|Acoustic impedance of micro-perforated panels under complex grazing flow condition
|Cheng, Li (ME)
Absorption of sound -- Measurement
Hong Kong Polytechnic University -- Dissertations
|Department of Mechanical Engineering
|xii, 67 pages : color illustrations
|Micro-perforated panels (MPPs), a new type of broadband sound absorption structure with high acoustic resistance and low acoustic reactance, are widely used for designing various noise control devices such as silencers. Nowadays, MPP exposed to a grazing flow is applied in many practical engineering areas and this problem attracts more and more attention from researchers. In spite of the large amount of efforts made in the past few years, many fundamental problems still remain unsolved. As an important metric to characterize the acoustic properties of MPPs, existing acoustic resistance formula with grazing of different flow speeds give very different results. More recently, a complete set of acoustic impedance formulae of MPPs with grazing flow in the low Mach number range has been established through numerical experiments. Later on, the proposed formulae have been compared with the experimental data reported in the literature for their validations. It is noted, however, when the Mach number of grazing flow exceeds 0.3Ma, an apparent discrepancy occurs between the proposed acoustic resistance formulae and experimental data, which suggests that the application range of the proposed formulae are limited to a certain Mach number range. Moreover, research on MPPs with high Mach number grazing flow under linear acoustic excitation region is scarce. This calls for a comprehensive investigation of these important issues in relation to major flow parameters, and eventually the proposal of a more reliable acoustic resistance prediction means, applicable to cases involving high Mach number grazing flow.
In this thesis, numerical analyses are carried out on a MPP with a backing cavity in a square duct by applying RANS (Reynolds-averaged Navier-Stokes) and LNSE (Linearized Navier - Stokes Equations) methods. Numerical simulations allow for 1). observing the flow field near the hole of the MPP and the coupled flow-acoustic field excited by the incoming acoustic waves; 2) identifying vortex shedding near the hole as the dominant mechanism of sound energy dissipation in a linearly acoustic excitation region (low SPL); and 3). identifying the friction velocity in the flow field as the fundamental flow parameter and showing its linear relationship with acoustic resistance in high Mach number grazing flow. Through a curve-fitting analysis, a new set of acoustic resistance formulae is proposed, which is applicable to the linear acoustic region within a certain flow range of high Mach number grazing flow. The proposed acoustic resistance formulae are validated through comparisons with experimentally measured data reported in the literature and numerically simulated results using appropriate simulation tools. Finally, the results show a good agreement with these data, thus validating the accuracy and the validity of the proposed acoustic resistance formulae.
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