圆孔声学非线性效应的数值模拟

本文发展了一种离散涡模型,模拟了在高声强声波作用下圆孔处发生的涡脱落过程。进而计算了圆孔中的声质点速度,并分析了孔中速度的畸变情况、最后给出了圆孔的非线性声阻和声抗的理论值。非线性声抗的理论预测是现有的研究圆孔声学非线性效应的准稳态模型没有满意解决的问题,因而所做的有关尝试是本文工作的特点之一。     

微圆孔非线性声阻抗及声整流现象的实验研究

在高声强下测量了微圆孔处声激发射流的速度和微圆孔的非线性声阻抗。随声压级的增加声激发射流的速度增大,实验中射流速度在 ０－１９ｍ／ｓ范围内变化,这表明出现一种强烈声整流现象;与此同时微圆孔声阻明显增大,而声抗减小,声抗最小值约是其线性值的０．７倍。此外实验结果还验证了一种微园孔声学非线性效应离散涡模型的合理性。     

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.     

A computational and experimental study of resonators in three dimensions

In a previous work by the present authors, a computational and experimental investigation of the acoustic properties of two-dimensional slit resonators was carried out. The present paper reports the results of a study extending the previous work to three dimensions. This investigation has two basic objectives. The first is to validate the computed results from direct numerical simulations of the flow and acoustic fields of slit resonators in three dimensions by comparing with experimental measurements in a normal incidence impedance tube. The second objective is to study the flow physics of resonant liners responsible for sound wave dissipation. Extensive comparisons are provided between computed and measured acoustic liner properties with both discrete frequency and broadband sound sources. Good agreements are found over a wide range of frequencies and sound pressure levels. Direct numerical simulation confirms the previous finding in two dimensions that vortex shedding is the dominant dissipation mechanism at high sound pressure intensity. However, it is observed that the behavior of the shed vortices in three dimensions is quite different from those of two dimensions. In three dimensions, the shed vortices tend to evolve into ring (circular in plan form) vortices, even though the slit resonator opening from which the vortices are shed has an aspect ratio of 2.5. Under the excitation of discrete frequency sound, the shed vortices align themselves into two regularly spaced vortex trains moving away from the resonator opening in opposite directions. This is different from the chaotic shedding of vortices found in two-dimensional simulations. The effect of slit aspect ratio at a fixed porosity is briefly studied. For the range of liners considered in this investigation, it is found that the absorption coefficient of a liner increases when the open area of the single slit is subdivided into multiple, smaller slits.     

圆孔非线性声阻抗三维时域仿真与特性分析

为研究有限振幅声波作用下圆孔的非线性声学特性,提出了基于三维时域计算流体动力学(CFD)仿真的圆孔非线性声阻抗提取方法,通过求解层流方程来模拟声信号在圆孔及上下游的传播,以及采用横向周期性边界条件来考虑高穿孔率时圆孔之间相互作用的影响。研究了不同幅值声波作用下孔径、厚度和穿孔率对声阻抗的影响规律,通过对质点振速幅值、频率和板厚等组成的无量纲参量进行非线性回归分析,得到了圆孔非线性声阻抗的拟合公式,并将其转换为可考虑多频声波影响的时域模型。最后结合声阻抗时域模型和有限差分方法计算了直通穿孔管消声器在小振幅和有限振幅声波作用下的传递损失,通过与实验测量结果的比较,验证了拟合公式的准确性和实用性。     

Vortex sound generation due to a flow impedance discontinuity on a flat surface

The sound generated by the unsteady motion of a vortex filament moving over a flat boundary with a sharp flow impedance discontinuity is studied theoretically. Theoretical results show that the vortex filament undergoes significant accelerating or decelerating motions and radiates sound at the instant when it moves across the plane of impedance discontinuity. The accelerations and decelerations of the vortex filament are shown to be the major mechanisms of sound generation. The sound so produced has a large low-frequency content such that the change in the flow impedance affects only the sound generation process but not the subsequent sound propagation to the far field.     

Nonlinear sound waves upon collapse of a vortex dipole

Acoustic emission by a vortex dipole decreases the dipole energy and changes the vortex velocity distribution. A relative shift of the dipole component occurs. Earlier, such a process of acoustic instability of the vortex was considered as applied to the case of weak acoustic emission described by the wave equation. Features of acoustic instability upon emission of a strongly nonlinear sound by a vortex are considered in this work.     

