Aeroacoustics of T-junctions—An experimental investigation

An experimental method for determining the aeroacoustic properties of side branch orifices allowing for any combination of grazing and bias flow is presented. The geometry studied, a T-junction, is treated as an active acoustic three-port. The passive properties, describing the reflection and transmission of an incident acoustic wave, are described by a system matrix while the active properties are described by a source vector. Expressions for the acoustic impedance under various mean flow and acoustic incidence configurations are developed. In addition, methods for identifying regions where the system can generate sound, by studying only the passive properties, are discussed. A self-sustained oscillation is triggered at one of the identified regions by coupling a resonant system to the three-port.     

流管实验装置中声传播计算的模态方法

流管实验装置是测量有流动情况下航空发动机消声短舱内声衬声阻抗的主要装置。本文发展了一种解析的模态匹配方法进行在平均流有声衬条件下矩形流管中声传播的计算。用同伦方法求解特征值问题,并与用环绕积分求解的结果进行比较。声场通过轴向阻抗间断面的声压和声质点速度积分相等计算。第一个算例是无流动、硬壁、有限长、考虑端口反射的情况,并与北航流管实验台测量数据进行了对比;第二个算例为有流动情况下有限长声衬管道不考虑端口反射的声场计算,它与文献中NASA流管实验结果和CAA计算结果符合得很好。     

Setting up an experimental apparatus for the study of multimodal acoustic propagation with turbulent mean flow

An experiment has been set up to study multimodal acoustic propagation inside a cylindrical duct in presence of a turbulent mean flow. This paper describes the preliminary work which has been found necessary for assembling the experiment together with first measurement results. In order to set up this experimental facility, a high level acoustic source was developed to generate higher propagating modes in the presence of mean flow. A microphonic antenna was designed for detecting the propagating modes. LDV measurements were performed and synchronous detection was used to extract both the mean flow and the acoustic components of the particle velocity. Results of aeraulic measurements are presented. Then, results of acoustic velocity measurements are compared to results obtained from the microphonic antenna.     

均匀流直通穿孔消声器的声学特性分析

将数值模态匹配法(NMM)扩展应用于计算有均匀流存在时直通穿孔管抗性和阻性消声器的声学特性,编写了相应的计算程序。对于圆形同轴穿孔管抗性和阻性消声器,应用数值模态匹配法计算得到的传递损失结果与实验测量结果吻合良好,从而验证了计算方法和计算程序的正确性。进而应用数值模态匹配法研究了运流效应和穿孔阻抗以及穿孔管偏移对穿孔管抗性和阻性消声器传递损失的影响。研究结果表明,马赫数越高,穿孔管抗性消声器在中高频的消声量越高,阻性消声器在整体频段内的消声性能越差;低马赫数时运流效应对穿孔管抗性消声器的影响可以忽略,马赫数较高时运流效应和穿孔阻抗的影响比较明显;对于穿孔管阻性消声器,穿孔阻抗对消声器声学特性的影响比运流效应的影响小,但是与真实值的差别不可忽略;穿孔管偏移对消声器声学特性的影响与频率和消声器结构均相关。     

Measurements of energy exchange between acoustic fields and non-uniform steady flow fields

This paper describes an experimental investigation of interactions between acoustic waves and a non-uniform steady flow field. Data have been obtained for a resonance tube having a vent at the center in the lateral boundary, an average flow being introduced at the ends. Experiments have been done for both circular and slot vents, over ranges of both frequency and Mach number. According to the one- dimensional linear stability analysis, the interactions between the longitudinal acoustic field and the mean flow in the vicinity of the vent cause a net transfer of acoustic energy from the average flow to the acoustic field. This result has been verified by the experiments reported here. The gain of energy measured is less than that deduced from a one-dimensional analysis.     

