Gérard Maze

Resonances of plates and cylinders: Guided waves

The study of the normal diffusion of an ultrasonic plane wave by cylinders and plates imbedded in the water shows resonances which are the natural modes of vibration. When a natural mode of an elastic target is excited, the energy which is stored during the forced excitation is emitted after the end of the forced excitation. The observation of backscattered spectra obtained by the Resonance Isolation and Identification Method (RIIM) from an aluminum cylinder shows supplementary resonances. The directivity pattern of the transducer is the cause of these supplementary resonances. The behavior of these resonances is analogous to the resonances of the plate. This leads us to study the natural modes of the cylinder. All the resonances which are experimentally detected may be considered as normal modes of the target. The results obtained on plates and cylinders have a common point: the generation of a guided wave by the excitation of a resonance.

Scattering of an obliquely incident acoustic wave by an infinite hollow cylindrical shell

Acoustic scattering from an isotropic elastic hollow cylindrical shell of infinite length excited by an obliquely incident plane acoustic wave is investigated. The form functions of an aluminum cylindrical shell immersed in water have been calculated by the direct summation of the Rayleigh series. Computations are made at angles (with the normal to the cylinder axis) between α=0° and α=35°. The results of the theoretical calculation are in good agreement with the results of experiments. The experimental results have shown in a frequency range of k1a=0 –20 that the resonances are related to three wave families: the circumferential wave (l=2) detected for angles smaller than the ‘‘angle of longitudinal wave in thin rods’’ (αl), the guided wave (p=1) detected for angles smaller than the second critical angle (αT), and the Scholte–Stoneley wave (l=0). The evolution of the resonance frequencies is followed for different angles and one can note experimentally, that at an angle superior to the Rayleigh critical ...

Ultrasonic measurement of milk coagulation time

Using a pulse reflection technique an ultrasonic system has been developed to monitor in situ the coagulation process of rennetted milk. The velocity and attenuation of ultrasonic waves through coagulating milk were continuously monitored. The observed changes in ultrasonic velocity during coagulation were used to predict the coagulation time. The coagulation time is indicative of the transition from the enzymatic phase to the physicochemical phase. The determination of coagulation time has a decisive role in determining the qualities of the end product in cheesemaking.

Acoustic scattering from submerged cylinders. MIIR Im/Re: Experimental and theoretical study

To recognize a target submerged in water, a spectroscopy based on the backscattering spectrum or on the resonance spectrum can be used. These spectra are experimentally obtained from the quasiharmonic or short pulse method of isolation and identification of resonances (MIIR). They show sharp amplitude variations that are related to the scatterer’s resonances. These resonances are related to the surface waves that propagate around the target. The backscattering spectrum and the resonance spectrum are power spectra. They are obtained from the computation of the modulus of the Fourier transform of the signal scattered by the target. In this paper, the computation of the phase tangent of the Fourier transform is presented. The spectra obtained are called the Im/Re spectra. The experimental results are compared with the computation of the ratio of the imaginary part over the real part of the complex pressure observed at a large distance from the target center. It is shown that the Im/Re spectra are highly usef...

Determination of the group and phase velocities from time–frequency representation of Wigner–Ville

Abstract The experimental measurement of the group and phase velocities of some circumferential waves propagating around a thin elastic tube is a still complex operation. In this study, we show that the dispersion velocity can be determined from a time–frequency representation. We use the Wigner–Ville method by virtue of its interesting properties. On some time–frequency images, the symmetric (S0) and antisymmetric (A1) circumferential waves are identified. The group velocity dispersion estimated from these images is compared with that computed by the proper mode theory method. A good agreement is obtained. The phase velocity is also determined from the group velocity.

