The acoustic scattering from a fluid-loaded stiffened cylindrical shell is described by using elasticity theory. The cylindrical shell is reinforced by a thin internal plate which is diametrically attached along the tube. In this model, cylindrical shell displacements and constraints expressed from elasticity theory are coupled to those of the plate at the junctions, where plate vibrations are described by using plate theory. The present model is first validated at low frequency range (k1a approximately 5-40) by comparison with a previous model based on the Timoshenko-Mindlin thin shell theory and by experimental results. Theoretical and experimental resonance spectra are then analyzed in a high frequency range (k1a approximately 120-200). Only resonances due to the S0 wave are clearly observed in this frequency range, and their modes of propagation are identified. Furthermore, A0 wave propagation is detected, because of the presence of the reflection of this wave at the shell-plate junctions.
Through an experimental approach, this paper explores the acoustic wave scattering processes involved in the acoustic backscattering from air-filled submerged cylindrical shells having axial discontinuity. The discontinuity is caused by lengthwise soldering along the tubes in the course of manufacture from flat sheets, and is represented as an internal axial mass layer. The tubes are excited by a plane wave at normal incidence. In the time domain, different echoes on short pulse responses (echo wave forms), are identified by the arrival times and related wave type. This enables the localization of additional wave generating sites and scattering centers due to the solder. A number of detected echo series are thus identified. In the frequency domain analysis, the influence of supplementary echoes and that of inter-echo interferences on both backscattering and resonance spectra is presented. The influence of the angular position of the solder, relative to the direction of the incident sound wave, on the amplitudes of spectra, is analyzed. The key phenomena in this study are wave generation at the solder when it is directly insonified (prevalent over classical wave generations of considered waves) and the wave type conversions observed when propagating waves encounter obstacles in the shell, geometric or material discontinuities (in this case the solder). The wave types studied are the A wave (Scholte–Stoneley) and the S0 compressional wave, in the midfrequency region, ka=25–90, where k=ω/C1 is the wave number in water, C1 is the sound velocity in water and a the external radius of the tube. This analysis of the influence of discontinuities in the propagation medium on the acoustic scattering is carried out with a view to the investigation of scattering of assembled objects such as cylindrical shells with hemispherical endcaps.
An analytical solution is derived for the acoustic response of submerged thin-walled ring cylindrical shell containing lengthwise stiffening members: internal stringers and walls. On the basis of the analysis of the acoustic pressure versus time diagrams the stiffener-borne wave-generation mechanisms are traced. Shown is that the shell/stiffener junctions act as additional entry and exit points of circumferential waves circulating in the shell and the fluid. The stiffening members cause transformations of circumferential waves from one propagation type to another.
An experimental study of sound scattering by a thin-walled cylindrical shell stiffened by an internal lengthwise rib is presented. The results show that, in the explored frequency band, the positions of the resonance frequencies of the wave S0 are not affected much by the presence of a stiffener. Measurements made during the transient state after the quasiharmonic excitation of the stiffened shell have shown that, at the S0 wave resonance frequencies, backscattering is at its maximum when the rib is located at the vibration node of the shell. On the other hand, when in the total backscattered pressure, forced vibration is also taken into account, the maxima are detected when the rib is located at the vibration antinodes. In the stiffened shell other additional resonances appear which are not excited in the case of an unstiffened shell. These resonances are attributed to the propagation of the flexural type waves which are not detected in an unstiffened shell. In a stiffened shell these resonances appear as a result of the interaction of the rib and the shell at their structural joint. These additional resonances are better observed when the rib is located at the ‘‘illuminated’’ part of the shell which indicates that most of the energy is scattered by the rib and shell joint. In the case of a short-pulse excitation, specular reflections from a joint can be observed. The experimental results correlate very well with the theoretical models.
A theoretical and experimental study of the acoustic response of a submerged stiffened cylindrical shell is presented. The internal rib is modeled as a clamped-free plate mounted inside the shell perpendicular to the shell surface. The stiffened shell is excited by a normally incident acoustic pressure wave. Wave propagation around the circumference of the shell and associated sound radiation are discussed. From the directivity of the monostatic scattering, the resonances in the scattered sound pressure field can be separated into three different types. A mechanical admittance is used to help identify the different types of resonances excited in the fluid-loaded stiffened shell. Each type of resonance is shown to be associated with a particular type of interaction between the shell and the rib in terms of the components of the coupling forces: i.e., the normal force, the transverse force, and the coupling moment. For kR ranging from 16 to 35, the normal coupling force is shown to control the symmetric flexural vibration field in the shell, while the coupling moment controls the antisymmetric vibration field. The rib interacts with the membrane vibrations in the shell via the transverse coupling forces.
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