An experimental study on swirling flow in a 90 degree circular tube by using particle image velocimetry

The study of swirl flow is of technical and scientific interest because it has an internal recirculation field, and its tangential velocity is related to the curvature of the streamline. The fluid flow for tubes and elbows of heat exchangers has been studied largely through experiments and numerical methods, but studies about swirl flow have been insufficient. Using the Particle Image Velocimetry method, this study found the time averaged velocity distribution, time averaged turbulence intensity with swirl and without swirl flow for Re=10,000, 15,000, 20,000 and 25,000 along longitudinal sections, and the results appear to be physically reasonable. In addition, streamwise mean velocity distribution was compared with those of Khodadai et al. and Jeong et al.     

水面船尾流的声散射实验研究

介绍尾流湖上实验,从水平和竖直两个方向探测尾流.用A式和B式两种显示方式绘出了尾流的轨迹图,测量了尾流的几何尺度.然后利用瞬时频率序列分析方法,对尾流回波(混响)进行了分析.最后,利用比较法测量了尾流的声散射强度.     

An experimental study of partially premixed flame sound

The occurrence of oscillating combustion and combustion instability has led to resurgence of interest in the causes, mechanisms, suppression, and control of combustion noise. Noise generated by enclosed flames is of greater practical interest but is more complicated than that by open flames, which itself is not clearly understood. Studies have shown that different modes of combustion, premixed and non-premixed, differ in their sound generation characteristics. However, there is lack of understanding of the region bridging these two combustion modes. This study investigates sound generation by partially premixed flames. Starting from a non-premixed flame, air was gradually added to achieve partial premixing while maintaining the fuel flow rate constant. Methane, ethylene, and ethane partially premixed flames were studied with hydrogen added for flame stabilization. The sound pressure generated by methane partially premixed flames scales with M⁵ compared to M³ for turbulent non-premixed methane flames. Also, the sound pressure generated by partially premixed flames of ethane and ethylene scales as M^4.5. With progressive partial premixing, spectra level increases at all frequencies with a greater increase in the high-frequency region compared to the low-frequency region; flames develop a peak and later a constant level plateau in the low frequency region. The partially premixed flames of methane, ethylene, and ethane generate a similar SPL as a function of equivalence ratio when the fuel volume flow rate is matched. However, when fuel mass flow rate is matched, the ethane and ethylene flames produce a similar SPL, which is lower than that produced by the methane flame.     

双声源位置和强度识别的三维声强方法

目前在远场识别声源空间位置和强度缺乏行之有效的方法。针对此问题,提出采用四传声器进行三维声强测量,从而构建出声强、声源坐标和声功率的非车线性方程组,求解方程得出声源空间坐标和强度的方法。以3个三维声强探头对两个同频率单极子声源的识别为例,分别利用数值仿真和半消声室内的实验进行方法验证,并对声源的识别空间分辨率做了测试,得出角度识别最大误差为3.83°,为真实值的8.5%,距离识别最大误差0.1 m,为真实距离的10%。结果表明采用该方法空间坐标和声功率识别均具有很高的准确度,双声源的空间位置分辨力也优于远场声全息方法。     

An experimental study of sound generated by flows around cylinders of different cross-section

The sound radiated by rigid cylinders placed transversely in a uniform stream has been measured in an anechoic wind tunnel over a range of Mach numbers (M=0.09-0.2). The cylinders have different cross-sections, e.g., circular, square, rectangular, elliptic, and circular with lateral ribs or knurled surfaces. Different length to diameter ratios of the cylinders are also investigated. Results are presented as narrow band spectra, measured in the far field (acoustic as well as geometric). All spectra are presented with dimensionless (scaled) axes, as well as the original dimensional scales. It is shown that elliptic cross-sections are less noisy, compared to all other cylinders, but noise abatement techniques such as lateral ribs and knurled surfaces can also reduce tonal radiated noise. Two practical applications of these experiments are the reduction of radiated noise from pantographs of high-speed railway trains, and from the landing gear of modern passenger aircraft.     

风冷冷冻柜中毛细管特性的试验研究

以某风冷冷冻柜为研究对象,分别从毛细管长度、制冷剂充注量、频率变化三个方面试验分析毛细管对制冷系统性能的影响。试验得出最佳毛细管长度为650mm;最佳充注量为1060g;系统高能效比的频率范围在20Hz～60Hz之间,对于在稳定工况下运行的机组,变频并不能起到很好的节能效果。通过标准毛细管和使用一定时间后的被测毛细管的对比分析,改变电流模拟分析系统启停,得到以库中央温度和冷凝器中央温度为代表的影响系统性能的变化曲线。实际运行中可通过监测库中央温度、冷凝器中央温度、压缩机吸气和排气温度、压缩机上下表面温度,与标准毛细管对比就可准确判断出系统运行状态是否正常。     

Blind extraction and localization of sound sources using point sources based approaches