Influence of the free modes of vibration on the acoustic scattering of a circular cylindrical shell

The scattering of an acoustic wave by an elastic circular cylindrical shell immersed in water depends on the natural modes of the target. Two groups of natural modes may be considered. The first group is related to circumferential waves which propagate around the tube. These resonances are provided by the resonance scattering theory. The second group is related to waves whose direction of propagation is parallel to the axis of the tube. The calculation of the natural modes and the experimental data obtained by the ‘‘method of isolation and identification of the resonances’’ agree very well.

Characterization of the syneresis and the firmness of the milk gel using an ultrasonic technique

A non-invasive ultrasonic method was used to control the change in physical properties of milk gel and the syneresis, which is an essential step in the manufacture of cheese. The velocity and the attenuation were recorded for ten hours. They provide a good indicator of syneresis occurring. The firmness of the milk gel increases with the variation in velocity (ΔV). The effects of the temperature, calcium chloride and rennet concentration on the syneresis were studied.

Experimental identification of finite cylindrical shell vibration modes

Acoustic scattering from a finite air-filled elastic cylindrical shell, immersed in water, is investigated. The shell is made of stainless steel and has a thickness to outer radius ratio of 17%. The considered dimensionless frequency range extends over 7 << k1a << 22 (k1: wave number in water, a: outer radius). Bistatic measurements are carried out to identify vibration modes related to the phase matching of the first guided wave, T0, propagating on the shell. Both transducers, the emitter and the receiver, are positioned at the same angular distance with regard to the normal axis of the shell. The emitter transducer is fixed at a given position. In order to identify circumferential modes of vibration, the receiver transducer is made to rotate in the azimuthal plane, normal to the shell axis. Results obtained are plotted in functions of dimensionless frequency and azimuthal angle. Vibration modes along the shells length are identified by moving the receiver transducer parallel to the shell axis. In this case, results are plotted in functions of dimensionless frequency and axial wave number. The experimental investigation is corroborated by theoretical results obtained from approximate calculations for thick finite cylindrical shells [Scot F. Morse et al., J. Acoust. Soc. Am. 103, 785-794 (1998)]. The evolution of the mode position with respect to the incidence angle is discussed so as to clarify peak patterns in backscattered resonance spectra.

Analysis of the circumferential acoustic waves backscattered by a tube using the time-frequency representation of Wigner-Ville

This paper presents a study of the group velocity dispersion of some circumferential waves propagating around an elastic tube. The dispersive character of the circumferential waves is theoretically known, but the experimental measurement of the group velocity in a dispersive medium is still a complex operation. We have determined the characteristics of the circumferential wave dispersion for aluminium and steel tubes using a time-frequency representation. Among these time-frequency techniques, the Wigner-Ville distribution (WVD) is used here for its interesting properties in terms of acoustic applications. The WVD is applied to the analysis of the dispersion of S0 symmetric and A1 antisymmetric circumferential waves propagating around a tube with a radii ratio equal to 0.95 (internal radius:external radius). This allowed us to determine their group velocities and reduced cutoff frequencies. The results obtained are in good agreement with the calculated values using the proper modes theory.

Behavior of first guided wave on finite cylindrical shells of various lengths: Experimental investigation

Acoustic backscattering from elastic cylindrical shells of finite lengths, immersed in water, is investigated. These objects, characterized by the ratio of length over diameter (L/2a = 9.76, 4.88, 2.44, a: outer radius), are excited by an obliquely incident plane acoustic wave. In the three cases studied here, the radii ratio b/a (b: inner radius) is fixed at 0.97. The investigated dimensionless frequency range extends over 10 k1a < or = 50 (k1 : wave number in water). The first guided wave, T0, is of particular interest here. The influence of the shells length on the backscattered pressure is experimentally observed in the time-angle and frequency-angle representations. In support of this experimental study, a time-domain representation is used by extending a theoretical model that provides a geometrical description of the helical propagation of the surface waves around the shell [Bao, J. Acoust. Soc. Am. 94, 1461-1466 (1993)]. Theoretical results on cylindrical shells considered as infinitely long, with identical characteristics, are compared with both experimental representations.