MS23.6 - Vibro-Acoustics

A semi-infinite thin shell is considered within the framework of 2D Koiter-Sanders theory. The focus is on the frequency domain near the lowest cut-off frequencies, being of order of the relative shell thickness, with the values tending to zero at a thin shell limit. Within this domain, the dynamic behaviour of the shell is strongly affected by its curvature and cannot be interpreted in terms of plate bending and plate extension even at a rough approximation. The solution is sought for as the composition of slowly decaying semi-membrane modes and a boundary layer in the form of a simple edge effect, e.g. see [1, 2] and references therein. In contrast to previous considerations in [3], a sophisticated iterative asymptotic process of satisfying the edge boundary conditions is not implemented this time. Instead, the dispersion relation for the sought for edge wave presented in the form the fourth-order determinant is subject to straightforward analysis using a symbolic algebra package. Numerical results are presented for several types of boundary conditions at the edge, including the case of a free edge. The effect of anisotropy is also addressed. References [1] Kaplunov, J., Manevitch, L. I., & Smirnov, V. V. (2016). Vibrations of an elastic cylindrical shell near the lowest cut-off frequency. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 472(2189), 20150753. [2] Kaplunov, J., & Nobili, A. (2017). A robust approach for analysing dispersion of elastic waves in an orthotropic cylindrical shell. Journal of Sound and Vibration, 401, 23-35. [3] Kaplunov, J. D., Kossovich, L. Y., & Wilde, M. V. (2000). Free localized vibrations of a semi-infinite cylindrical shell. The Journal of the Acoustical Society of America, 107(3), 1383-1393.

In this work, we study the application of the Herschel–Quincke (HQ) principle to the case of bending waves in a beam structure. This type of device is well known in acoustic tubes where an HQ filter consists of the bifurcation of a primary tube into two tubes of different lengths placed in parallel. The resulting phase shift creates a destructive interference and so a zero transmission at selected frequencies. To adapt this principle to bending vibrations, a homogeneous solid beam is divided into two different strands with different thicknesses and of same length, so that the phase difference created between these two strands of different wave celerities also leads to the same interferential effects. A Timoshenko wave model is derived to analyse the scattering properties of such HQ filter for bending waves. Notably, the drops of the transmission coefficient are related to the complex modes trapped inside the HQ filter. The results are well confirmed with reference finite element simulations as well as measurements done on an experimental beam demonstrator.

The reciprocity, orthogonality and bi-orthogonality relations are well established for symmetric waveguides, with the most known example being the Rayleigh-Lamb problem of wave propagation in an infinite elastic layer. In this paper, these relations are derived for non-symmetric waveguides using as an example the canonical problem of wave propagation in an infinite elastic plate loaded by a fluid layer of a finite height with uniform mean flow parallel to its surface. The links between formulations of dispersion equation, reciprocity/bi-orthogonality relations and energy flux are demonstrated and discussed. The Green’s function for stable regimes of wave motion in the presence of a flow is derived using the bi-orthogonality relation, and partition of the energy flux between transmission paths is analyzed.

Leaks in water distribution systems are a major problem as a large amount of water is wasted between treatment plants and consumers. Water supply companies use different strategies to identify and locate leaks in their pipe networks. Among these are vibro-acoustic methods and devices (i.e. listening sticks, geophones, and leak noise correlators). Although listening devices are cheap and simple to use, they rely on the operator's hearing ability to locate leaks from single point measurements. Moreover, this may lead to a search based on trial and error as only vibration intensity is used, so no information is obtained regarding the direction of the leak from the measurement position. Leak noise correlators, however, use two sensors positioned on the pipe or at access points, and the leak can be located by estimating the time of flight between the two measured signals and the speed at which the leak noise propagates. This paper concerns an investigation of a new technique where leak noise signals measured on the ground surface using a camera (non-contact measurements) are pre-processed using computer vision techniques to extract meaningful information concerning the location of the leak. A preliminary study is reported in which measurements are made on a test bench that simulates the vibration response due to a buried pipe at two positions. Tests are conducted to evaluate the camera sensitivity for leak noise measurements regarding the distance from the vibration source, lighting, contrast intensity, and the relation between image resolution and leak detection are performed. The obtained results are compared to classic contact measurements using accelerometers showing that the results from the proposed technique are promising.