Time-domain Simulation And Analysis Of Acoustic Characteristics Of Perforated Silencing Structures

The common computational methods for predicting the acoustic characteristics of perforated silencing structures can be divided into the frequency-domain method and time-domain method. By comparison, the time-domain method can better take the effect of nonlinear factors into account, such as the thermal viscous of air flow, high sound pressure level of environmental noise and complex flow. However, the time-domain method requires much calculation costs to predict the acoustic performance of the perforated silencing structures for rather complicated structure with a large number of small holes and large computational model with long upstream and downstream connecting pipes to separate the incident and transmitted signals. The improved method of the time-domain method is investigated in this dissertation,as well as the simulation and analysis for the acoustic property of perforated silencing structures.The research for an improved time-domain impulse method based on plane wave decomposition and non-reflecting boundary condition is carried out. The basic principles and the data processing are introduced. Appling non-reflecting boundary condition on the inlet and outlet of the computational model, the reflected waves can be eliminated on the inlet and outlet. After the accomplishment of the time-domain computation, the incident signal can be separated by plane wave decomposition on the upstream monitoring point of the computational model, while the transmitted signal can be directly acquired on the downstream monitoring point. After that, the incident and transmitted signals in time-domain can be transformed into the frequency domain, and then the acoustic behavior of silencers is determined. Compared to the traditional time-domain impulse method, the computational model can be smaller and the corresponding calculation efficiency is higher in the improved method. Meanwhile, the rationality of the application of plane wave decomposition is discussed in detail by studying the variations of acoustic quantities on the monitoring points at the upstream and downstream. At last,the improved method is applied to predict acoustic attenuation of straight-through perforated tube silencers with and without flow. The improved method is verified by the comparison between the predicted and the experimental results.In order to investigate the nonlinear effect of high sound pressure levels, the time-domain method is used to study the nonlinear phenomenon of acoustic characteristics for straight-through perforated tube silencers. The time-domain method based on white-noise is described in detail and then applied to calculate the transmission loss of a straight-through perforated tube silencer under the condition of high sound pressure levels. The calculation results are validated by the experimental results. After that, the time-domain method is employed to study the nonlinearity of acoustic attenuation for a certain straight-through perforated tube silencer in the cases with different high sound pressure levels of the incident sound wave, and the conclutions are summarized.The time-domain method is then extended to study the acoustic impedance of perforated plates without flow. The time-domain CFD computational models for the acoustic impedance of the perforated plates are established from the experiment. And then, the time-domain method and the CFD computational models for calculating the acoustic impedance of perforated plates are verified by FEM. After that, the approximate expression for the acoustic impedance of perforated plates is derived from the model with a single hole and the temperature-related parameters are analyzed. Taking the computational efficiency and accuracy into account, qualitative research is conducted and selection principles on the number of holes for the CFD computational models with different porosities are obtained.Finally, the time-domain method and the CFD computational model with appropriate holes are applied to predict the acoustic impedance of perforated plates with different porosities under different temperature conditons.