Rijke热声不稳定性的有源控制

从理论上推导出一个确定带有控制系统的Rijke管的稳定性的条件,并研究了管中声模式的复本征频率与放大倍数和相移的关系。然后通过分析管末端声波反射条件的变化,阐述了有源控制热声不稳定性的机理。最后对有源控制进行了实验研究.该研究说明了起源于线性扰动(声学量级)的非线性的不稳定增长过程,可以用一个线性的小扰动去控制,使系统达到稳定.我们的理论与实验结果再一次证明了有源控制流体动力不稳定性的可能性。     

A study of the influence of mean flow on the acoustic performance of Herschel-Quincke tubes

In this paper, a simple flow model is used in order to assess the influence of mean flow and dissipation on the acoustic performance of the classical two-duct Herschel-Quincke tube. First, a transfer matrix is obtained for the system, which depends on the values of the Mach number in the two branches. These Mach numbers are then estimated separately by means of an incompressible flow calculation. Finally, both calculations are used to study the way in which mean flow affects the position and value of the characteristic attenuation and resonances of the system. The results indicate the nontrivial character of the influence observed.     

A study of the transition to instability in a Rijke tube with axial temperature gradient

A horizontal Rijke tube with an electric heat source is a system convenient for studying the fundamental principles of thermoacoustic instabilities both experimentally and theoretically. Given the long history of the device, there is a surprising lack of accurate data defining its behavior. In this work, the main system parameters are varied in a quasi-steady fashion in order to find stability boundaries accurately. The chief purposes of this study are to obtain precise values of the system parameters at the transition to instability with specified uncertainties and to determine how well the experimental results can be explained with existing theory. Measurement errors are reported, and the influence of experimental procedures on the results is discussed. A form of hysteresis effect at stability boundaries has been observed. Mathematical modelling is based on a thermal analysis determining the temperature of the heater and the temperature field in the air inside the tube, which, consequently, affects acoustical mode shapes. Solutions of the linearized wave equation for a non-uniform medium, including losses and a heat source term, determine the stability properties of the eigen modes. Calculated results are compared with experimental data and with results of the modelling based on the common assumption of a constant temperature in the tube. The mathematical model developed here can be applied to designing thermal devices with low Mach number flows, where thermoacoustic issue is a concern.     

Duct acoustics—A numerical technique for the higher order mode solution of three-dimensional problems with rigid walls and no flow

A three-dimensional, time dependent, finite difference technique is developed for the steady state solution of the acoustic wave equation in the absence of flow. The acoustic characteristics of a mitred bend are used to illustrate the use of the technique and numerical results are corroborated by experimental data.     

Acoustic radiation from a fluid-filled, subsurface vascular tube with internal turbulent flow due to a constriction

The vibration of a thin-walled cylindrical, compliant viscoelastic tube with internal turbulent flow due to an axisymmetric constriction is studied theoretically and experimentally. Vibration of the tube is considered with internal fluid coupling only, and with coupling to internal-flowing fluid and external stagnant fluid or external tissue-like viscoelastic material. The theoretical analysis includes the adaptation of a model for turbulence in the internal fluid and its vibratory excitation of and interaction with the tube wall and surrounding viscoelastic medium. Analytical predictions are compared with experimental measurements conducted on a flow model system using laser Doppler vibrometry to measure tube vibration and the vibration of the surrounding viscoelastic medium. Fluid pressure within the tube was measured with miniature hydrophones. Discrepancies between theory and experiment, as well as the coupled nature of the fluid-structure interaction, are described. This study is relevant to and may lead to further insight into the patency and mechanisms of vascular failure, as well as diagnostic techniques utilizing noninvasive acoustic measurements.     