This paper presents theoretical models for blind sound source localization and separation of the signals emitted by arbitrary point sources in free space. Source localizations are achieved by a model based approach that accounts for the spherical spreading of an acoustic wave and utilizes an iterative triangulation, based on the signals measured by a three-dimensional microphone array. Once source locations are determined, the source signals are separated by using the point source separation (PSS) method, which is valid for all types of signals, including harmonic, continuous, transient, random, narrowband and broadband. General solutions for signals separation are presented. Theoretically, PSS can reconstruct the individual source signals exactly. This is because it employs the free-space Green's function, which defines the exact correlation among individual sources and measurement microphones. To validate PSS, numerical simulations are carried out and results are compared with those obtained by FastICA (Independent Component Analysis) code. The impacts of various parameters such as the microphone configuration, type of source signals, signal to noise ratio, number of microphones and source localization errors on the quality of signals separation by using PSS and FastICA are examined. The advantages and disadvantages of PSS and FastICA are compared and discussed.     

Simple sound transmission loss measurements using a modified impedance tube technique

A method of comparing the sound transmission characteristics of various materials, and combinations of materials, is presented, using a modified impedance tube technique. The procedure proved to be relatively quick and inexpensive in comparison with standard reverberation suite tests, and is therefore particularly useful for the qualitative ranking of multiple samples.The limitations of the technique are discussed in some detail, and particular emphasis is given to the problems of small sample size and method of mounting in the apparatus.     

受控应力-应变过程中沉积物声速变化的实验研究

各类海底沉积物在受控应力-应变过程中,声速随应变过程发展而变化;分析表明,最终有3种结果:(1)声速最大值是在应力最大时出现。(2)声速最大值是在未加载时出现。(3)声速最大值是在应变过程结束时出现。这些结果与沉积物的物理力学性质和结构特征等有关。     

An experimental investigation on the directional distribution of incident energy for the prediction of sound transmission loss

It is suggested from a previous theoretical study that an angular distribution function can be successfully used for describing the directional distribution of incident energy on the wall, instead of the field incidence assumption being used in the prediction of sound transmission loss. This study is devoted to an experimental investigation on the directionality of incidence sound field for validating the underlying assumption in the theoretical work. To this end, the directional magnitudes of incident energy on a wall in a reverberation chamber are measured by employing the sound intensimetry. The experimental results suggest that the transmission coefficient formula should be revised by introducing a proper angular distribution function as a weighting function for describing the directional energy density distribution, which replaces the conventional field incidence method. The revised formula is applied to the prediction of transmission loss for a double-leaf panel with an air cavity, in which its effectiveness is strongly supported by good agreements between measured and predicted values.     

The measurement of structure-borne sound transmission using impulsive sources

A simple method to measure structure-borne sound transmission is described. Measurement is made of the level difference in the acceleration between two structural elements using a plastic headed hammer as a noise source. The method is at least as accurate as conventional measurements made under steady-state conditions using continuous noise sources and can be carried out with less instrumentation on site and in about a tenth of the time.The portability of the source greatly simplifies the measurements as a hammer can be used to hit structures in a wide variety of positions whereas shakers can only be used in limited situations. In addition, attaching a shaker to a wall can damage the wall surface whereas, with care, a hammer hit will not.     

Identification of noise sources in factory’s sound field by using genetic algorithm

Noise control is important and essential in factory, where the noise level is restricted by the Occupational Safety and Health Act. Before noise abatement being performed, the identification work in searching for the location and sound power level (SWL) of noisy sound sources is absolutely prerequisite. Several researches on new techniques of single noise control have been well addressed and developed; however, the research work on sound identification for the existing multi-noise plant is hardly found sufficient. Under the circumstance of unrecognized noises, the noise control work will expectedly be extravagant and fruitless. Therefore, the numerical approach in distinguishing noises from a multi-noise plant becomes crucial and obligatory.In this paper, the novel technique of genetic algorithm (GA) in conjunction with the method of minimized variation square will be adopted and used in the following numerical optimization. In addition, various sound monitoring systems in detecting the noise condition within the plant area will also be introduced. Before noises identification, the accuracy of mathematical model has then been proved to be in good agreements comparing to the simulated data of SoundPlan, a commercialized simulation package in sound field. Moreover, three kinds of multi-noise plants have been fully discussed and acknowledged by GA optimization. The results reveal that the relevant locations and sound power levels (SWLs) of noises can be precisely recognized. This paper surely provides a rapid methodology in the noise identification work for a multi-noise plant.     

The effect of stationary diffusers in the measurement of sound absorption coefficients in a reverberation room: An experimental study

It has not yet been established if stationary diffusers are effective in the measurement of the sound absorption coefficients of materials in a reverberation room. Although the results of such measurements may be strongly influenced by the use of diffusers, the conditions under which diffusing panels must be used and the number of such panels required have still not been ascertained exactly.In this paper we report the results of a series of measurements on different kinds of materials, performed in a reverberation room in both the presence and absence of diffusers, in order to establish the effect of diffusing elements on the absorption coefficient values. In some cases variations greater than 40 per cent were observed.     

