虚拟声屏障在变压器低频降噪中的实验研究

针对开口房间内的变压器,在开口处布放若干扬声器和误差传声器构成虚拟声屏障,实验研究了虚拟声屏障对通过开口向外辐射的低频线谱噪声的控制效果。将15个次级源近似均匀分布在面积为2 m′ 2.7 m的开口面上,左右间距约58.5 cm、上下间距45-65 cm,15个误差传声器分别位于对应的次级源正前方1 m,系统采用自适应谐波降噪算法。结果表明：虚拟声屏障系统在误差点100 Hz、200 Hz和300 Hz的平均降噪量分别达到12.7 dB、19.9 dB和22.2 dB,虚拟声屏障对100 Hz、200 Hz、300 Hz线谱噪声的控制效果与单层封闭窗户相当,且内部合成参考信号,无需外接参考传声器。采用虚拟声屏障对开口房间内的变压器降噪的好处是实现室内外的自然通风,便于变压器的散热。     

人头散射对虚拟声屏障的性能影响分析

在噪声来自于多个方向的普通房间中,通过立体结构的虚拟声屏障(Virtual Sound Barrier,VSB)系统能够产生比人头大的静区。实际应用中人头处于系统包围的静区内,必须考虑人头的散射作用对系统的影响。数值模拟表明,由于人头的散射作用,在人头附近,声压降低量分布更为均匀,系统性能可能变好也可能变坏,与误差传感器包围区域的半径及噪声频率有关。系统性能随系统物理配置的变化趋势,与未引入人头时是一致的。人头可以在系统包围的静区内移动,随着人头偏离系统中心,降噪效果会下降,但即使人头偏离至系统包围静区的边缘,仍有10 dB以上的降噪。实验给出一种实用的圆柱状分布的16通道的VSB系统,引入人头后系统性能变好了。当人头在该系统包围的静区内移动时,即使频率达到500 Hz,降噪效果最差仍达13.3 dB。     

用于开口声辐射控制的虚拟声屏障实现 方法及机理研究*

建筑物通常留有开口以便人员物料的进出及室内的自然通风采光,但这些开口也是噪声传播的途径。传统被动噪声控制方法需要将开口封闭,且对低频噪声的控制效果不好,故引入有源噪声控制技术降低室内声源通过开口的声辐射。基于惠更斯原理,均布开口的次级源和误差传声器构成的平面型虚拟声屏障可以实现对开口声辐射的有效控制,数值仿真和实验已证明其有效性。将次级源安装在开口边界更有利于保留开口的功能且方便实际安装,但这样的单层边界虚拟声屏障降噪效果存在上限,仅能在低频段实现全局控制。和单层边界次级源相比,双层边界次级源可显著提高降噪量和有效降噪频率上限。该文回顾了开口声辐射有源控制的相关工作,并讨论了未来可能的研究方向。     

声屏障和有源噪声控制声屏障

概述了声屏障降低噪声的原理和常用方法，提出了存在的不足，介绍了有源声屏障的降噪性能与发展动态，提出了进一步研究的设想。     

声悬浮的实验研究和数值模拟分析

介绍了自行研制的一种磁致伸缩式单轴超声悬浮装置,用此装置实现了尺寸1—7mm的若干种金属、半导体和有机物在空气中的稳定悬浮和定位,可悬浮样品的密度高达11.3g/cm³.为了优化谐振腔设计,采用边界元方法结合两类不同结构的谐振腔模型研究了谐振腔几何形状对声场分布以及悬浮定位能力和悬浮稳定性的影响. 关键词：     

高架桥声屏障高度对列车气动特性影响的数值模拟

采用计算流体力学方法对高架桥声屏障高度影响高速列车空气动力特性进行数值研究.通过网格划分、湍流模型选取、边界条件设置等来提高数值计算精度.结果表明,当高速列车运行在下风向时,头车、中间车上的侧向力随着声屏障高度增加而逐渐下降.头车所受的侧翻力矩在整车中最大,且随着声屏障高度的增加而逐渐减小.随着声屏障高度的增加,上风向工况下中间车受到的侧翻力矩要大于下风向工况.上、下风向工况下高速列车气动特性差异主要是由于流动空腔中列车所处的相对位置不同,改变了车体表面的压力分布,从而改变了车体所受到的气动力、力矩.     

基于气泵的声管实验及分析

为研究声管的发声频率,进行了实验设计和分析.声管转动发声是由于空气在声管中的流动,考虑实际的实验条件,设计了气泵吹气代替声管转动的实验方式,改变吹气速率并测量发声频率.通过对实验结果的分析发现,此时的声管不符合两端开口即两端为波腹的条件,而是一端波节、一端波腹.声管的发声频率取决于其长度,符合驻波特性,发声时吹气速率与波纹长度须满足共振条件.研究结果有助于对驻波的理解和相关内容的实验设计.     