Thermoacoustic oscillations in ducted domestic heating systems

Thermoacoustic oscillation is a significant problem in ducted domestic heating systems fitted with fully premixing burners as well as in combustion systems of industrial scale. This paper reports experiments using a burner closely representative of practical designs in which high-intensity free-field sound has been used to simulate the feedback which may be excited naturally in installations with different acoustic properties. This experimental approach facilitates study of the non-linear response of the burner/flame sub-system to feedback of different frequencies and direct measurement of the acoustic impedance of the burner under operational conditions. A function proportional to the Rayleigh integral has been evaluated directly from the experimental results. It is found that the head of this type of burner divides the typical installation into a quarter-wave tube and a Helmholtz resonator which must couple unstably for thermoacoustic oscillations to be autoexciting. It is further found that there is a strong interaction between the acoustic field and the mean flow which modifies significantly the behaviour of the burner as a Helmholtz resonator and thus the stability of the whole system.     

A computer simulation study of compressor tuning phenomena

Basic thermofluid processes of a positive displacement compressor are strongly dependent upon the acoustic behavior of the manifolds. The tuning process of such a compressor is fairly complex as increases in the mass flow rate may not correspond with higher energy efficiencies. In this paper a computer simulation program is described, which includes the manifold back pressure effect, developed to investigate and explain the tuning phenomena for a single or two-cylinder reciprocating compressor. A symmetric suction manifold system for a two-cylinder refrigeration compressor has been considered as the example case in this tuning study. Results for flow efficiency, energy efficiency, and pressure pulsations at the valve exit are presented in terms of the acoustic natural frequencies of the manifold system. Predicted results compare reasonably well with experimental data. Based on this study, it is possible to choose optimal manifold dimensions which will provide higher efficiencies with lower pressure pulsations.     

泡状流局部界面浓度实验与理论研究

应用高速数字摄像系统,首先对双头电导探针测量泡状流局部界面浓度的几种模型进行了标定和评价。使用标定好的双头电导探针技术,对垂直上升管内气液两相泡状流局部界面浓度和含气率分布特性进行了深入的实验研究。试验段为内径40 mm的透明有机玻璃管。根据试验数据的分析结果,发展了一种垂直上升管内气液两相泡状流局部界面浓度预测模型。在本文实验条件下,该局部界面浓度模型预测结果能够与实验数据很好地符合。     

Sound transmission in pipes with porous walls

Pipes with porous (permeable) walls have received the attention of several authors as a noise control element in automotive intake systems; however, a closed theory of sound transmission including the effect of the coupling of the internal and external acoustic fields and the presence of mean flow does not appear to be available. The present paper proposes an integro-differential system for the propagation of plane sound waves in pipes with porous walls, and presents its general numerical solution, as well as an approximate analytical solution. The predicted effect of the coupling between the internal and external acoustic fields in a circular pipe made of reinforced woven fabric walls is shown, and the transmission loss predictions are compared with the existing experimental data in the literature.     

基于相干光时域反射型的光纤分布式声增敏传感研究

开展了基于相干光时域反射型的光纤分布式声增敏传感研究,提出了单端固定开口波纹薄筒光纤声增敏方法,建立了光纤声增敏装置波节间距、单波节轴向刚度、光纤长度等参数对光纤相位灵敏度的影响理论模型.制作了3种规格的光纤声增敏传感装置进行声传感实验.实验结果表明,声增敏传感装置相位灵敏度达到2.975 rad/Pa,最小声探测信号达到60.1 dB,3种规格的声增敏传感装置的灵敏度测试值与理论分析基本一致.研究结果为高灵敏度的光纤分布式声传感的进一步发展提供了理论和实验基础.     

