On the effects of reflections on time delay estimation for leak detection in buried plastic water pipes

This paper is concerned with the way in which wave reflections in a fluid-filled pipe affect the cross-correlation function of two leak noise signals used to detect and locate leaks in buried water pipes. Propagating waves generated by leak noise reverberate in a pipe network system, as they encounter features such as changes in section, and resistance such as valves, and pipe junctions. A theoretical model of a straight pipe with discontinuities, which cause reflections, is developed and incorporated into a model of the cross-correlation function. The reasons why the reflections and the low-pass filtering properties of the pipe can be largely removed by the generalised cross-correlation (GCC) phase transform (PHAT) are determined. Using the analytical model, theoretical predictions of the basic cross-correlation function (BCC) and the GCC PHAT are compared with experimental data from a specially constructed test site in Canada.     

利用海浪噪声自相关实现散射体无源探测

提出了一种利用海浪噪声自相关实现散射体无源探测的新方法.将各接收器记录噪声信号的自相关减去所有接收器记录噪声信号自相关的平均值,得到散射信号的到达结构,然后结合基尔霍夫移位算法实现对散射体的探测.与利用背景噪声互相关提取格林函数从而实现散射体探测的方法不同,自相关无需考虑各个接收器之间的大量数据传输及时间同步问题,这为相距较远的多接收器和移动平台目标探测提供了极大的方便.将所提出的方法应用于实验数据中,最终探测结果与实际测量结果相比差别不大,验证了方法的有效性.     

管道弯头对低频纵向导波传播特性影响分析

管道弯头显著改变了导波传播特性,影响了对检测信号的解读,研究弯头对导波传播特性的影响是实现复杂管道系统导波检测的基础。采用半解析有限元法计算弯管导波频散曲线,分析了弯管导波频散曲线所呈现的不同特征,并基于弯管导波频散曲线,以低频L(0,1)模态导波为研究对象,实验研究了低频L(0,1)模态导波通过管道弯头时的模态变换特征。研究结果发现,当L(0,1)模态导波通过管道弯头时,不仅会发生L(0,1)到F(1,1)的模态变换,还会模态变换出反向L(0,1)模态导波,即弯头反射现象,且随着激励频率的降低和弯头弯曲半径的减小,弯头反射现象愈发明显。研究结果将深化对弯管导波传播特性的认识,推动导波检测技术在复杂管道系统检测中的应用。      

黏弹介质包裹的充液管道中低频轴对称波传播特性

研究埋地充液管道中低频轴对称波传播特性.将土壤考虑为黏弹介质,结合Kennard薄壳方程和Kelvin-Voigt线性黏弹性模型,引入土壤载荷矩阵,推导出土-管滑移情形下流体主导波和管壁压缩波的相速度表达式.通过数值模拟计算得到流体主导波和管壁压缩波的频散和衰减曲线并进行可靠性验证,分析两种波引起的管壁径向位移之比,讨...     

一维粗糙地表面下方埋藏目标SAR成像

提出一种快速实现一维粗糙地表面下方埋藏金属目标的成像算法.用层内波传播算法结合谱加速的前后向迭代算法(PILE+FB-SA)计算一维粗糙面下方埋藏金属目标的后向散射;用得到的后向散射数据结合后向投影算法实现二维SAR(synthetic aperture radar)成像,研究窗函数的选取,讨论目标位置及尺寸大小变化的影响.用具有高斯谱的粗糙面模拟实际地表面,并采用锥形波入射以减小人为截断粗糙面引起的边缘效应.由于散射数据由快速数值算法得到,该算法不受地面和目标参数限制,可以实现任意粗糙地表面和复杂目标成像,对地下目标探测具有重要的应用价值.     

Vibration characteristic analysis of buried pipes using the wave propagation approach

A theoretical analysis for the free vibration of simply supported buried pipes has been investigated using the wave propagation approach. The pipe modeled as a thin cylindrical shell of linear homogeneous isotropic elastic material buried in a linear isotropic homogeneous elastic medium of infinite extent. The vibrations of the pipe are examined by using Flüggle shell equation. The natural frequencies are obtained for the pipes surrounded by vacuo or elastic medium. The results are compared with those available in the literature and agreement is found with them. It is found that the free vibration frequency of the pipe does not appear for some of the axial or circular vibration modes and the real natural frequencies of the pipe are significantly dependent on the rigidity of the surrounding medium.     

