Reseach On The Acoustic Characteristic Of The Cylindrical Shell With Periodic Internal Plates Or Lengthwise Ribs

A cylindrical shell with internal plates or lengthwise ribs is the simplified main structure of underwater vehicles,considering the lengthwise distribute periodically along the circumferential direction of the cylindrical shell.The influence of the internal plates or lengthwise ribs to the vibroacoustic behavior of the cylindrical shell is significant.The in-depth study of acoustic characteristics of a cylindrical shell with the internal plates or lengthwise ribs could benefit the target detection and identification technology which is significant in underwater acoustic.The study of this thesis has an important theoretical value and a wide engineering application.The elastic waves of the cylindrical shell can be classified as the subsonic waves and supersonic waves.The wavenumbers of the subsonic waves are larger than that of the exteriorfluid acoustic wave or the phase speed of the subsonic waves are lower than that of the exteriorfluid acoustic wave.The wavenumbers of the supersonic waves are lower than that of the exterior-fluid acoustic wave or the phase speed of the supersonic waves are larger than that of the exterior-fluid acoustic wave.For the empty cylindrical shell,according to the Snell law,the subsonic waves can neither be generated by incidence wave nor radiate to the far-field efficiently due to phase-matching.However,the supersonic waves can both be generated by the incidence wave and radiate to the far-field efficiently due to phase-matching.The study of this thesis is based on the vibration and acoustic characteristic of empty cylindrical shell and the generation,propagation and radiation of elastic wave in the cylindrical shell on the circumference direction.For study the influence of the internal plates or lengthwise ribs to the vibroacoustic behavior of the cylindrical shell,the theoretical prediction and experimental verification are used.The main research works are listed as following:1.Considering the thickness ratio of internal plate and cylindrical shell,the dynamics of the internal plates are neglected,the attachments of the internal plates to the shell are regarded are rigid.The shell motion is described by the Donnell equations.The analytical expressions for the backscattering of a cylindrical shell with multiple internal plates are derived.Based on the frequency-angle spectra,the effect of the attachments of the plates to the cylindrical shell are analysis in deep.The study shows that besides the specular reflection,when the attachments in the illuminated region are insonified by the incident wave,some acoustic energy will reflect immediately,and it will interfere with the specular reflection wave to induce interference fringes in the frequency-angle spectra.Moreover,the remaining acoustic energy will generate flexural waves that cannot radiate due to phase matching.Only through the attachments can the subsonic flexural wave of the attachments,the radiation flexural wave can also interfere with the specular reflection wave and induce a different series of interference fringes in the frequency-angle spectra.If more than two attachments are in the illuminate region simultaneously,the trajectories of the re-radiation flexural waves will be covered up by the reflection waves from the attachments.2.As for the experimental aspect,the three air-filled cylindrical shells studied are used for the experimental.For obtain the transient information and steady state information of the models,the short pulse and long pulse LFM signal are used,respectively,and the transient timeangle spectra and steady state frequency-angle spectra are obtained.Since the energy of short pulse is low,only the trajectories of the reflection waves from the attachments can be observed clearly,and the trajectories of the re-radiation flexural waves from the attachments cannot be observed.When the incident wave is long pulse,the trajectories of the reflection waves and the re-radiation flexural waves can both be observed clearly.Besides that,since the internal plates of the experimental models are elastic,there will be a complex phase shift for the flexural waves at the attachments of the internal plates to the shell.As a result,the trajectories of the reradiation flexural waves can hardly predict accurately in the experiment.3.The mathematical model for vibration and radiation from an immersed cylindrical shell with periodic lengthwise ribs is established.The vibration of the shell is described by the Donnell equation,and the lengthwise rib is modeled as an elastic beam whose motion is decomposed into longitudinal and flexural vibrations.Analytical expressions are obtained for the shell motion via a circumferential mode expansion based on the periodicity in the circumferential direction and the Fourier transform in the longitudinal direction.Furthermore,the far-field radiated pressure is obtained via the stationary-phase approach.The shell vibration at point force excitation and sound radiation are analyzed using discrete circumferential wavenumbers(DCW).Multi-order flexural Bloch waves(FBWs)exist in the circumferential direction when the cylindrical shell is equipped with periodic lengthwise ribs.The supersonic components of the flexural Bloch waves radiate efficiently and lead to acoustic radiation resonances in the far field.Because of the dispersive property of flexural Bloch waves,there is a low-frequency limit for the effective frequency of flexural Bloch wave radiation in the far field.That frequency limit decreases as the rib spacing is increased.4.Based on the mathematical model for vibration and radiation from an immersed cylindrical shell with periodic lengthwise ribs,the mathematical model for the scattering from an immersed cylindrical shell with periodic lengthwise ribs is established.The ratio of circumferential mode order to cylindrical shell radius is regarded as discrete circumferential wavenumber.Based on the discrete circumferential wavenumber spectra,the vibration of a cylindrical shell with periodic lengthwise ribs at plane acoustic wave incidence is studied.The study shows that the subsonic flexural wave in the shell cannot be generated by the incident wave,however the incidence wave can generate the vibration of the shell,and the vibration of the shell will drive the motion of the ribs.As a result,the forces of the ribs to the shell will generate the subsonic flexural waves.The same as the point force excitation,Multi-order flexural Bloch waves exist in the circumferential direction when the shell is incidence by the acoustic wave.The flexural Bloch waves of different orders will resonant in the circumferential direction in the corresponding order partial wave form function,the complex resonant peaks are contribution by the resonant peaks from multi-order flexural Bloch waves.Besides that,degeneracy phenomenon can be observed in the scattering of a periodically ribbed cylindrical shell.The study of this thesis makes a further understanding of the characteristic of the cylindrical shell with internal plates or lengthwise ribs.The effects of the internal plates or lengthwise ribs on the generation,propagation and radiation of the elastic wave are discussed detailly.Besides that,the contributions of the flexural Bloch waves,which is caused by the periodicity of the lengthwise ribs,on acoustic field are also studied detailly.It provides a theoretical basis for actual engineering prediction.