Theoretical analysis about eigenfrequencies of "aligned double flute

I.IntroductionFluteisanancientmusicinstrument.Itsprincipa1structureisasegmentofcircu1arpipewithaseriesofaperturesonthewa11.Amongtheseapertures,oneisca11edblowingapertureandothersarecalledtuningapertures.Duringperformance,thep1ayerb1owsbythecdgeofblowingapertureandcontro1sthetuningapertureswithcertainfingeringtoexcitedifferentpuretones.Forapopularflute,onetcrmina1ofthcfluteisopenandtheothcrisclosedwithapiston.Thetuningaperturesaresett1edneartotheopenterminalandtheblowingapertureneartotheclose…     

具有流动和侧向Helmholtz共振器的驻波管内声场研究Ⅱ.实验验证及结果讨论

利用建立的具有流动和侧向Helmholtz共振器的驻波管的实验装置,成功地证实了作者所作的理论分析,包括有流动驻波管和Helmholtz共振器连接处的间断条件、驻波管末端的反射条件、切向流时Helmholtz共振器声阻抗,并实验确定了末端修正系数和等效半径系数。接着,应用理论分析和实验得到的结果讨论了流动对Helmholtz共振器性能和驻波管声场的影响,结果表明,流动会降低一些消声效果,但在具有流动的驻波管中采用Helmholtz共振器仍然是一种降噪的有效方法。     

Modeling sound transmission through the pulmonary system and chest with application to diagnosis of a collapsed lung

A theoretical and experimental study was undertaken to examine the feasibility of using audible-frequency vibro-acoustic waves for diagnosis of pneumothorax, a collapsed lung. The hypothesis was that the acoustic response of the chest to external excitation would change with this condition. In experimental canine studies, external acoustic energy was introduced into the trachea via an endotracheal tube. For the control (nonpneumothorax) state, it is hypothesized that sound waves primarily travel through the airways, couple to the lung parenchyma, and then are transmitted directly to the chest wall. In contradistinction, when a pneumothorax is present the intervening air presents an added barrier to efficient acoustic energy transfer. Theoretical models of sound transmission through the pulmonary system and chest region to the chest wall surface are developed to more clearly understand the mechanisms of intensity loss when a pneumothorax is present, relative to a baseline case. These models predict significant decreases in acoustic transmission strength when a pneumothorax is present, in qualitative agreement with experimental measurements. Development of the models, their extension via finite element analysis, and comparisons with experimental canine studies are reviewed.     

Identification of aero-acoustic scattering matrices from large eddy simulation. Application to a sudden area expansion of a duct

A methodology is presented which allows to determine the coefficients of transmission and reflection of plane acoustic waves at flow discontinuities in piping systems by combining large eddy simulation (LES) of turbulent compressible flows with system identification. The method works as follows. At first, an LES with external, broadband excitation of acoustic waves is carried out. Time series of acoustic data are extracted from the computed flow field and analyzed with system identification techniques in order to determine the acoustic scattering coefficients for a range of frequencies. The combination of broadband excitation with highly parallelized LES makes the overall approach quite efficient, despite the difficulties associated with simulation of low-Mach number compressible flows. The method is very general, here it is applied to study the scattering behavior of acoustic waves at a sudden change in cross-section in a duct system. The complex aero-acoustic interactions between acoustic waves and free shear layers are captured in detail by high resolution compressible LES, such that the scattering coefficients can be determined accurately from first principles. In order to demonstrate the reliability and accuracy of the method, the results for the scattering behavior and the acoustic impedance are presented and physically interpreted in combination with several analytical models and experimental data.     

Investigation on nonlinear thermo-acoustic characteristics of Rijke tube

Based on the energy conservation relationship,nonlinear thermo-acoustic effects of Rijke tube including instability range,saturation processes and higher harmonics modes were investigated.With coupling between the external flow and the inner space of a Rijke tube, the acoustic characteristics of self-excited oscillation were simulated.The experimental study was also carried out and the results were compared with those from simulation.The nonlinear factors which distort the acoustic waveform distortion were analyzed.From the results,it is seen that varying size of the nozzle outlet changes the acoustic impedance in the boundary, and leads to reduction of the nonlinear effects.The results show that the modes of self-excited oscillation could be influenced by the position of higher harmonics.In the large amplitude oscillation,the distortion of pressure wave within Rijke tube could be induced by the acoustic losses due to vortices on nozzle.It is found that the waveform distortion could be avoided by the shrinkage of nozzle.