Research On Wet Modal Characteristics Of A Solid-Liquid-Flexible Coupling Structure

The solid-liquid-flexible(SLF)coupling structure which can be applied to underwater vehicles is composed of metal shell,flexible bladder and filled liquid studied.The structure can provide the functions of buffer,damping and passive buoyancy compensation for underwater vehicles.In order to guide practical application,the modal characteristics of SLF coupling structure are analyzed from three aspects:analytical calculation,experimental modal analysis and finite element simulation.Through the study of its modal characteristics,the change rule of its modal characteristics is mastered,which provides reference for its practical application.By modifying the additional mass factor of the cylinder,the expression of the additional mass of the internal and external parts of the structure is obtained.Using the calculation method of the first modal frequency of the free vibration beam model,the first dry modal frequency analytical expression of the SLF coupling structure is obtained.The first order wet modal frequency analytical expression is obtained by introducing the mass ratio coefficient.The analytical expression is in good agreement with the experimental modal analysis and the finite element simulation results.The physical model is designed and manufactured,and the wet modal frequency and mode shape change rules of the SLF coupling structure are measured as the model parameters change.The results show that the higher the filling volume and pressure are,the lower the modal frequency is.Neglecting the change of the bladder's thickness,the filling volume has a negative correlation with the modal frequency,while the filling pressure has a positive one.Therefore,the influence of the filling volume and the filling pressure on the modal frequency should never be ignored.By comparing the frequency response functions of different excitation positions,it is found that the SLF coupling structure has a good vibration absorption effect at low frequency.Based on the ANSYS Workbench software platform,the wet modal analysis of the SLF coupling structure is carried out by using the acoustic-structure coupling algorithm.The results show that the wet modal frequency is much lower than the dry modal frequency.The external water pressure and internal filling pressure of the structure have a negative correlation with the modal frequency,while the thickness of the bladder has a positive one.The shape with small aspect ratio has higher modal frequency,and the structure with larger size is more likely to be excited by the low frequency vibration source.The effects of three kinds of rib added bladders on the structural frequency are studied.It is found that all of them have a good effect of vibration suppression and can be used for structural optimization.