轻轨桥架声屏障降噪效果的测量与分析

在轻轨桥架两侧设置声屏障是控制轨道交通噪声的主要措施之一，对上海轻轨明珠线桥架两侧设置不同高度声屏障实际降噪效果的测量与分析，对声屏障的实用设计有重要的参考价值。     

管道声传播数值计算与实验结果比较

本文对管道声传播数值计算提出了一组判据,用来衡量数值计算的可靠性;对于具有壁面声阻抗间断的声传播问题,提出了一种处理方法,可保证数值计算的准确性。本文还给出了一组硬壁-软壁-硬壁二维管道声传播数值计算结果与实测结果比较曲线及几个算例,说明本文采用的数值计算模型是合适的,提出的判据及间断点处理方法是成功的。     

Performance analysis of the virtual sound barrier system with a diffracting sphere

Since a practical virtual sound barrier (VSB) system has to be implemented surrounding a human’s head, the effects of such a diffracting sphere on the performance of the system are considered in this paper. It is found that the performance of the VSB system with a diffracting sphere might be improved or decreased depending on the size of the zone surrounded by the error sensors and the noise frequency, and the noise reduction distribution around the sphere tends to be uniform due to the diffraction of the sphere. Numerical simulations show that the tendency of the control performance with respect to the configuration of the system with a diffracting sphere is similar to that without the sphere. The diffracting sphere might move in the quiet zone of the VSB system, and the performance decreases with the increase of the distance between the sphere and the center of the system. However, the noise reduction level can remain more than 10 dB even when the sphere is near the border of the quiet zone. Experiments with a 16-channel cylindrical VSB system in a normal room validate the above conclusions.     

有源噪声控制在隔声罩中应用的初步实验研究

本文以对液压泵降噪为例,对有源噪声控制系统在隔声罩中的应用进行了初步的实验研究。噪声的低频段由有源噪声控制系统降低,中高频段由隔声罩降低,从而发挥了有源噪声控制和隔声罩各自的优点。附加了有源系统后的隔声罩使液压泵的噪声从112.4dB降到了80.0dB,整个系统的降噪量达到了32dB。其中有源噪声控制系统采用了6个次级控制源,在600Hz以下频段新增13dB的插入损失。     

A numerical study of transient flow around a cylinder and aerodynamic sound radiation

The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×10⁴ based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions.     

Numerical and experimental study on sound absorption by pseudostochastic diffusors

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

Experimental and numerical study on the propagation of impulsive sound around buildings

Propagation of impulsive sound around buildings and induced structural loading are investigated experimentally and numerically. Experiments were conducted on a rectangular building at Virginia Tech using sonic booms generated by shaped charges with an explosive weight of 0.78 kg, constructed from detonation cord. These experiments were simulated with a three-dimensional numerical model, in the context of geometrical acoustics (GA), by combining the image source method for the reflected field (specular reflections) with an extension of the Biot–Tolstoy–Medwin (BTM) method for the diffracted field. In this model, it is assumed that the acoustic propagation is linear and that all surfaces are acoustically rigid. This numerical model is validated against a boundary element (BE) solution and experimental data, showing a good overall agreement. The key advantages of this GA modeling approach for this application include the ability to model large three-dimensional domains over a wide frequency range and also to decompose the sound field into direct, reflected, and diffracted components, thus providing a better understanding of the sound-propagation mechanisms. Finally, this validated numerical model is used to investigate sound propagation around a cluster of six rectangular buildings, for a range of elevated source positions simulating sonic booms from aircraft.     

A numerical study on the flow and sound fields of centrifugal impeller located near a wedge

Centrifugal fans are widely used and the noise generated by these machines causes one of the serious problems. In general, the centrifugal fan noise is often dominated by tones at blade passage frequency and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cut-off in the casing. However, only a few researches have been carried out on predicting the noise because of the difficulty in obtaining detailed information about the flow field and considering the scattering effect of the casing. The objective of this study is to understand the generation mechanism of sound and to develop a prediction method for the unsteady flow field and the acoustic pressure field of the centrifugal impeller. A discrete vortex method is used to model the centrifugal impeller and a wedge and to calculate the flow field. The force of each element on the blade is calculated by the unsteady Bernoulli equation. Lowson's method is used to predict the acoustic source. In order to consider the scattering and diffraction effects of the casing, Kirchhoff-Helmholtz boundary element method (BEM) is developed. The source of Kirchhoff-Helmholtz BEM is newly developed, so the sound field of the centrifugal fan can be obtained. A centrifugal impeller and wedge are used in the numerical calculation and the results are compared with the experimental data. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. The radiated acoustic field shows the diffraction and scattering effect of the wedge.     

Performance of an independent planar virtual sound barrier at the opening of a rectangular enclosure

Planar virtual sound barrier systems have been used successfully to reduce noise radiation through an opening without affecting natural ventilation and lighting. However, the complexity of a fully coupled control system grows at the rate proportional to the square of the number of channels and this make the system implementation become impractical for enclosures with large openings. To reduce the system complexity, this paper proposes an independent planar virtual sound barrier, which is a multi-channel system consisting of many independent single channel active noise control systems. Each single channel system is “independent” in the sense that the control source output of the system is updated only with the signal from its own error sensor. Based on the analytical model of sound radiation through the opening of a rectangular enclosure, the transfer functions from both primary and control sources are calculated first. Then the noise reduction performance, the stability, and the convergence behavior of both fully coupled and independent planar virtual sound barrier systems are investigated. It is found that the independent system with no control output constraint becomes inherently unstable at some frequencies; however its stability can be improved by applying some control output constraint. Reducing the number of channels and the distance between secondary loudspeakers and error microphones can also increase system stability but at the cost of smaller noise reduction. When the system is inherently stable and there is no constraint on control output, the independent system can provide the same noise reduction as the fully coupled one but with faster convergence speed.     

A numerical and experimental study of the high-enthalpy high-speed cavity flow

Results of an experimental and numerical study of supersonic turbulent high-enthalpy flow in a channel with cavity are reported. On the basis of wind-tunnel tests performed in the IT-302M short duration wind tunnel, data on the flow structure and on the distribution of static pressure along the model walls were obtained. These data were subsequently used to verify the numerical algorithm. In the calculations, a parametric study of the effects of Mach number, cavity configuration, and temperature factor on flow quantities was performed. It was numerically shown that variation of the above parameters leads to a transition of the flow regimes in the vicinity of the cavity.