Reseach On The Low Frequency Vibroacoustic Behavior Of Near-surface Target

A cylindrical shell or open cylindrical shell is prevalent in engineering,such as mooring or surface sailing submarine,which can be approximated as a partially immersed cylindrical shell,and floating ships,which are often approximated as a partially immersed open cylindrical shell for the simplest case.For near-surface targets,the free surface not only affects the external fluid loading but also changes the radiated sound field,which complicates the vibroacoustic characteristics.The in-depth study of the vibroacoustic behavior of these shells can provide theoretical support for the acoustic property evaluation and measurement of the near-surface target.Therefore,modeling for the near-surface targets and studying of their vibroacoustic characteristics have important theoretical value and extensive engineering application.The foundation of this study is the establishing of the analytical model for the partially immersed,closed and open cylindrical shells.The analytical model is comprehensively verified by the experimental and numerical results.Based on the generation,propagation and radiation of the elastic wave in shell,the acoustic radiation mechanisms of the partially immersed shell(including closed and open)undergoing point-force excitation are explained.The main works of this thesis are listed as following:1.An analytical model of the vibroacoustic partial coupling for a partially immersed cylindrical shell is developed based on the thin shell theory,wavenumber transformation and separation of variables approaches.The effects of free surface on the radiated sound and radial velocity are analyzed using frequency–depth spectra(FDSs),and a series of regular oblique bright lines and weak interference fringes can be observed in the FDSs.For a partially immersed cylindrical shell,the subsonic flexural wave (60 can radiate energy into the fluid from air-fluid demarcation points on the shell surface,which is a new acoustic radiation mechanism,and produce a series of resonant bright lines in the pressure spectra when the shell resonates in the circumferential direction.In addition,interaction between the radiated waves,which propagate on parts of shell above and below the free surface(on the ‘‘dry part” and ‘‘wet part”,respectively),produces up-and down bending interference fringes in the FDSs of sound pressure.2.Considering the boundary conditions at two ends of a finite cylindrical shell,we establish an analytical vibro-acoustic coupling equation for a semi-submerged finite cylindrical shell using the wavenumber transformation and separation of variables for the sound pressure and the mode expansion for the shell's motion.The modal frequencies,radial velocities,sound power,and sound pressure(including the circumferential and axial directivity patterns)are calculated and measured.The comparisons of these results between a semi-submerged and submerged finite cylindrical shell also provide new insight into the vibro-acoustic problem.Based on the new acoustic radiation mechanism,below the 0-th resonant frequency of compressional wave,the circumferential and axial directivity patterns of a semisubmerged finite cylindrical shell can be approximated as a superposition of two inphase dipoles formed by the air-liquid demarcation points and as a superposition of two in-phase dipole line sources,respectively.Simple formulas are derived to predict the circumferential and axial directivity patterns.This is a new and simplified approach for predicting the directivity patterns of a semi-submerged finite cylindrical shell.3.To verify the analytical model,a series of water tank experiments for the partially immersed cylindrical shell are carried out.The radial velocity and radiated sound pressure of the shell are measured during the shell is lowered continuously,and this process results in the FDSs for the corresponding measurement.In addition,the sound power,circumferential direction and axial sound pressure directivity of a semisubmerged finite cylindrical shell are measured using a circular(arc)hydrophone array.The experimental results verify the analytical model.The oblique resonant bright spots and interference fringes generated by the flexural wave in the pressure spectra can be clearly observed in the experimental results,and accurately predicted by the simple formulas.4.An analytical vibro-acoustic coupling equation for a partially immersed open cylindrical shell are established,and its accuracy is verified by the experimental results.Only the circumferential and axial symmetric modes are observed for a finite open cylindrical shell undergoing a symmetric point-force excitation.The FDSs of radiated sound pressure and radial velocities are calculated and measured,and a series of regular oblique bright lines and weak interference fringes can be observed.Based on the new acoustic radiation mechanism,the resonant frequency of an open cylindrical with different immersion depth can be predicted using a simple formula.5.Considering small regular water waves,an analytical model of the circumferential vibroacoustic behavior of a partially immersed cylindrical shell under point-force excitation is established.Experimental verification is carried out in a towing tank.The effect of wave period and amplitude on the vibroacoustic behavior of this system is studied theoretically and experimentally.Both the theoretical and experimental results show modulation of the vibration and sound radiation of the shell by the water waves.These modulation effects are determined by the wave parameters,namely,(i)the fluctuation period of the resonant peaks in the radial velocity and sound pressure is determined by the wave period;(ii)the bandwidth of the modulation is determined by the wave amplitude.Based on the new mechanism,the wave modulation is explained,and a simple formula is derived to predict the modulation of the resonant frequency of the radial velocity and the sound pressure using the phase velocities of the subsonic flexural waves on the dry and wet parts of the shell.The work of this thesis presents a further understanding of the vibroacoustic behavior of a partially immersed shell,especially the find of a new acoustic radiation mechanism for a partially immersed structure.The subsonic flexural wave (60 can radiated out though the airfluid demarcation points,which are provided by the free surface on the shell,to produce the regular oblique bright lines and weak interference fringes on the FDSs of sound pressure.Based on this new radiation mechanism,the prediction formulas of sound pressure directivity for a partially immersed finite cylindrical shell are derived;furthermore,the resonant bright lines and interference fringes are also explained and predicted in the FDSs of sound pressure of a partially immersed open cylindrical shells.However,the resonant condition of a circumferential closed cylindrical shell is different from an open one.Finally,by considering the effects of regular water waves on the resonance radiation of the shell,a simple formula is proposed to describe the modulation of resonance frequencies which should be useful in engineering applications.