Torsional wave experiments with a new magnetostrictive transducer configuration

For the efficient long-range nondestructive structural health inspection of pipes, guided waves have become widely used. Among the various guided wave modes, the torsional wave is most preferred since its first branch is nondispersive. Our objective in this work is to develop a new magnetostrictive transducer configuration to transmit and receive torsional waves in cylindrical waveguides. The conventional magnetostrictive transducer for the generation and measurement of torsional waves consists of solenoid coils and a nickel strip bonded circumferentially to test pipes. The strip must be premagnetized by a permanent magnet before actual measurements. Because of the premagnetization, the transducer is not suitable for the long-term on-line monitoring of pipes buried underground. To avoid the cumbersome premagnetization and to improve the transduction efficiency, we propose a new transducer configuration using several pieces of nickel strips installed at 45 degrees with respect to the pipe axis. If a static bias magnetic field is also applied, the transducer output can be substantially increased. Several experiments were conducted to study the performance of the proposed transducer configuration. The proposed transducer configuration was also applied for damage detection in an aluminum pipe.     

Global dynamics of a pipe conveying pulsating fluid in primary parametrical resonance: Analytical and numerical results from the nonlinear wave equation

The chaotic dynamics of nonlinear waves in the harmonic-forced fluid-conveying pipe in primary parametrical resonance, is explored analytically and numerically. The multiple scale method is applied to obtain an equivalent nonlinear wave equation from the complicated nonlinear governing equation describing the fluid conveyed in a pipe. With the Melnikov method, the persistence of a heteroclinic structure is shown to be satisfied and its condition is given in functional form. Similarly, for the heteroclinic orbit, using geometric analysis, a condition function of the stable manifold is derived for the orbit to return to the stable manifold from the saddle point. The persistent homoclinic structures and threshold of chaos in the Smale-horseshoe sense are obtained for the fluid-conveying pipe under both conditions, indicating how the external excitation amplitude can change substantially the global dynamics of the fluid conveyed in the pipe. A numerical approach was used to test the prediction from theory. The impact of the external excitation amplitude on the nonlinear wave in the fluid-conveying pipe was also studied from numerical simulations. Both theoretical predications and numerical simulations attest to the complex chaotic motion of fluid-conveying pipes.     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

小管径气液两相流空隙率波传播的多尺度相关性

空隙率波是气液两相流系统的特殊物理现象,理解空隙率波的传播特性对揭示两相流流型转变与流速测量物理机理具有重要意义.本文首先考察了典型非线性系统的多尺度互相关特性,发现去趋势互相关分析方法可有效揭示系统的多尺度非线性动力学特征;然后,通过采集垂直上升小管径气液两相流电导传感器阵列上下游空隙率波动数据,提出采用多尺度去趋势互相关分析方法探测空隙率波传播的多尺度互相关特性,并提取了低尺度空隙率波互相关水平增长率;另外,通过计算空隙率波空间衰减因子,考察了气液两相流空隙率波传播的结构不稳定行为.结果表明,空隙率波结构的多尺度互相关特性与其空间衰减特性具有较好的物理关联性:对于气液两相流过渡流型,低尺度空隙率波互相关水平增长率较高,且与较为稳定的空隙率波传播特性相对应;而当气液两相流空隙率波明显衰减或放大时,空隙率波互相关水平增长速率一般较低.     

