Florence Lecroq

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 ...

Acoustic scattering from finite cylindrical elastic objects

The acoustic scattering from infinite elastic cylinders or spheres is well known. It is possible to characterize these targets by their resonance spectra. The resonances are established by the generation of surface waves that propagate around the circumference of the targets. The resonances originate from the phase matching of repeatedly circumnavigating surface waves. Experimentally, it is possible to characterize a target with a complicated shape, but it is not easy to explain the spectra theoretically because the geometry is not separable and the usual analytical methods to calculate the far‐field pressure cannot be used. In this paper, resonance spectra and angular diagrams obtained from a target consisting of a finite cylinder with hemispherical endcaps are obtained experimentally. To explain the resonance spectra, an integral phase matching condition is used. Upon incidence normal to the cylinder axis, resonances due to the phase matching of surface waves traveling along a circumference or along a m...

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.

Resonance identifications of a solid axisymmetric finite length target

Acoustic resonance scattering from a water‐immersed, solid tungsten carbide target in the form of a finite‐length circular cylinder terminated by two hemispherical endcaps has been studied experimentally, and analyzed theoretically. The resonance frequencies have been determined from the condition of phase matching of the generated surface waves encircling the object; for the surface waves propagating over its cylindrical portion in the axial direction, the dispersion has been properly taken into account, as well as that of surface waves on the spherical surfaces. Surface displacements corresponding to the standing waves at each resonance were determined, and were used to calculate the angular patterns of the scattered acoustic field, in excellent agreement with the observed angular distributions. The pattern of lobes is found to provide information on the mode N of the standing surface waves, although in a less straightforward fashion than this had been the case in previous studies for the simple example...

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.

Acoustic scattering from immersed axisymmetric objects. Cylindrical shell limited by hemispherical endcaps

The study of the acoustic scattering from axisymmetric objects is based on the works that have been previously carried out in this field for objects with ‘‘simple’’ shapes (plates, cylinders, cylindrical shells). The objects under investigation consist of a cylindrical shell limited by two hemispherical shells. The experimental results presented in the form of resonance spectra are obtained by the MIIR (method of isolation and identification of resonances). A complete study for various excitation angle allows one to locate particular directions where the resonant phenomenon is important. For axial excitation, the results are interpreted by means of the condition of resonance formation (deduced from the phase matching) in the meridian plane of the objects. This condition takes into account the phase velocities of the waves propagating on the hemispherical parts and on the cylindrical part. For normal excitation experimental results are compared to those obtained from infinite cylindrical shell. Other parti...

Scholte interface wave on a cylindrical shell

The fluid‐borne interface wave on an evacuated cylindrical shell immersed in water is known as Scholte–Stoneley wave, or also as A wave [Talmant et al., J. Acoust. Soc. Am. 86, 278 (1989)]. The resonances of this wave, caused by phase matching upon its multiple circumnavigations of the scattering object, can be observed in the backscattering spectra of incident sound waves within a frequency window that depends on the shell thickness, for thicknesses less than 40% of the cylinder radius. The interaction between the flexural wave A0 in the shell, and the Scholte wave is studied in detail as a function of shell thickness. Dispersion curves for the A and the A0 waves are presented, indicating a region of repulsion between these two curves corresponding to a change of the physical nature of the two waves.

ACOUSTIC SCATTERING FROM CYLINDRICAL SHELLS BOUNDED BY HEMISPHERICAL ENDCAPS

The acoustic scattering from infinite cylinders or spheres has shown the great influence of surface wave propagation. Two types of surface waves are distinguished: the Rayleigh or Whispering Gallery waves and the Scholte–Stoneley waves. In this study, experimental results obtained on the finite cylindrical shells bounded by two hemispherical endcaps are presented. These objects are made of stainless steel and filled with air. Their radius is 27 mm and their radius ratio b/a is 0.97 (b is the inner radius and a the outer radius). The direction of insonification is parallel to the main object axis. The reduced frequency range k1a of the study varies from 10 to 40. These experimental results are explained by writing the stationary condition on the large circumference of objects, in the meridian plane. This condition depends on the phase velocity in the length of the cylindrical shell and in the spherical shell. These two velocities are computed separately in relation to the frequency and used to apply the st...

Radiation of a guided wave at the end of the 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 waves generated at an incident angle α on a determined area of a cylindrical shell immersed in water and filled with air, propagate in a direction parallel to the cylinder axis and re‐emit their energy during the propagation at an angle α. The various velocities of propagation of helical waves allows one to record separately the resonance spectrum of each type of waves. Then, the radiation, in the far field, can be observed for each wave, at the end of a shell of semi‐infinite length, with a quasiharmonic method. In this case, no phenomenon of stationary waves can occur in the cylinder length. The axial displacement is analyzed and the use of the Rayleigh integral allows one to determine a model of the pressure radiated by the circular section of the tube. The results of the theoretical calculation are in agreement with the results of experiments.