一种以涡声理论为基础的声衬的实验研究与分析

本文对一种新型声衬一通气声衬的声学特性进行了实验研究。首先,根据实验装置的实际结构给出了通气声衬反射系数的理论计算公式,并使用不同孔径和穿孔率的穿孔板考察了通气声衬的特性参数随频率和气流速度的变化规律,理论与实验符合得较好,其结果表明:适当地选择声衬的几何参数和气流速度可以使通气声衬在共振频率处的吸声系数达到或接近于1.0。其次,本文对共振式声衬和通气声衬的特性进行了比较,对于大孔径穿孔板,前者的吸收频带较窄,而通过合理选择通气速度,则可以设计出高吸声系数和宽频带的通气声衬。最后,本文还对相同几何参数的穿孔板进行了吹气和吸气的对比实验。     

流体通过涡激振动机翼的声辐射研究

为了有效地降低涡激噪声,研究了粘性流体通过涡激振动机翼的声辐射。采用Navier-Stokes方程描述二维机翼的流固耦合运动,用弹簧系统代替实际固体变形产生的回复力和力矩,翼型的运动是两个自由度,即垂直于来流的振荡和转动振荡;为了模拟涡激振动,机翼的初始攻角取得比较大,以便产生周期性的旋涡脱落及周期性的流体动力,后者与弹簧系统相耦合,引起振动,用Lighthill声比拟方法研究了由此引起的声辐射。计算结果表明:当涡脱频率和机翼的固有的振动频率一致时,发生锁定的现象,此时的声辐射达到最大。     

The acoustic impedance of perforated plates under various flow conditions relating to combustion chamber liners

The absorption of sound by cavities lined with perforated sheets depends crucially on the impedance of the orifices in the sheets. Although the theory for that absorption in the absence of a mean flow was well-developed in 1926, the presence of either a ‘bias’ flow through the orifices, or of a flow ‘grazing’ the sheet and deflecting the acoustic jets, radically alters the absorption. There are many theoretical and experimental treatments of the various cases, some of which are reviewed here. However, there has been little attempt to show how these data relate to one another, and this is also undertaken. The frequency dependence of the impedance is here expressed in terms of a Helmholtz number and used as the prime parameter for comparison. Theories for the cases where the mean flow is negligible are naturally based on the viscous penetration depth, whereas those for bias flow have a Strouhal number as the main parameter and are independent of viscosity. It is found that there are major uncertainties in the impedance for higher Strouhal numbers, when the bias flow is small. A criterion for transition to the no-bias flow theory is proposed. Theories and correlations for grazing flow rationally feature a Strouhal number based on the friction velocity in the duct, since this determines the boundary layer characteristics, but there should be a smooth transition to the case where the grazing flow can be considered negligible. Criteria for this are also proposed, based on the available experimental data. When both types of flow are present, particularly when the grazing velocity is larger than the bias velocity, the available data are very limited.     

Scattering and absorption of sound at flow duct expansions

The scattering of plane acoustic waves at area expansions in flow ducts is analysed using the vortex sheet model where the flow at the expansion is modelled as a jet, with a vortex sheet emanating from the edge. Of particular interest is the influence of the flow field on acoustic scattering and absorption.First, it is demonstrated that the stability properties of the shear layer can be simulated by modifying the edge condition within the vortex sheet model. To this end the accuracy for the region where the shear layer is changing from unstable to stable is improved by introducing a gradually relaxed Kutta edge condition with empirically derived coefficients. For low Strouhal numbers the vortex sheet model applies and for higher Strouhal numbers the two models converge.Second, it is demonstrated that the acoustic transmission through the jet expansion region can be determined by neglecting the scattering there. Incorporating this assumption, the vortex sheet model reproduces well the experimental results on transmission and absorption for an area expansion. This result supports the assumption that the main part of the scattering occurs at the area expansion at least for the low-frequency range. Furthermore, the influence of the flow field is particularly strong for small Strouhal numbers.     

