Advanced numerical techniques for characterising obstructions in sewer pipes

英国的地下下水道系统长约30万公里，更换费用估计为1040亿英镑。下水道系统由私营水务公司拥有，这些公司有法律责任维持其下水道系统的结构和运作状况，包括减少水浸事件。在大约80%的情况下，水浸事件是由管道壁损坏或大量沉积物和/或脂肪沉积造成的堵塞造成的。检测和清除障碍物应成为任何维护计划的一部分，尽管目前由于缺乏快速和可靠的方法，这方面的能力受到限制。这个项目将集中于使用声波来检测和描述下水道中的障碍物。在这里，扬声器产生沿下水道传播的压力脉冲；该脉冲通常会被遇到的任何障碍物强烈反射，通过使用麦克风捕获反射的能量信息，可以快速轻松地捕获有关障碍物的信息。因此，该方法为大型排水系统的监测提供了一种快速、客观的方法。本研究旨在对EPSRC项目(EP/D058589/1)中开发的原型声学装置进行阶梯式改进。目前的设备依赖于新的声强测量和存储的已知下水道缺陷的测量之间的相互关系；然而，这种方法受到可能进行的实验研究的数量和解释测量强度数据的困难的限制。此外，目前的方法无法说明障碍物的几何形状或表面特征，也没有证据表明在测量数据和障碍物特性之间存在唯一的联系。拟议的研究试图通过使用数学模型来帮助开发一种新的测量方法来解决这些问题，该方法将声强视为一个复数，而不是使用当前设备中采用的传统实值表示法。这里，与实值强度表示法相比，复数声强有可能从散射声场中揭示出更多的信息，而声导中复数强度的测量形成了本方案的重点。尽管复数强度测量有可能提供更多的信息，但它们还没有被很好地理解，特别是对于声波导中障碍物的散射。因此，为了更好地了解复杂的强度，需要建立数学模型，这里提出了频域和时域模型。频域模型基于有限元方法，以适应下水道系统中常见的不规则几何形状；时域模型基于对频域计算进行逆傅立叶变换，并且还将利用逆分析来解决诸如测量数据的唯一性等问题。理论预测将与在实验室条件下获得的时间平均和瞬时复杂强度测量进行比较。这样，在将这些知识应用于开发新的下水道系统测量方法之前，将对复杂强度有一个更一般的理解。此外，为了保持与实际排水系统的相关性，还将研究已知的影响现场测量准确性的问题，如检修孔、裂缝、接缝和管道表面粗糙度。因此，通过数学模型和实验室测量发展起来的理解将被用来开发一种新的原型实验方法，适用于重建真实下水道系统中障碍物的几何形状和表面特征。

The underground sewer system in the U.K. is approximately 300,000 km long, for which the replacement costs are estimated to be 104 billion. The sewer system is owned by the privatised water companies who have a legal duty to maintain the structural and operational conditions of their sewer systems, and this includes reducing flooding incidents. In approximately 80% of cases, flooding incidents are caused by obstructions arising from the deterioration of a pipe wall, or from large deposits of sediment and/or fat. The detection and removal of obstructions should form part of any maintenance programme, although the ability to do this is currently restricted by the lack of a fast and reliable method. This project will focus on using sound waves to detect and characterise obstructions in sewers. Here, loudspeakers generate a pressure pulse that travels down a sewer pipe; this pulse is normally strongly reflected by any obstruction it encounters and by using microphones to capture the reflected energy information about the obstruction may be captured quickly and easily. Accordingly, this method offers a fast and objective way to monitor large sewer systems.The proposed research aims to deliver a step change improvement to a prototype acoustic device developed in a previous (experimentally based) EPSRC project (EP/D058589/1). The current device relies on cross-correlation between new acoustic intensity measurements and measurements stored for known sewer defects; however, this methodology is limited by the number of experimental studies it's possible to undertake and difficulties when interpreting measured intensity data. Furthermore, the current method can say nothing about the geometry, or surface characteristics, of an obstruction, and there is no proof that a unique link exists between the measured data and the properties of the obstruction. The proposed research seeks to address these issues by using mathematical models to aid in the development of a new measurement methodology that treats the acoustic intensity as a complex quantity rather than using the traditional real valued representation adopted in the current device. Here, complex acoustic intensity has the potential to uncover significantly more information from scattered sound fields when compared to a real valued intensity representation, and it is the measurement of complex intensity in acoustic waveguides that forms the focus of this proposal.Although complex intensity measurements have the potential to deliver significantly more information, they are not well understood, especially for scattering from obstacles in an acoustic waveguides. Accordingly, to gain a better understanding of complex intensity it is desirable to develop mathematical models and here both frequency and time domain models are proposed. The frequency domain model is based on the finite element method in order to accommodate those irregular geometries typically found in sewer systems; the time domain model is based on taking an inverse Fourier transform of the frequency domain calculations and will also utilise an inverse analysis in order to address issues such as the uniqueness of measured data. Theoretical predictions will be compared with time-averaged and instantaneous complex intensity measurements obtained under laboratory conditions. In this way, a more general understanding of complex intensity will be developed before this knowledge is applied to the development of a new measurement methodology for sewer systems. Furthermore, to maintain relevance to real sewer systems problems known to affect the accuracy of field measurements, such as manholes, cracks, joints and pipe surface roughness will also be studied. Accordingly, the understanding developed with the mathematical models and laboratory measurements will be used to develop a new prototype experimental methodology suitable for reconstructing the geometry and surface characteristics of obstructions in real sewer systems.