Vibro-acoustic Characteristics Of A Finite Cylindrical Shell Submerged In Bounded Acoustic Media

Cylindrical shell is usually assumed to be immersed in infinite flow field in most available researches on the vibro-acoustic characteristics of submerged cylindrical shell.However,the presence of fluid boundaries,i.e.,free surface,wwater bottom,tank wall,will bound the flow field,and the reflection of acoustic waves at fluid boundaries will make a difference on the vibro-acoutic characteristics of submerged structures.Recently,this problem receives more and more attentions.Here,a new analytical approach is put forwarded to investigate the vibro-acoustic chracteristics of a finite cylindrical shell submerged in fluid field bounded by acoustic boundaries with combination of the wave propagation approach and image method,and the modal added mass theory ia also applied to give an explanation for the acoustic boundary effect.Conclusions in this work will provide theorectical guide for the wide application of cylindrical shell in engineering.Firstly,a sound-structure interaction model is built for a finite cylindrical shell ideally submerged in infinite flow filed,and modal added mass is also put forward in the view of sound-structure interaction.The motions of cylindrical shell and acoustic medium are described with the Flugge thin shell theory and the acoustic wave equation,respectively,and the expressions of shell displacements and sound pressure are expanded in wave propagation form.The relation between structural vibration and acoustic field is reflected with continuity condition at the sound-structure interface,then the sound-structure interaction model for a finite cylindrical shell submerged in infinite flow field is built.By gain nspiration from the virtual added mass in the traditional fluid-structure interaction problem,the modal added mass is proposed following the sound-sturucture interaction procedure.Then,the vibro-acoustic chracteristics of a submerged cylindrical shell with finite depth below the free surface,and both the effect of free surface and hydrostatic pressure.In detail,the free surface effect is taken into consideration by setting an image with the image method,and the influence of hydrostatic pressure is accounted in terms of pre-stress in the motion euqtion of cylindrical shell.The sound-structure interaction model is built for a finite cylindrical shell submerged below the free surface using the wave propagation approach and the effects of free surface and hydrostatic pressure are separately studied and explanaed with the modal added mass theory.Compared with the vibration of cylindrical shell immersed in infinite flow field,the presence of free surface will result in the increase of coupled modal frequencies,while the hydrostatic pressure effect will lead to the resutions of coupled modal frequencies and higher level of sound pressure for submerged structures.When the immersion depth of cylindrical shell increases,the influence of free surface on vibro-acoustic characteristics of submerged shell will be reduced,while the hydrostatic press effect will be more and more significant.When the hydrostatic pressure is big enough,the submerged cylindrical shell will be unstable.Furthermore,the vibro-acoustic problem of a finite cylindrical shell partially submerged in flow field is also studied.The expression for sound pressure is given by making some simplication of the free surface boundary.The wave propagation approach is also extended to build the sound-structure interaction model for a finite cylindrical shell partially submerged in flow field.In numerical anslysis,the effect of immersion angle on vibro-acoustic chracteristics of cylindrical shell submerged in fluid is investigated,and it's also explaned with the modal added mass theory.Later,taking a finite cylindrical shell submerged in shallow water for example,the effect of multi-acoustic boundaries on vibro-acoustic charcteristics of submerged structures is studied.Anlysis on the rigid bottom effect on structural vibration is firstly carried,then the wave propagation is extended for the vibro-acoustic problem of a finite cylindrical shell submerged in shallow water.Through comparative analysis on vibro-acoustic characteristics of finite shell submerged in shallow water and in half-space bounded by free surface or rigid bottom,it's shown that the effects of shallow water boundaries is acturally a combination of the effects of both the upper and lower acoustic boundary.Finally,by making reasonalble experimental plan,an exppernment is conducted to investigate the natural vibration of a finite cylindrical shell submerged in shallow water.Comparative analysis on natural frequeies of cylindrical shell from the experiment and theoretical analysis is also doncted to better verify the accuracy of the present method.Some useful conclusions are made from numerical analysis for the vibro-acoustic chracteristics of a finite cylindrical shell submerged in flow field bounded by acoustic boundaries.When the shell is close to fluid boundary,the acoustic boundary effect on vibro-acoustic characteristics of submerged shell is significant when the shell is close to fluid boundary,but it will be reduced when the shell far away.When the distance between cylindrical shell and fluid boundary is more than 4 times of shell radius,the acoustic bundary effect on coupled modal frequency,input power flow and sound radiation power could be neglected,but it drcreases much slower for the far-field sound radiation characteristics of submerged cylindrical shell.When cylindrical shell is submerged in shallow water,the shallow water boundary effect is a combination of both the upper and lower fluid boundary effect.These conclusions could guide reasonable application of cylindrical structures in engineering.