基于平面声源进行结构声辐射有源控制的实验研究

采用分布式平面声源作为次级声源,对振动钢板的声辐射进行了抵消实验,验证了以往研究中的一系列关键理论。实验研究结果表明:一个平面声源可以控制钢板奇-奇模态的声辐射,两个平面源可以控制结构偶-奇或奇-偶模态的声辐射,同时也可以控制结构奇-奇模态的声辐射;平面声源的面积和布放位置对降噪效果有重要影响,采用单个平面声源控制时,平面声源面积越大,控制效果越好;基于近场声压的误差传感策略是有效可行的,实际中,将近场测量面的声功率作为有源控制的目标函数与总声功率作为目标函数是一致的;控制后远场声压和声强都得到有效降低,部分区域的声能向声源流动,近场声压及声强分布也发生显著变化。     

Numerical and experimental study on sound absorption by pseudostochastic diffusors

I.IntroductionPseudostochasticdiffusorisaperiodicsurfacestructurewhichconsistsofaseriesofwellsofthesamewidthbutofdifferentdepths,whicharedeterminedaccordingtoapseudthrandomsequence(forexample,quadraticresiduesequence).ItisverifiedfromtheoryandexPerimelltthatthescatteredenergyfromthestructureplaneisindependentofangle.Intheotherhand,theunexpectedhighsoundabsorptionparticularlyatIowfrequencieswasreported.Severalauthorsattemptedtoexplainthephysicalmechanism,however,thesatisfiedpredicationhasnotb…     

Acoustic transmission performance of double-wall active sound packages in a tube: Numerical/experimental validations

A method based on the combination of the (u, p) formulation and the finite element approach is proposed for calculating the transmission loss of double-wall active sound packages with porous-material cores in a rectangular tube. The (u, p) formulation based on the displacement in the solid phase and the pressure in the fluid phase is rewritten to investigate sound propagation in porous materials, and the coupled boundary conditions and related parameters involved are given. The transmission loss of the double-wall active sound packages excited by a plane wave is calculated via the COMSOL environment with passive and active control. Moreover, a two source-location method is developed to measure the transmission loss of these double-wall active sound packages in a rectangular tube. Some results from numerical examples are shown to be in good agreement with the measured data. It is concluded that with active control, the transmission loss of double-wall active sound packages can be improved by more than 10 dB around resonance frequencies.     

基于平面声源实施结构声辐射有源控制的理论研究

摘要研究了利用分布式平面声源对结构声辐射进行有源控制的问题。首先建立了系统的数学模型,然后推导了有源控制条件下次级声源的强度和声功率降低的计算公式。在实际应用中,次级声源参数(面积大小、安放位置、个数等)对控制效果有重要影响,本文基于有源控制的物理机理和数值仿真研究这些问题。结果表明: -般情况下,次级声源板的振动模态分布与初级结构振动模态分布不相同,因此,在低频范围内,需要至少4个分布式次级声源,方能有效地控制初级结构声辐射。     

Active sound control by directional sources in the free field

TheprojectissupportedbyAeronauticalScienceFoundation.I.IntroductionThecontro1ofsoundfieldbyintroducingacousticsourcesisatopicofnoisecontrolinwhichanincrcasinginterestisbeingshown.Gcnerallytherearemain1ytwoaspectsinthere-searchesonActiveNoiseContro1(ANC)--theoriesofactivesoundattenuationwhichisthebasementofthetCchnique,anddesignofthecontrolsystem.Recent1y,anewtheorywhichisconsideredtohewc11suitcdtotheanalysisofpracticalproblemshasbeenpresentedbyP.A.Nc1sonetal.[ll.Itshowsthattheminimumpowe…     

低温翅片管换热器的传热试验研究

为了研究翅片管换热器在低温工况下的传热性能,进行了翅片管汽化器的气化试验。采用低温热电偶连续测量翅片管汽化器8个不同位置的温度,得到8条温度曲线。分析曲线表明气化过程可归结为非稳态与稳态两个阶段,并经历液相、汽液两相及气相三个传热区。试验结果为同类换热器的分段设计提供了依据。     

壁面分布次级声源对管道中高阶模式声波的有源控制

针对管道中高阶模式声波的有源控制问题,探究了管道壁面布放次级声源的控制机制。首先根据管道声场模式理论和有源控制准则,推导了无限长管道内次级声源激励声场实现控制的必要条件,然后论证了次级声源沿管道壁面分布的可控性,并提出将空间分布离散化后的控制策略;最后通过数值仿真验证了沿壁面分布的有效性。结果表明,当次级声源在管道壁面的周向分布满足空间采样定律且沿轴向分布具有足够长度时,可以取得显著的有源控制效果.