Investigation on the aero-acoustic behaviour of orifice under grazing flow and design of compact microperforated panel absorbers for ducted system

Abstract

Micro-perforated plates (MPPs) are widely employed in mufflers for the noise reduction of fluid systems; they have been extensively studied. With hole sizes typically in the sub-millimetre range, MPPs impart a high acoustic resistance and low reactance to the structure itself without other absorption materials. In real applications, the MPP component is often backed by a cavity or honeycomb structure; this produces an excellent reactive response. In addition, MPP elements are typically exposed to grazing medium flows, which considerably influences the acoustic properties of the MPP absorber. The flow near and inside the orifice is complex; hence, many efforts have been made to theoretically investigate and establish prediction formulae for the acoustic properties of the orifice under flow conditions; however, most are inconsistent and have poor generalizability. Recently, the development of meta-materials and -surfaces with MPP has allowed absorbers to be reduced to practicable (and even deep-subwavelength) scales and also contributed to an attractive and broadband noise-reduction performance. However, the existing impedance model (pertaining to the aero-acoustic behaviours of the orifice) cannot accurately predict the acoustic properties of the abovementioned meta-structures, because most research operates on the assumption of either a bulky cavity or no cavity at all. The effects of flow inside the cavity (particularly in shallow cavities) are often overlooked. The complex natures and the energy-dissipation mechanisms of these flow acoustic interactions have not been fully investigated. Therefore, a comprehensible investigation on the flow effects on the impedance of MPP needs to be performed, including theoretical modelling, numerical calculation and experimental investigation. A theoretical model is firstly proposed to investigate the grazing flow effects on the acoustic behaviour of a micro aperture of MPP. In this model, the shear layer was modelled as a span of the vortex sheet. The velocity continuity condition was applied to match the motion of flow on both sides of the upper surface of the orifice. The effects of resistance and reactance of micro orifice in MPP were considered by introducing the effective length (leff) of the hole. Moreover, the end correction terms were added to the impedance model by the assumption that the air mass of end correction above the upper surface of orifice is totally blown away by the grazing flow. The proposed impedance model was also verified by the comparison with the data measured by Malmary et al. (2001) and Allam and Åbom (2011). In order to comprehensively investigate the grazing flow effects upon the MPP, including the in-orifice and in-cavity flow dynamics, a 3D time-domain CFD approach is performed to characterise the acoustic behaviour variations of an orifice under the collective effects of the in-orifice and in-cavity flow dynamics for the compact configuration. In addition, the hybrid Rayleigh’s end correction model for a T-shaped resonator was employed to modify the reactance when calculating for a shallow cavity. The acoustic performances of single- and double-MPP absorbers under grazing flows were also studied numerically via the FEM and experiments. For achieving low-frequency and wide-band noise attenuation in duct system, an MPP-Matryoshka absorber was proposed. To evaluate the sound absorption performance of the proposed absorber, the theoretical model was presented according to the acoustic impedance circuit. Meanwhile, the numerical model was proposed and compared with the theoretical model; consistent predicted results were obtained by both methods. The numerical result revealed that the superior sound absorption performance of the MPP Matryoshka absorber is contributed from the collective effects of the MPP and orifice inside its cavity. Moreover, in order to simultaneously achieve better low-frequency and broadband noise damping, the compact parallel absorber was proposed and investigated numerically. Finally, the acoustic performances and flow effects of the MPP-Matryoshka absorber under normal and grazing incident were studied via FEM and experiments. Results show a good agreement between the proposed model and experimental data.