A computational and experimental study of resonators in three dimensions

Abstract

In a previous work by the present authors, a computational and experimental investigation of the acoustic properties of two-dimensional slit resonators was carried out. The present paper reports the results of a study extending the previous work to three dimensions. This investigation has two basic objectives. The first is to validate the computed results from direct numerical simulations of the flow and acoustic fields of slit resonators in three dimensions by comparing with experimental measurements in a normal incidence impedance tube. The second objective is to study the flow physics of resonant liners responsible for sound wave dissipation. Extensive comparisons are provided between computed and measured acoustic liner properties with both discrete frequency and broadband sound sources. Good agreements are found over a wide range of frequencies and sound pressure levels. Direct numerical simulation confirms the previous finding in two dimensions that vortex shedding is the dominant dissipation mechanism at high sound pressure intensity. However, it is observed that the behavior of the shed vortices in three dimensions is quite different from those of two dimensions. In three dimensions, the shed vortices tend to evolve into ring (circular in plan form) vortices, even though the slit resonator opening from which the vortices are shed has an aspect ratio of 2.5. Under the excitation of discrete frequency sound, the shed vortices align themselves into two regularly spaced vortex trains moving away from the resonator opening in opposite directions. This is different from the chaotic shedding of vortices found in two-dimensional simulations. The effect of slit aspect ratio at a fixed porosity is briefly studied. For the range of liners considered in this investigation, it is found that the absorption coefficient of a liner increases when the open area of the single slit is subdivided into multiple, smaller slits.impedance would be free of empirical constants. In addition to the possibility of becoming an impedance prediction tool, CAA offers an enhancement to semi-empirical model prediction by better quantifying constants that heretofore have necessitated elaborate experimentation, thereby providing a means to investigate and better understand the mechanisms by which an acoustic liner dissipates incident sound waves. The openings of an acoustic liner are quite small. This makes it difficult to perform experimental observations and measurements of the fluid flow and acoustic fields in and around an acoustic liner. On the other hand, small liner openings are no hindrance to CAA simulations. Moreover, numerical simulations using CAA methodologies can provide a complete set of space-time data. This is an invaluable asset for flow physics investigation and analysis. At the present time, the goals of using CAA methodology for first-principles impedance prediction and for investigation of acoustic wave dissipation mechanisms have not been fully attained. Nevertheless, significant advances have been made. Further efforts...

Numerical investigation of a honeycomb liner grazed by laminar and turbulent boundary layers

“…More recently, fully three-dimensional numerical simulations of sound interacting with resonators have been performed with different geometries. Tam et al (2008a) observed the formation of vortex rings in their simulations of a rectangular orifice liner and noted that their earlier results from slit liners had a very different flow topology, a result confirmed by Roche et al (2009). Xu, Li & Guo (2014 extended earlier slit liner simulations into a fully three-dimensional simulation and found the absorption coefficients agreed very well with the previous two-dimensional simulations.…”

Measurements and computational fluid dynamics predictions of the acoustic impedance of orifices

“…Accurate results were obtained up to a frequency of 2.5 kHz. Tam et al also used DNS in their numerical investigation of resonators in three dimensions [11]. By virtue of its high-fidelity non-modelling approach, DNS is able to reproduce the small scale details of acoustic phenomena (such as dissipation mechanisms) accurately, even for high frequency ranges.…”

Investigation of flow and acoustic performances of suction mufflers in hermetic reciprocating compressor

“…In terms of the acoustic performance, numerous holes in the modified design are expected to enhance the acoustic performance of the muffler by acting as slit resonators, which are known to convert acoustic energy into vortical energy by generating vortices (Tam et al, 2005(Tam et al, , 2010). Fig.…”