Vortex dynamics of a trapezoidal bluff body placed inside a circular pipe

The influence of Reynolds number and blockage ratio on the vortex dynamics of a trapezoidal bluff body placed inside a circular pipe is studied experimentally and numerically. Low aspect ratio, high blockage ratio, curved end conditions (junction of pipe and bluff body), axisymmetric upstream flow with shear and turbulence are some of the intrinsic features of this class of bluff body flows which have been scarcely addressed in the literature. A large range (200:200,000) of Reynolds number (Re_D) is covered in this study, encompassing all the three pipe flow regimes (laminar, transition and turbulent). Four different flow regimes are defined based on the distinct features of Strouhal number (St)–Re_D relation: steady, laminar irregular, transition and turbulent. The wake in the steady regime is stationary with no oscillations in the shear layer. The laminar regime is termed as irregular owing to irregular vortex shedding. The vortex shedding in this regime is observed to be symmetric. The emergence of separation bubble downstream of the bluff body on either side is another interesting feature of this regime, which is further observed to be symmetric. Two pairs of mean streamwise vortices are noticed in the near-wake regime, which are termed as reverse dipole-type wake topology. Beyond the irregular laminar regime, the Strouhal number falls gradually and vortex shedding becomes more periodic. This regime is named transition and occurs close to the Reynolds number at which transition to turbulence takes place in a fully developed pipe. The turbulent regime is characterised by a nearly constant Strouhal number. Typical Karman-type vortex shedding is noticed in this regime. The convection velocity, wake width formation length and irrecoverable pressure loss are quantified to highlight the influence of blockage ratio. These results will be useful to develop basic understanding of vortex dynamics of confined bluff body flow for several practical applications.     

Stability and three-wave interaction of disturbances in supersonic boundary layer with mass exchange on the wall

The interaction of disturbances in a boundary layer of the compressible gas is considered in the linear and nonlinear approximation (the weakly nonlinear theory of stability) in the presence of mass exchange (gas blowing or suction) on the surface. The regimes of moderate (the Mach number M = 2) and high (M = 5.35) supersonic velocities of the flow are considered. The suction from the surface is shown to lead to a considerable variation of the linear evolution of disturbances: the vortex disturbances of the first mode and the acoustic disturbances of the second mode are stabilized, the rate of variation is determined by suction intensity. The nonlinear interactions in three-wave systems between the vortex waves in asymmetric triplets at M = 2 and between the waves of different nature (acoustic and vortex waves) ?? in the symmetric ones at M = 5.35 are considered. The planar acoustic wave is the excitation wave in the latter, which excites the three-dimensional subharmonic components of the vortex nature. It is shown that one can delay considerably the transition region with the aid of suction, thereby one can reduce the skin-friction drag. In the gas blowing regime, strong deformations of the mean fields of boundary layers occur, which lead to the destabilization of the vortex and acoustic waves in the linear region and activate the nonlinear processes in transition region. One can expect that this will lead to the acceleration of tripping in supersonic flow.     

Mechanism of controlling turbulent channel flow with the effect of spanwise Lorentz force distribution

A direct numerical simulation(DNS) is performed to investigate the control effect and mechanism of turbulent channel flow with the distribution of spanwise Lorentz force. A sinusoidal distribution of constant spanwise Lorentz force is selected, of which the control effects, such as flow characters, mean Reynolds stress, and drag reductions, at different parameters of amplitude A and wave number k_x are discussed. The results indicate that the control effects vary with the parameter A and k_x. With the increase of A, the drag reduction rate D_r first increases and then decreases rapidly at low k_x,and slowly at high k_x. The low drag reduction(or even drag increase) is due to a weak suppression or even the enhancements of the random velocity fluctuation and mean Reynolds stress. The efficient drag reduction is due to the quasi-streamwise vortex structure induced by Lorentz force, which contributes to suppressing the random velocity fluctuation and mean Reynolds stress, and the negative vorticity improves the distribution of streamwise velocity. Therefore, the optimal control effect with a drag reduction of up to 58% can be obtained.     

