Study On The Acoustic Characteristics Of Perforates By Using The Three-dimensional Time-domain CFD Approach

The perforated components are widely used in the duct silencing devices to improve the acoustical attenuation performance in the frequency range of interest via the impedance mismatch and energy dissipation.As a part of the intake and exhaust silencing systems,the acoustic characteristics of the perforated components is inevitably influenced by the gas flow.In order to predict accurately the acoustic attenuation performance of the duct silencing devices,the acoustic impedance of the perforated components needs to be obtained firstly.Therefore,the determination of acoustic impedance of perforates in the presence of gas flow becomes a research project with engineering application and theoretical significance.In view of the complex flow field and sound field near orifices,as well as the effect of flow-sound coupling,it is difficult to accurately compute the acoustic impedance of perforates by using the commonly used frequency-domain acoustic numerical methods.Therefore,the three-dimensional time-domain CFD approach is employed to investigate the acoustic impedance of perforates in the presence of gas flow in this thesis.The basic idea is that the flow field governing equations is solved directly in the time-domain,and the time-histories of acoustic pressure and particle velocity are obtained.The corresponding frequency-domain signals could be obtained by using the Fast Fourier Transform(FFT).Finally,the acoustic impedance of perforates are computed according to the acoustic theory.Considering the flow pattern near orifices,the acoustic impedance of perforates in the presence of grazing flow,bias flow,and grazing-bias mixing flow is investigated separately.The present study establishes the computational models for extracting the acoustic impedance of perforates under different flow conditions,and the extraction methods of acoustic impedance are determined based on the characteristics of the sound field near orifices.The acoustic impedance of perforates could be obtained by following certain computation steps.The predictions from the three-dimensional time-domain CFD approach are compared with the experimental measurements,which verified the accuracy of the three-dimensional time-domain CFD approach for computing the acoustic impedance of perforates in the presence of flow.The process of vortex shedding near the orifice is captured in the presence of grazing flow and bias flow by using the three-dimensional time-domain CFD approach,and the impact mechanism of shed vortex on the acoustic impedance is revealed.The effects of flow Mach number,porosity,the ratio of plate thickness to orifice diameter,and the Reynolds number near the orifice on the non-dimensional acoustic impedance are investigated in detail,thereby the expressions of acoustic impedance of perforates in the presence of grazing flow and bias flow are obtained by using the numerical fitting tool.The present study also computes the acoustic impedance of perforates in the presence of grazing-bias mixing flow and analyzes the distribution of the acoustic impedance versus the frequency.Then the generally applicable rules were summarized.The transmission loss of straight through perforated-tube mufflers,cross flow perforated-tube mufflers,H-Q tube,and partially plugged flow perforated-tube muffler is predicted by using the expression of acoustic impedance of perforates with grazing or bias flow and the related rule for the grazing-bias mixing flow via finite element computation.The good agreements between predictions and measurements verified the accuracy of the acoustic impedance expressions of perforates with grazing flow and bias flow as well as the validity of acoustic impedance rule for the case with grazing-bias mixing flow.The present study provides the expression of acoustic impedance of perforates with sound-absorbing material attached according to the acoustic theory.The experimental set-up is built for measuring the acoustic attenuation performance of muffler.The transmission loss of perforated-tube dissipated mufflers is measured by using the two-load method.The transmission loss predicted by the finite element method via the acoustic impedance expression of perforates agree well with the experimental measurements.The effects of perforation parameters,flow Mach number,and filling-density of sound-absorbing material on the acoustic attenuation performance of perforated-tube dissipatted muffler are analyzed in detail.