充水管道声分隔片消声性能研究Ⅰ.理论分析

本文对在充水管道中加入声分隔片的消声结构的消声性能进行了理论研究,用模式匹配法计算了有限和蔗声分隔片的传递损失和功率反射系数。研究结果表明,声分隔片的消声效果优于通常所采用在管壁加吸声衬怪的结构,并且其功率反射系数很小。     

An algorithm for the efficient acoustic analysis of silencers of any general geometry

Transfer matrix analysis provides a very efficient means to analyse the linear plane-wave acoustic performance of silencer systems in the frequency domain. However, the nature of the algorithm for combination and reduction of the matrices is different for different combinations of elements. This paper describes an efficient algorithm for acoustic analysis of any general silencer system. The basic format of the algorithm is the identification of sub-systems of two-port acoustic elements. Computational time is also reduced by recording the order in which all of the elements are analysed and the sub-systems are reduced. Examples of the analysis of several complex silencer systems are presented. The gain in efficiency over a general global matrix approach is exceptional.     

一种防尘、耐高温消声器的研究

一种新型的消声器能应用在高温,含尘的条件下,消声器设计采用了四分之一波长吸声,其内部消声构造与防尘,耐高温有机结合起来,实现了防尘和消声的较佳效果。     

Design and performance of an open jet wind tunnel for aero-acoustic measurement

This paper presents the design and performance of an open jet, blow down wind tunnel that was newly commissioned in the anechoic chamber at the ISVR, University of Southampton, UK. This wind tunnel is intended for the measurement of airfoil trailing edge self-noise but can be extended to other aeroacoustic applications. With the primary objectives of achieving acoustically quiet and low turbulence air jet up to 120 m/s through a 0.15 m × 0.45 m nozzle, several novel noise and flow control techniques were implemented in the design. Both the acoustical and aerodynamic performances of the open jet wind tunnel were examined in detail after its fabrication. It is found that the background noise of the facility is adequately low for a wide range of exit jet velocity. The potential core of the free jet is characterized by a low turbulence level of about 0.1%. Benchmark tests by submerging a NACA0012 airfoil with tripped and untripped boundary layers at 0° and 10° angles of attack respectively into the potential core of the free jet were carried out. It was confirmed that the radiating airfoil trailing edge self-noise has levels significantly above the rig noise over a wide range of frequencies. The low noise and low turbulence characteristics of this open jet wind tunnel are comparable to the best facilities in the world, and for its size it is believed to be the first of its kind in the UK.     

Laboratory measurement of the acoustical and airflow performance of interior natural-ventilation openings and silencers

This paper discusses laboratory measurements of the acoustical and airflow performance of interior natural-ventilation openings and silencers (‘ventilators’). The objective was to create and characterize a purpose-built test facility, and use it to measure the combined acoustical and airflow performance of a number of ventilators of interest, to understand and optimize it, and provide design guidelines to practitioners. The paper discusses the characterization of ventilator performance, and methods and theory for measuring it. The design and performance of a purpose-built, two-room laboratory facility are described. The facility was used to investigate the performance of a non-acoustical grille, an acoustical louver, slot ventilators, crosstalk silencers and a novel door-vent silencer. The results identify a number of best practices for successful ventilator design: non-acoustical grilles should be avoided; the addition of a glass-fiber absorptive liner to the surface adjacent to a slot ventilator increases acoustical performance by STC 3–6; acoustically-lined crosstalk silencers can be very effective – the straight configuration is best and performance increases with the length of the flow path; acoustical liners should be at least 50-mm thick. A prototype door-vent silencer showed very promising performance, but needs to be optimized.     