Reflection of sound at area expansions in a flow duct

An analytical model for scattering at area discontinuities and sharp edges in flow ducts and pipes is presented. The application we have in mind is large industrial duct systems, where sound attenuation by reactive and absorptive baffle silencers is of great importance. Such devices commonly have a rectangular cross-section, so the model is chosen as two-dimensional. Earlier solutions to this problem are reviewed in the paper. The modelling of the flow conditions downstream of the area expansion, with and without extended edges, and its implications for the resulting acoustic modes are discussed. Here, the scattering problem is solved with the Wiener-Hopf technique, and a Kutta condition is applied at the edge. The solution of the wave equation downstream of the expansion includes hydrodynamic waves, of which one is a growing wave. Theoretical results are compared with experimental data for the reflection coefficient for the plane wave, at frequencies below the cut-on for higher order modes. Influence of the interaction between the sound field and the flow field is discussed. A region where the reflection coefficient is strongly Strouhal number dependent is found.     

Vorticity dynamics and sound generation in two-dimensional fluid flow

An approximate solution to the two-dimensional incompressible fluid equations is constructed by expanding the vorticity field in a series of derivatives of a Gaussian vortex. The expansion is used to analyze the motion of a corotating Gaussian vortex pair, and the spatial rotation frequency of the vortex pair is derived directly from the fluid vorticity equation. The resulting rotation frequency includes the effects of finite vortex core size and viscosity and reduces, in the appropriate limit, to the rotation frequency of the Kirchhoff point vortex theory. The expansion is then used in the low Mach number Lighthill equation to derive the far-field acoustic pressure generated by the Gaussian vortex pair. This pressure amplitude is compared with that of a previous fully numerical simulation in which the Reynolds number is large and the vortex core size is significant compared to the vortex separation. The present analytic result for the far-field acoustic pressure is shown to be substantially more accurate than previous theoretical predictions. The given example suggests that the vorticity expansion is a useful tool for the prediction of sound generated by a general distributed vorticity field.     

Experiments on sound generation in corrugated pipes with flow

The article reports acoustic measurements on short corrugated pipes with flow. Such pipes might generate high sound levels associated with length resonances. One of the main objectives of the study was to estimate the location of the effective sources by studying the energy flow through the pipes. It was found that a short section of corrugations will only produce sound effectively when placed at the inflow end, while for fully corrugated pipes, the sound-producing regions are located around the pressure maxima of the observed standing waves. It was further found that the net energy flow is in the upstream direction for nearly the complete length of pipe.     

流体通过二维振动翼型的声辐射研究

推导有均流时二维运动物体的声辐射公式,研究了粘性流体通过二维翼型的声辐射。数值计算结果表明:在不同雷诺数时,声压随时间的变化不同;小雷诺数时,具有规则性,而大雷诺数时,具有非规则性,这是由于多个涡之间的相互作用而引起的;流体通过绕弹性中心的振动翼型引起的声辐射大于流体通过固定翼型的声辐射,单级子项对声场的贡献不容忽略。     

有流条件下环形管道中的声传播特性

本文从线性波动方程出发,根据界面处声压连续和质点位移连续的条件,导得有流条件下无限长环形吸声管道中声传播的特征方程,并且具体分析了管道衰减系数与气流速度、壁面特性、截面几何尺寸和声波频率等参量的相互关系。研究表明,管道衰减系数随着气流流速的增加,管壁吸声系数的减小、管道截面几何尺寸的增加而减小。同时,随着声波频率从低频到高频的变化,衰减系数从小到大,再从大到小地变化,存在一个最佳峰值。     

Extraordinary sound screening in perforated plates

We report extraordinary effects in the transmission of sound through periodically perforated plates, supported by both measurements and theory. In agreement with recent observations in slit arrays, M. H. Lu et al. [Phys. Rev. Lett. 99, 174301 (2007)10.1103/PhysRevLett.99.174301], nearly full transmission is observed at certain resonant frequencies, pointing out similarities of the acoustic phenomena and their optical counterpart. However, acoustic screening well beyond that predicted by the mass law is achieved over a wide range of wavelengths in the vicinity of the period of the array, resulting in fundamentally unique behavior of the sound as compared to light.     

