Application of orthogonality-relation for the separation of Lamb modes at a plate edge: numerical and experimental predictions.

In this study the orthogonality relation-based method for post-processing finite element (FE) predictions and experimental measurements is applied in order to separate Lamb modes at a plate edge at normal incidence. The scattered wave field from the free edge is assumed to be a superposition of all the eigenmodes of an infinite plate. The eigenmode amplitudes of the reflected wave field are determined by implementing the orthogonality-based method on the measured plate edge displacements. Overlapping wavepackets of Lamb modes at a plate edge are simulated by using the FE model and the experiment in the case of an incident S0 mode in a plate with a notch. In the experiment a 3D Scanning Laser Doppler Vibrometer (3D SLDV) (Johansmann and Sauer, 2005) is used to measure 3 dimensional vibrations and thus the edge two-dimensional displacement components simultaneously. It is demonstrated that it is possible to extract signals of various propagating and non-propagating modes in time-domain. The influences of the errors in practical measurements on the extraction procedure have also been studied.

Acoustic scattering from a finite plate: generation of guided Lamb waves S(0), A(0) and A.

In the domain of renewable energies, marine current turbines constitute one of the possibilities of producing electrical energy. Naked-eye inspection, or with the aid of video monitoring systems of these machines to ensure their perfect working order, can be difficult in a turbid environment. Acoustic methods are conceivable. The study focuses on the blades of these machines, by considering rectangular plates. The propagation of Lamb waves in a plate is studied by analyzing experimental time signals obtained from acoustic scattering. These signals are analyzed employing the ray theory. In vacuum, the flexural wave is the A(0) Lamb wave, whilst in water this wave splits in a bifurcation: the A wave with a phase velocity always smaller than the sound speed in water, and the A(0) wave with a phase velocity always higher than the sound speed in water. In the central bandpass of the transducers used in the experiments, mainly the A and S(0) waves exist. However, signals observed in the third harmonic bandpass of the transducers are also analyzed. In order to complement these results, resonance frequencies of the plate studied are calculated taking into account the boundary conditions and compared with the resonance frequencies of the experimental spectra.

Edge resonance in semi-infinite thick pipe: numerical predictions and measurements.

This paper presents theoretical and experimental studies of axisymmetric longitudinal guided wave L(0,2) interaction with the free edge of the pipe. A numerical method based on normal mode superposition is applied to predict the edge resonance by an analysis of dispersion relations of separate modes. In parallel, the finite element analysis and experimental measurements prove the existence of edge resonance in the pipe in case of L(0,2) wave incidence. It is shown that the edge resonance is mainly caused by the first pair of complex modes. Additionally the behavior of edge resonance phenomenon as a function of the curvature of the pipe is studied. The displacement amplitudes measured at the edge demonstrate that the edge resonance is affected by the frequency and thickness to midradius ratio of the pipe, and it is losing its strength in thicker pipes, as the growing difference between the outer and inner radii destroys symmetry. The reflected energy amplitudes show that at the resonance frequencies the incident wave is strongly converted to L(0,1) and L(0,3) modes, depending also on the curvature parameter of the pipe.

Acoustic scattering by an elastic elliptic cylinder in water: numerical results and experiments.

The acoustic scattering from an elastic elliptic cylinder immersed in water and excited by a normally incident plane wave is considered in this paper. The purpose is to determine, theoretically and experimentally, the pressure scattered by this cylinder. A model based on the theory of elasticity is described briefly. It consists in carrying out expansions in Fourier series of the expressions relating to the conditions of continuity (displacements and constraints) at the surface of cylinder. These expressions form a system of equations. The resolution of this system enables us to obtain the scattering coefficient, then the pressure scattered by the cylinder. The numerical results obtained from this model are compared with experimental results obtained by means of an experimental short-pulse method presented in the literature. An good agreement between the results is noted.