The study of reactive silencers by shape and parametric optimization techniques

The aim of this work is to show the application of shape and parametric optimization techniques in the study of reactive silencers with extended inlet and outlet ducts. Parametric optimization is employed to evaluate the appropriate size of the inlet and outlet ducts. Shape optimization is employed to establish the proper profile of these ducts in order to improve the acoustic features of these mufflers in a specific frequency range. The objective function used in the optimization processes is defined through the average transmission loss (TL) for the desired frequency range. This type of objective function is strongly non-linear and the genetic algorithm, GA, was chosen as a mathematical method for determining the maximum of this function. The Finite Element Method with an axisymmetric formulation along with the modified four-parameter method are used to calculate the TL(ω). The Hermite polynomials were used in the shape optimization in order to obtain local boundary approximations with C¹ continuity. The results showed the optimization efficiency of the inlet ducts profile for acting in specific frequency ranges with gains up to 20 dB with respect to silencers without shape optimization. The numerical analyses agree well with experimental results.     

The acoustic performance of ventilated window with quarter-wave resonators and membrane absorber

Noise and air pollution problems become significantly in a busy city such as Hong Kong since buildings usually located close to the heavy traffic lines. Traditional openable window cannot fulfill all the functions of noise reduction, lighting and natural ventilation. A new ventilated window combines the multiple quarter-wave resonators (silencer) and the new wing wall designs aim to make a balance between acoustic and ventilation performances at the same time. Furthermore, the use of multiple-wave resonators and membrane absorber which made plexi-glass plastic sheet replace absorption material can improve the durability; avoid small particle emission and light transparency.The acoustic and ventilation performance of new ventilated window were examined in this study. Noise attenuation of the new ventilated window design has improved significantly by combine flexible absorber and quarter-wave resonator effects. Transmission loss of 10–22 dB can be achieved in the frequency range of 500 Hz–4 kHz band. Outlet air flow velocity of ventilated window design is higher than that of “an open window”. Thus, both the acoustics and ventilation performance of the new ventilated window is essential. Wind-driven natural ventilation is an effective strategy in maintaining the comfort and health of the indoor environment.     

Numerical simulation of the flow field of a diffused pneumatic silencer

A diffused pneumatic silencer had been widely used in the pneumatic fields due to its small dimensions and high level of performance in noise reduction. A numerical simulation of its interior and exterior flow field was important for studying the gas flow in the silencer and the flow structure outside the silencer, as well as for understanding the mechanism of the silencer’s noise reduction. A porous media model and the Darcy–Forchheimer principle were used as the basic theoretical models in this paper. The unified governing equations were used here to describe the compressible flow in and out of the silencer. A robust numerical scheme was used to discritize the equations, and the TDBC (Time-dependent boundary conditions) was used to treat the non-reflecting boundaries. The detailed structures of the inner and outer flow fields of the diffused pneumatic silencer were obtained. The simulation results displayed the characteristics of the flow in the silencer. The nature of the flow outside the silencer, comparable with the experimental data, was also obtained.     

A network approach for analysis of silencers with/without absorbent material

The object of this work is to establish a general approach that can analyze the performance of most of the silencers with/without sound absorbent material. Under the assumption of plane wave propagation, the transfer matrices between the two ends of straight pipe and two-duct perforated section are derived and taken as the basic elements. Based on the conditions of continuity of pressure and of mass velocity, the silencer is modeled as a network formed by the two basic elements. Then the sound attenuation characteristic of the silencers can be investigated. With this scheme the multiply connected acoustic filters can also be analyzed. Further, the porous sound absorbent material is also included in this scheme. The effect of sound absorption material on the performance of silencers is analyzed and discussed.     

An adaptive quarter-wave tube that uses the sliding-Goertzel algorithm for estimation of phase

This paper describes an adaptive quarter wave tube used to attenuate a tone from the exhaust noise of a large diesel engine. A sliding-Goertzel algorithm was used to calculate the phase angle of the transfer function between a microphone in the adaptive quarter wave tube and in the main exhaust duct. The control system adjusted the length of the adaptive quarter wave tube until the phase angle was −90° and caused the sound pressure level at the cylinder firing frequency in the exhaust duct to be minimized. The system was able to adapt to changes in engine speed, exhaust gas temperature, and load applied to the engine. The results demonstrate that the sliding-Goertzel algorithm can be used effectively to estimate the phase angle in an adaptive–passive acoustic control system.