Reduction of sound in a low velocity flow duct by the use of Bragg reflections

This paper presents the results of some experiments on the use of structures of aerodynamically-shaped elements as narrow-band sound reflectors in a duct. The structures are tuned to give maximum attenuation for frequencies determined by the Bragg reflection condition, and are thought to be of special value where space considerations prohibit the use of conventional acoustic wave filters.     

Self-induced harmonic generation in a storage-ring free-electron laser

Coherent radiation from a relativistic electron beam is a valuable way to overcome the present limitations of conventional lasers and synchrotron radiation light sources. The typical scheme has electrons, directly from a linac, in a single-pass interaction with a laser pulse in the presence of a static undulator magnetic field. We demonstrate that a storage-ring free-electron laser can also achieve harmonic generation (down to 36.5 nm), presenting both experimental and theoretical results, and offer a reliable interpretation of the peculiar underlying physical processes involved.     

Experimental investigations of sound reflection from hot and subsonic flow duct termination

The knowledge of the reflection properties of open end jet ducts is important for different applications, where the flow and high temperature conditions are involved and add complexity to the problem. In this paper, the magnitude of the reflection coefficients together with the respective end-corrections is experimentally determined for hot flow duct openings. A Mach number range up to 0.3 for cold jets and up to 0.12 for a jet temperature of 200 °C is treated. The experimental results are compared with the numerical model proposed by Munt (Acoustic transmission properties of a jet duct with subsonic jet flow: 1. The cold jet reflection coefficient, Journal of Sound and Vibration 142 (1990) 413–436) and a good correlation in plane-wave region is demonstrated. To reduce experimental uncertainty, the sound reflection properties at the duct opening are obtained by using an overdetermined two-microphone technique with the implementation of a three pressure transducer array. By introducing a modified multistep version of the stepped sine excitation, the accuracy of data acquisition process is improved without compromising the measurement time.     

Aerodynamic sound generation caused by viscous processes

With the aim of resolving some questions arising from previous studies by Morfey, Kempton, Hardin, and Kambe and Minota, the effects of viscosity on aerodynamic sound generation by unheated, low Mach number flow are investigated theoretically. First, an analytical solution in terms of a multipole expansion is derived by means of the method of matched asymptotic expansions. This solution is especially well suited to compare the results obtained by the various authors mentioned above. Second, an estimation of the importance of viscous effects on aerodynamic sound generation is given by which one can show that viscous effects have been overestimated in the past.     

Broadband sound generation by confined turbulent jets

Sound generation by confined stationary jets is of interest to the study of voice and speech production, among other applications. The generation of sound by low Mach number, confined, stationary circular jets was investigated. Experiments were performed using a quiet flow supply, muffler-terminated rigid uniform tubes, and acrylic orifice plates. A spectral decomposition method based on a linear source-filter model was used to decompose radiated nondimensional sound pressure spectra measured for various gas mixtures and mean flow velocities into the product of (1) a source spectral distribution function; (2) a function accounting for near field effects and radiation efficiency; and (3) an acoustic frequency response function. The acoustic frequency response function agreed, as expected, with the transfer function between the radiated acoustic pressure at one fixed location and the strength of an equivalent velocity source located at the orifice. The radiation efficiency function indicated a radiation efficiency of the order (kD)2 over the planar wave frequency range and (kD)4 at higher frequencies, where k is the wavenumber and D is the tube cross sectional dimension. This is consistent with theoretical predictions for the planar wave radiation efficiency of quadrupole sources in uniform rigid anechoic tubes. The effects of the Reynolds number, Re, on the source spectral distribution function were found to be insignificant over the range 20002.5. The influence of a reflective open tube termination on the source function spectral distribution was found to be insignificant, confirming the absence of a feedback mechanism.     

Self-induced generation of an off-axis frequency shifted radiation from atoms

The transient resonant linear response at wavelength λ_a of an N two-level atom vapor driven by a strong pulse with wavelength λ_f = λ_a - |Δλ| is shown to promote an emission of radiation peaked at wavelength λ_c = λ_a + |Δλ| in a conical shell around the propagation axis of the incident beam. In the limit of weak excitation, i.e. for an incident Rabi frequency much smaller than the detuning, the cone angle is found to be equal to 2λμ(2N/ch|Δλ|)^{$\text{1}\text{2}$} where μ is the transition dipole moment.