Transition from thermally activated to regular flow of magnetic flux vortices in HTSC

The effect of structural inhomogeneities in a superconductor on a vortex medium flow in weak magnetic fields at temperatures varying from 78 to 83 K for various bias current densities is investigated by using transport measurements of Bi₂Sr₂CaCu₂O_8+x thin-film microbridges. The results obtained are analyzed on the basis of the theories of flux creep and the regular flow of vortices. It is shown that the current dependences of the effective potential for vortex pinning can be satisfactorily described in the framework of two statistical models, one of which was proposed earlier by the authors. Both models cover the regimes of thermally activated and regular flow of vortices as limiting cases. The wide transition region in which the creep and regular vortex flow processes simultaneously occur due to a large dispersion in the pinning energy distribution. It is found that when the magnetic field exceeds a certain value, the average value and dispersion of the pinning potential decrease sharply, so that the conditions of regular flow set in even for small values of the bias current. This fact is attributed to the destruction of vortex lines into two-dimensional segments.     

Nature of inlet turbulence and strut flow disturbances and their effect on turbomachinery rotor noise

Results of an investigation in which turbomachinery rotor sound spectra were correlated with aerodynamic measurements of the inlet turbulence, strut wake, and vortex flow strengths are reported. Aerodynamic measurements included mean velocity profiles, turbulence intensity, and axial length scales. Inlet turbulence data indicate that the major effect of flow contraction appears to be the elongation of turbulent eddies. Eddies of this size dominate the blade passing frequency (BPF) tones. Decreasing eddy size by use of a grid revealed vortex flow strength to be the second major sound source. A doubling of vortex flow strength produced a 6 dB increase in the SPL of the first BPF. The sound pressure level showed less than a 2 dB change with doubling of strut wake turbulence intensity or velocity defect. A discussion of the relative importance of various sources of noise due to flow non-uniformities at the inlet is given.     

Behavior of rigid-porous layers at high levels of continuous acoustic excitation: theory and experiment

A model for the propagation of high amplitude continuous sound through hard-backed rigid-porous layers has been developed which allows for Forchheimer's correction to Darcy's law. The nonlinearity associated with this is shown to be particularly important in the range of frequencies around layer resonance. The model is based on the introduction of particle velocity dependent flow resistivity into the equivalent fluid model expression for complex tortuosity. Thermal effects are accounted for by means of a linear complex compressibility function. The model has been used to derive analytical expressions for surface impedance and reflection coefficient as a function of incident pressure amplitude. Depending on the material parameters, sample thickness, and frequency range the model predicts either growth or decrease of reflection coefficient with sound amplitude. Good agreement between model predictions and data for three rigid-porous materials is demonstrated.     

Vortex sound in the presence of a low Mach number flow across a drum-like silencer

The sound generated by a vortex propagating across a two-dimensional duct section with flexible walls (membranes) in an infinitely long rigid duct conveying a flow is investigated numerically using the matched asymptotic expansion technique and the potential theory. The effects of the initial vortex position, the mechanical properties of the flexible walls, and the mean flow on the sound generation are examined in detail. Results show that the presence of a vortex inside a uniform mean flow can strengthen or attenuate the sound generation, depending on the phase of the membrane vibration when the vortex starts vigorous interaction with the membranes and the strength of the mean flow. The results tend to imply that there is a higher chance of sound amplification when a vortex stream is moving closer to the lighter membrane under a relatively strong mean flow or when the mean flow is weak. The chances of sound amplification or attenuation are equal otherwise.     

湍流热对流大尺度环流反转时的角涡特性

本文采用DNS方法计算二维方腔Rayleigh–Bénard热对流.在软湍流区热对流场呈现大尺度环流和两个反向转动的角涡,并出现了大尺度环流的反转现象.连续的温度等值线和流线图清晰地描述了反转现象的全过程.在反转过程中,角涡的大小尺度变化起到重要的作用.对角涡大小尺度变化的分析发现,在反转现象中其角涡尺度随时间的变化出现剧烈的振荡,而没有反转现象的热对流场中角涡尺度变化只有小幅的脉动.对反转过程前后的角涡大小尺度、典型位置速度及角点附近温度等流动特性进行了探讨和分析,发现反转是在瞬间完成的,角涡内速度脉动较小、温度较高,反转前角涡尺度与角涡侧壁垂向速度变化具有同步性.     