Chatter suppression by parametric excitation: Model and experiments

Chatter vibration leads to challenges in precise machining due to its harmful effect on productivity and surface quality. In this study, a chatter suppression method based on parametric excitation was developed. The effect of parametric excitation on self-excited vibration was investigated based on a model of a van der Pol-Mathieu-Duffing oscillator with a time delay. It reveals that there can be a zero solution for the oscillator under the effect of parametric excitation, while it is impossible to have a stable zero equation without parametric excitation. The stability of a parametrically excited vibration system regarding the regenerative effect in the cutting processes was studied by the averaging method. The stability analysis shows that parametric excitation with an appropriate frequency and large amplitude has a chatter suppression effect no matter whether the waveform is a sinusoidal wave, square wave or triangular wave. To validate the effect of parametric excitation for chatter suppression, experiments were conducted based on a magnetorheological (MR) fluid-controlled boring bar, which can generate high-frequency parametric excitation based on the quick response of the MR fluid. Cutting experiments with an excitation current of different waveforms and diverse frequencies show that chatter can be significantly suppressed by the effect of parametric excitation.     

长骨中振动声激发超声导波的方法

为了实现一定频段内任意低频下在长骨中激励导波信号,本文提出一种采用聚焦高频(5 MHz)超声换能器在长骨皮质骨中激发低频(150 kHz)超声导波的振动声方法.首先介绍了板状超声导波理论和双声束共聚焦法与单声束调幅法激发振动声的基本原理;进而采用三维有限元仿真方法分析振动声激发低频超声导波的基本现象,然后结合牛胫骨板离体实验,验证振动声激发低频超声导波的可行性.结果均表明,双声束共焦与单声束振动超声均可在骨板中激发低频超声导波.相关研究方法有助于提高空间域长骨中超声导波测量精度,以及在一定频段内实现任意频率激励等,对发展低频超声导波在体测量长骨皮质骨的新技术具有一定的指导意义.     

Accelerometer measurements of acoustic-to-seismic coupling above buried objects

The surface velocity of sand inside a large PVC container, induced by the sound pressure from either a large loudspeaker radiating into an inverted cone and pipe or a Bruel and Kjaer point source loudspeaker mounted with its axis vertical, has been measured using accelerometers. Results of white noise and stepped frequency excitation are presented. Without any buried object the mass loading of an accelerometer creates resonances in the spectral ratio of sand surface velocity to incident acoustic pressure, i.e., the acoustic-to-seismic (A/S) admittance spectra. The A/S responses above a buried compliant object are larger and distinctive. The linear A/S admittance spectra in the presence of a buried electronic components box have been studied as a function of burial depth and sand state. The nonlinear responses above the buried box have been studied as a function of depth, sand state, and amplitude. Predictions of a modified one-dimensional lumped parameter model have been found to be consistent with the observed nonlinear responses. Also the modified model has been used to explain features of the A/S responses observed when using an accelerometer without any buried object.     

Leak noise propagation and attenuation in submerged plastic water pipes

Detection of water leaks in buried distribution pipes using acoustic methods is common practice in many countries. Correlation techniques are widely used in leak detection, and these have been extremely effective when attempting to locate leaks in metal pipes. However, a number of difficulties have been highlighted when trying to determine the position of leaks in plastic pipes. Of particular interest here is what happens to the leak noise when the pipe passes through an expanse of water, such as across a river or lake.In this paper, the low-frequency acoustic propagation and attenuation characteristics of a submerged plastic water pipe are investigated experimentally in the laboratory, supported by predictions from a theoretical model. It is found that, whilst the signal attenuation for a submerged pipe is increased relative to that for a similar in-vacuo pipe, energy does not, in fact, radiate into the water; furthermore, the attenuation is small compared with that for a pipe buried in soil.     

强耦合表面极化子的激发能量

采用线性组合算符方法及幺正变换方法研究了电子与表面光学(SO)声子和体纵光学(LO)声子均为强耦合的表面极化子的激发态性质.计算了体系的有效哈密顿量、振动频率和体系由基态向第一激发态跃迁所需的激发能量.     

The effect of pipe bend on low-frequency longitudinal mode guided wave propagation

The pipe bend significantly changes the propagation characteristics of guided wave,and makes the interpretation of the received signals difficult.Therefore,better understanding of guided wave propagating in bended pipe is essential for the inspection of pipeline comprising bends.First of all,the different features of dispersion curves derived with the semi-analytical finite element method for guided wave in bended pipes are summarized.Secondly,based on the dispersion curves for guided wave in bended pipes,experiments are performed to investigate the mode conversions of L(0,1) mode guided wave traveling through pipe bends.It is found that,except for the mode conversion from L(0,1) to F(1,1),the L(0,1) reflections of bends are also observed in some cases,which are proven to be the mode converted negative L(0,1)mode guided wave,and the negative L(0,1) mode guided wave becomes more obvious with the decrease of excitation frequency and bending radius.The findings of this paper will provide some insight for guided wave behavior in bended pipe,and generalize the application of guided wave inspection in practical pipelines.     

