G. Maze

Methode impulsionnelle numerisee (MIN) pour l'isolement et l'identification des resonances de tubes immerges

Abstract The method of isolation and identification of resonances (MIIR) allows a direct verification of “resonance scattering theory” (RST). Indeed it provides the resonance spectra of aluminum elastic cylindrical shells, insonified by plane acoustic waves and it also allows the experimental determination of the mode number of each resonance of the target. At this day, when the shells are insonified by a short pulse it is only possible to obtain the resonance spectrum with a fast Fourier transform of the backscattered response. This paper presents an impulse method which allows the isolation of resonances and besides the identification of each eigenmode of the target.

Acoustic scattering from an immersed plane multilayer : application to the inverse problem

The authors deal with acoustic scattering from a plane multilayered structure. This structure is composed of a first plastic elastic layer, a thin water layer, and a second aluminum elastic layer. A pulse excitation is used to obtain the scattered spectra and the resonance spectra at normal and oblique incidence. These spectra give information about the structure. The spectra of resonances due to the guided waves provide information about the resonant character of the two elastic solids. The farther the guided waves propagate in a layer, the more resonant the layer is. These guided waves are Lamb waves of the solid layers. The backscattered signal is formed by a series of echoes which arise from the different layers of the structure. With simple experiments, involving in particular a temporal filtering of the reflected signal, it is shown that it is possible to obtain three of the four parameters which characterize the solid layers: the phase velocities of the longitudinal waves, thicknesses, and densitie...

Acoustic scattering from a cylindrical shell bounded by hemispherical endcaps. Resonance interpretation with surface waves propagating in cylindrical and spherical shells

The studies of the acoustic scattering from infinite cylinders or spheres have shown the strong influence of the propagation of surface waves. Two types of surface waves are distinguished: the Rayleigh or Whispering Gallery waves and the Scholte–Stoneley wave. In this paper, theoretical and experimental results obtained on objects constituted by a cylindrical shell bounded by two hemispherical shells are presented, when these objects are insonified in the direction parallel to their main axis. To explain the experimental results, the stationary condition is written in the meridian plane of these objects. To apply this condition, the phase velocities of the waves propagating in the cylindrical and in the spherical parts must be known. First, the phase velocity of waves propagating along the length of cylindrical shells is theoretically and experimentally studied. Then an experimental study of the phase velocities of the waves propagating on spherical shells is carried out. Finally, the results of these two...

Acoustic scattering from an air‐filled cylindrical shell with welded flat plate endcaps: Experimental and theoretical study

Many authors have already studied the acoustic scattering from infinite cylinders or cylindrical shells both theoretically and experimentally. When the shells are insonified in a direction perpendicular to its axis, some circumferential waves propagate. When the insonification is not perpendicular to the axis of the shell, some helical waves propagate. In this paper, the influence of the length limitation of the shell on the different waves which propagate around the shell is studied. In the frequency range used in this work with a normal insonification, there was no difference between an infinite and a finite length shell. With an oblique incidence (10°), there were several resonances with the same vibration mode n but with different frequencies that were close to each other. A calculation, described in this paper, explains these phenomena.

Experimental analysis of phase spectrum of cylindrical or plane targets : a new global method of isolation of resonances

Abstract Up to now, resonance isolation by means of a pulse method has been performed by the calculation of the FFT of the free elastic response of the insonified target immersed in water. We propose a new method which allows the resonance detection from the FFT of the whole backscattered signal, involving the imaginary and real part spectra and in some cases, the phase spectrum. Experiments are carried out on several targets at normal incidence. This method seems to be the appropriate one to detect the resonances in the case of a “very absorptive material/elastic material” double layered plane structure.

The resonances of finite-length elastic cylinders and elastic spheroids excited by sound scattering

The predictions of resonance frequencies for elongated elastic cylinders and spheroids, based on various methods of approach, were discussed in a number of papers in recent years. In the present study, the regions of applicability of the phase matching method for surface waves and of the longitudinal bar wave approximation are examined by comparing the predicted results with the results of T-matrix calculations or with the results of scattering experiments.

Diffraction of Lamb waves at the end section of a plate

An experimental study is presented of the diffraction of Lamb waves at the end section of a plate imbedded in water. The amplitude of the diffracted wave is plotted versus angle of diffraction for different Lamb modes. The S modes have maxima of diffraction in their direction of propagation, as opposed to A modes that are associated with minima of diffraction in their direction of propagation. For all modes, the diffraction pattern is symmetric about the direction of propagation of the incident (Lamb) wave, and the peaks associated with maxima of the diffracted amplitude widen as the angle of observation increases. Those results are in good agreement with those obtained by a simple calculation of interferences between two sources, the relative phases of which are determined by the type (A or S) of diffracting Lamb wave.

Acoustic scattering from a finite plate: Generation of guided Lamb waves S0, A0 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.

Experimental acoustic resonance study on finite length cylindrical targets

Abstract The acoustic resonances of cylindrical finite length targets are investigated experimentally. The targets have different length-to-diameter ratios η:η=1.33 for a solid aluminium cylinder, η=1.66 for an aluminium cylindrical shell (with a ratio between the inner radius b and the outer radius a of 0.89), η=1.85 for a stainless steel cylindrical shell ( b a =0.94 ). The insonification is performed at normal or slightly oblique incidence. Resonances related to Whispering Gallery, guided waves and Scholte-Stoneley waves are identified. For a given vibration mode n it is possible to identify two resonances at frequencies close to each other. A comparison is made with theoretical values at normal incidence. The experimental resonances related to Whispering Gallery and guided waves are shifted towards the high frequencies; resonances related to Scholte-Stoneley waves are shifted towards the low frequencies.

Acoustic scattering from a circular cylindrical shell excited by a short pulse in oblique incidence: Helical waves

The theoretical and experimental spectra, obtained from an infinite pipe insonified with a plane wave in oblique incidence, show resonances which are related to three types of helical waves: the circumferential waves (Ai or Si waves), the transversal guided waves (Ti waves), and the Scholte wave (A wave). The resonances of the Scholte wave are detected in a frequency window and their frequency slowly increases when the incidence angle increases, whereas the ones of the other waves increase towards infinity when the incidence angle tends towards the transversal critical angle. At oblique incidence, the experimental results obtained with the MIIR show resonances which are related to the helical waves S0, T0, T1, and A. This method uses a long pulse with many sinusoid periods, a steady state takes place in a part of the infinite shell. In this presentation, the excitation is a short pulse and the scattered echoes are detected when the helical wave emission is in front of the receiver. Between each echo, the ...