at the need for proper systematic parameter tuning in order to achieve the noise control target. As an illustration, a few selected optimization problems are discussed to highlight the efficacy of the PTF approach alongside the proposed acoustic impedance prediction formulae established in this thesis. Meanwhile, the improved capability and efficiency of the improved PTF approach based on coordinate transformation are also demonstrated by comparing with the optimization results from hybrid theoretical-numerical treatment.
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