基于声发射和GAN-CNN的铝合金管道法兰 连接松动泄漏检测*

面向管道法兰连接松动引起的泄漏检测需求,为解决数据样本不足和减少特征指标手动选取的繁琐环节。本文,考虑到生成性对抗网络（GAN）作为数据扩充工具,已被证明能够生成与真实数据相似的样本数据。同时,卷积神经网络（CNN）作为一种深度学习方法,为自动提取数据的特征提供了一种有效的方法。开展了基于GAN和CNN的铝合金管道法兰连接松动泄漏检测研究。首先,搭建管道泄漏标定和数据采集实验台,利用声发射技术获取不同等级的原始泄漏信号。其次,采用GAN生成样本数据扩充原始数据。同时,为了评估生成模型的性能,引入统计特评估生成质量。最后,将生成的样本数据与原始数据设置为不同训练集,基于卷积神经网络构建智能分类检测模型,应用于管道泄漏检测。同时,分类检测结果与小样本智能分类方法SVM进行了比较,实验结果表明,基于GAN和CNN构建的智能分类模型可显著提高管道法兰连接松动泄漏检测精度。     

Pipe wall damage detection by electromagnetic acoustic transducer generated guided waves in absence of defect signals

Most investigators emphasize the importance of detecting the reflected signal from the defect to determine if the pipe wall has any damage and to predict the damage location. However, often the small signal from the defect is hidden behind the other arriving wave modes and signal noise. To overcome the difficulties associated with the identification of the small defect signal in the time history plots, in this paper the time history is analyzed well after the arrival of the first defect signal, and after different wave modes have propagated multiple times through the pipe. It is shown that the defective pipe can be clearly identified by analyzing these late arriving diffuse ultrasonic signals. Multiple reflections and scattering of the propagating wave modes by the defect and pipe ends do not hamper the defect detection capability; on the contrary, it apparently stabilizes the signal and makes it easier to distinguish the defective pipe from the defect-free pipe. This paper also highlights difficulties associated with the interpretation of the recorded time histories due to mode conversion by the defect. The design of electro-magnetic acoustic transducers used to generate and receive the guided waves in the pipe is briefly described in the paper.     

浅埋三舱管廊甲烷爆炸的地面响应规律

地下综合管廊可燃性气体爆炸事故时有发生,给地面人员的生命和财产造成了巨大损失。依托重庆市某地下综合管廊试点工程,基于物质点法,采用点火增长模型模拟浅埋管廊泄漏甲烷气体爆炸冲击管廊本体结构和围岩的过程,研究爆炸作用下地面压强与位移的响应特性。研究发现:泄爆作用下管廊及围岩会出现因接触面反射和折射产生的次生应力波,管廊横向方向次生波振幅随距起爆点水平距离的增大而增大,而管廊纵向方向产生的次生波振幅较小,且随距离增大变化较小;爆炸作用造成整体地面沉降,但在起爆点中心附近地面隆起,这种隆起由管廊本体结构破裂,气体直接冲击岩土体形成的剧烈隆起和管廊整体震动形成的轻微隆起两部分组成。     

激光声源特性研究及海洋应用

激光声作为水下声源的一种新的激发方法,具有高声强、窄脉冲、宽频谱等优点。基于激光声激发的线性及非线性理论,建立了激光声信号特性的初步模型,研究了激光在水中激发声波的特性,研究表明：在热膨胀激发状态,增大激光光强可以提高激光声的转换效率;光强大于击穿阈值后,激光声的激发机制为非线性的光击穿机制。激光声具有的高距离分辨能力和海洋抗干扰能力,对水下目标探测、空载平台等领域有着较广的应用前景。