Experimental investigation of holes interaction effect on the sound absorption coefficient of micro-perforated panels under high and medium sound levels

This paper experimentally investigates the holes interaction effect on the sound absorption coefficient of micro-perforated panels under high and medium sound levels. The theoretical formulations are based on a semi-empirical approach and the use of Fok’s function to model the acoustic surface impedance. For the high sound level regime, an empirical power law involving three coefficients is adapted. It is shown theoretically and experimentally that these coefficients can lead to optimized absorption performance and particularly, a formula relating the critical Reynolds number (Reynolds number value after which the absorption coefficient decreases with the increase of sound level) and the center-to-center distance between the perforations is derived. It is demonstrated that the first coefficient of the nonlinear acoustic resistance strongly depends on the separation distance between the apertures and decreases with a decrease of this latter distance. Analysis of the data reveals the fact that even with Holes Interaction Effect (HIE), the nonlinear reactance dependence on velocity is still very low compared to the resistance-velocity dependence. Four perforated panels of 1.5 mm thickness with different separation distances between the holes (from widely to closely separation) were built and tested. Experimental results performed with an impedance tube are compared with the described model for HIE. To test the dependence of the coefficients on frequency, the experiments are carried out for two different excitation frequencies (292 Hz and 506 Hz). The results can be used for designing optimal perforated panels for ducts, silencers and for the automotive industry.     

Role of anisotropy on the domain wall properties of ferromagnetic nanotubes

In this paper we investigate the role of magneto-crystalline anisotropy on the domain wall (DW) properties of tubular magnetic nanostructures. Based on a theoretical model and micromagnetic simulations, we show that either cubic or uniaxial magneto-crystalline anisotropies have some influence on the domain wall properties (wall size, propagation velocity and energy barrier) and then on the overall magnetization reversal mechanism. Besides the characterization of the transverse and vortex domain wall sizes for different anisotropies, we predict an anisotropy dependent transition between the occurrence of transverse and vortex domain walls in tubular nanowires. We also discuss the dynamics of the vortex DW propagation gradually increasing the uniaxial anisotropy constant and we found that the average velocity is considerably reduced. Our results show that different anisotropies can be considered in real samples in order to manipulate the domain wall behavior and the magnetization reversal process.     

高超声速椭圆锥边界层横流转捩特性大涡模拟

横流效应显著影响高超声速飞行器的三维边界层转捩过程, 深化对该流动机制的认识有助于提升和改善飞行器气动性能及热力学环境. 针对HIFiRE5椭圆锥绕流问题, 采用大涡模拟方法计算分析了超声速边界层横流转捩特性, 并揭示其中的流动机理. 参考HIFiRE5风洞模型试验条件, 数值模拟中椭圆锥来流入口处施加人工速度扰动以激发边界层内不稳定扰动波, 进而预测了高超声速边界层流动横流失稳、转捩过程等基本流动特征, 并基于转捩热流分布形态对比, 获得了与试验数据基本吻合的计算结果. 研究发现, 椭圆锥中心线流动汇聚形成的流向涡结构非常容易失稳, 另外在中心线及侧缘之间的中部区域存在较强的横流不稳定性, 两种机制共同作用影响边界层转捩过程. 此外, 分析了来流扰动幅值对边界层横流失稳转捩的影响, 并发现静来流条件下, 横流区域出现两组独立的定常横流涡结构, 而强噪声来流条件下, 中心线主涡和中部横流涡均发生失稳转捩, 且在椭圆锥表面形成多峰状的转捩阵面. 最后, 深入分析流场的压力脉动动力学特性, 揭示了三维边界层发生失稳转捩的非线性演化机制.      

Effect of flow on quasi-one-dimensional acoustic wave propagation in a variable area duct of finite length

The general equation for the velocity potential of quasi-one-dimensional acoustic wave motion in a variable area, finite duct with one-dimensional flow is derived by using a perturbation technique. The non-linear second-order partial differential equation is linearized and then solved, by either a power series expansion method or the Runge-Kutta fourth-order method, for harmonic time dependence. The boundary condition taken at the duct mouth is that of matching the impedance of the duct sound field to that of the radiation field at the duct opening. Three axial Mach number variations along the duct axis are considered and the results obtained are compared with those for the case of constant Mach number, to determine the influence of the axial velocity gradient on sound propagation. The effect of flow on the radiation impedance is also considered.     

Nonsteady waves in distributed dynamical systems

The paper is devoted to the study of one-dimensional and two-dimensional transient wave regimes in nonlinear systems of the reaction-diffusion type. In a one-dimensional case the process of collision of two travelling waves is considered. It is demonstrated that in the case of a nondispersive nonlinear system, where a steady regime of the collision is not possible, the process can be described by means of an approximation which is nonuniform in a spatial coordinate. The collision results, in a general case, in formation of an oscillatory shock wave moving with varying velocity. In a two-dimensional situation the transition of a rotating vortex into a rotating spiral wave in the case of dispersive systems and the inverse transition in the case of nondispersive systems are considered.