Study On Applications Of Time Domain Impulse Method To Acoustic Computations Of Ducts And Silencers

The silencers are widely used to reduce the propagation of noise in ducts.For the acoustic prediction and analysis of silencers,there are two kinds of methods available:the frequency domain approach and the time domain approach.The former has the faster computational speed,but the effects of complex flow and viscosity on the propagation and attenuation of sound are usually simplified or neglected in the computations.The time domain approach directly solves the nonlinear fundamental balance equations of mass,momentum and energy,which may included the influences of complex flow,viscocity and thermal conductivity in the simulation.Therefore,the time domain approach will be employed to investigate the acoustic attenuation performance of silencers and duct systems in this thesis.For the cases without flow,the time domain predictions of transmission loss for simple expansion chamber,expansion chamber with extended inlet/outlet,double expansion chamber silencer,straight-through perforated tube silencer,cross flow perforated tube silencer and hybrid expansion chamber silencer are compared with experimental measurements and finite element method(FEM)predictions,and good agreements between the present predictions and experimental results show that the time domain impulse method could predict accurately the acoustic attenuation characteristics of reactive silencers.In the FEM,the acoustic behavior of perforations is simulated by the acoustic impedance,the accuracy and applicability of which has a direct influence on the transmission loss predictions of perforated tube silencers.In the time dimain impulse approach,the perforation may be simulated directly and the nonlinear effect in the perforations could be included in the simulation,which leads to the improvement in the accuracy.In addition,the porous media model is used to simulate sound absorbing material,the time domain predictions are depended on the determination of viscous and inertial resistance coefficients and the mesh quality in porous media.The effects of air flow and temperature on the acoustic attenuation performance of the perforated tube silencers are examined next.The comparisons show that the time domain predictions match closely the experimental results throughout the frequency range of interest,which is better than the FEM predictions.The numerical results demonstrated that the air flow and temperature have a significant influence on the acoustic attenuation behavior of the perforated tube silencers.The mean flow increases the acoustic attenuation at most frequencies,while increasing the air temperature shifts the transmission loss curve to higher frequency and lowers the resonance peaks somewhat.The transmission loss of double expansion chamber silencer without and with flow is measured with the existing testing bed in the laboratory,and the measured results are compared with the predictions from time domain impulse method.Good agreements between the numerical results and experimental measurements are observed in the whole frequency range,but the measured curve has serrated fluctuations,which is deteriorated with air flow.Therefore,the testing bed is modified locally,and the rapid sine sweep and synchronization time averaging techniques are implemented to increase the signal-to-noise ratio.The comparisons and analyses show that the transmission loss measurements of straight-through perforated tube silencer agree well with the time domain predictions,and the fluctuations in thetransmission loss measurements decrease somewhat.Then,the reflection coefficient magnitudes of unflanged pipe without and with flow are calculated by the time-domain impulse approach,which are compared with the experimental measurements and BEM predictions.In the absence of air flow,the time domain impulse approach could predict accurately the acoustic characteristics of duct termination,but there are small discrepancies in the low frequency range between the numerical results and measurements with air flow.The differences may be attributed to the fact that the pressure outlet boundary condition,which is a reflection boundary condition,is applied to the outlet of free atmosphere zone.It could also be found that the flow velocity and oblique termination have a significant influence on the reflection coefficient for the unflanged pipe.For the case of air flow,the reflection coefficient initially increases with Helmholtz number to a peak value above unity and eventually drops below unity as the Helmholtz number increases.And the value of Helmholtz number corresponding to the maximum of reflection coefficient increases with Mach number.Compared with the perpendicular termination of duct,the oblique termination reduces the reflection coefficient of the pipe.And the ducts with smaller angle provide lower reflection coefficient at higher Helmholtz numbers.Based on the User-Defined Functions in software Fluent to complement the calculation codes of water density,the time domain impulse method is developed to compute the acoustic attenuation characteristics of water-filled silencers.The transmission loss of simple expansion chamber,expansion chamber with extended inlet/outlet,double expansion chamber silencer,straight-through perforated tube silencer and cross flow perforated tube silencer are calculated by time domain method and FEM in the absence of flow,respectively,and good agreement between them are observed in the frequency range of interest,but there are small discrepancies between them in resonant frequencies.The reason may be attributed to the fact that the sound speed in the FEM computations is different from the time domain impulse method,because the viscosity and compressibility of water are not considered in the FEM computations.Because the FEM could not simulate perfectly the convective effect in calculation,the time domain impulse method is used to examine the influence of volume flow rate on the acoustic attenuation behavior of straight-through and cross-flow perforated tube silencers.The predictions demonstrate that the volume flow increases the acoustic attenuation at most frequencies.