Vibration Mode And Acoustic Radiation Response Of Damped Structures

In the field of ship and ocean engineering,with the gradual improvement of people's requirements on the safety,advancement and comfort of ship structure,the vibration and noise control of ship has been continuously concerned by researchers,and it also has very important significance in theoretical research and engineering application.Passive control and laying of damping materials should have good damping performance,vibration frequency band width,applicability,and many other advantages can be widely used in large ships,naval ships,torpedoes and other military and civilian industrial vibration noise reduction measures,so the analysis influence the effect of damping material reducing vibration noise reduction effects factor is particularly important.For damped materials,the loss factor of materials is the main factor that dissipates the energy of vibration noise.Secondly,different working environments will also influence the damping effect of damped materials.So this paper mainly studies the structure of damping materials with different loss factor in the structure vibration characteristics and response characteristics of different working environment,with the aid of finite element simulation and acoustic boundary element numerical analysis method combined computing implementation of solving this problem of theory analysis,through the analysis of the finite element model of structure vibration modal and sound power level line diagram analysis of loss factor of the damping layer on the numerical difference of structure vibration characteristics and response characteristics of inhibition,under different working conditions effects on the vibration and sound radiation characteristic,It has a certain engineering practical application value and theoretical research significance.First explained the vibration noise for production people about the dangers of life and response measures,combed the related theory of modal analysis of the structure and the research progress of sound radiation transmission theory,observes the current damping structure simplified model to the research of damping materials,performance of viscoelastic damping materials and composite structure movement differential equation also made the corresponding description.Then,the theory of modal analysis of damped structure is described in detail,which lays a solid theoretical foundation for further simulation.In actual simulation,finite element modeling and simulation calculation was carried out on the air and underwater cylindrical shell respectively vibration modal analysis and comparison of work,and compares the result with the analytic solution is given in the literature,the simulation results with the literature results perfectly,used in this paper,by use of theory and operation method can be used to analyze the damping of the structure of the vibration modal analysis,the late for paper work ready to the numerical simulation.In this paper,the cylindrical shell in the air is taken as the research object,and the modal calculation of five self-built structural models is carried out by finite element modeling.The bottom-up modeling method is adopted to verify the influence of different ratios on the vibration modes of damped structures by laying damping materials with different damping ratios.Secondly,in order to study the influence of the fluid medium and the structure damping configuration on the structure complex mode,the complex mode analysis of the damping plate is carried out.It is found that the damping layer with a higher damping ratio can reduce the vibration amplitude of the structure more rapidly.However,too high a damping factor will change the inherent mode of the structure and the mode order of the structure,making the structure more susceptible to the interference of low-frequency excitation.At the same time,it is also found that because of the different boundary constraints of the engineering structures in reality,the vibration of the cylindrical shell structure is likely to occur only along the axial direction at the end face,while the vibration mode of the circular side of the cylindrical shell structure does not appear.Through the model of structural damping and frequency of relation curve of cylindrical shell structure can be found in a relatively stable modal vibration mode,and when the vibration frequency across the relatively stable frequency,modal vibration mode then the structural damping ratio and natural frequency changes by leaps and bounds,is similar to the curve is in a state of "rupture";The influence of the fluid medium on the vibration pattern is mainly reflected in the decrease of the natural frequency,the decrease of the vibration amplitude and the change of the order of the vibration pattern.From the perspective of the influence on the complex modes of the damped structure,the fluid medium and the unevenly damped layer laid on the structure will produce complex modes of the corresponding structure,and the influence of the two on the real part of the complex modes of the vibration structure is relatively small,but the influence on the phase of the vibration particle is greater.The influence of fluid medium on the complex mode of structure is not obvious,while the influence of damped configuration on the complex mode of damped structure is greater.Under certain complex modes,the MPC and MCC values will suddenly decrease or increase(the change value is over an order of magnitude),especially when the half-wave number in the width(length)direction of the structure is fixed,while the half-wave number in the length(width)direction of the structure will increase or decrease.From the complexity indexes of MCC and MPC,it can be seen that the vibration structure is no longer complex mode vibration when approaching the high order mode,but has more real mode characteristics for the high order mode.The fluid medium can make the uniformly laid damping plate appear partial complex mode.Sometimes the MPC value of the complexity index shows that the structure is complex mode vibration,but the MCC value cannot well show that the structure is complex mode vibration.Therefore,further study is needed in how to select the modal complexity index to measure the modal complexity of the structure.Study of acoustic radiation response features,mainly using the boundary element method to calculate the various structural model of sound field,by extracting the structure of simulation calculation of finite element modeling surface node space information,method of structure vibration velocity,the surface node to the displacement response information,and then using the boundary element method for calculating and analyzing the sound field using the direct boundary element method of corresponding software calculation module,calculation of cylindrical shell structure vibration acoustic characteristics of radiated acoustic power level curve.It is found that the damping ratios of different values can affect the mode order of the cylindrical shell structure,change the mode mode of the structure under low frequency vibration,and increase the modal complexity of acoustic radiation.At the same time,the laying of the damping layer,especially the laying of the damping material layer with high damping factor,can effectively reduce the radiation power of the acoustic radiation of the cylindrical shell.In terms of reducing sound power of acoustic radiation,the noise reduction effect of cylindrical shell with damping material layer laid in air is more obvious than that of cylindrical shell with the same damping material layer laid in water.By grasping the line spectrum characteristics of the whole sound power level curve macroscopically,it can be found that the change of the sound power level curve is relatively slow in the low-frequency vibration stage,but after the vibration frequency reaches a certain mode of mode,the sound power level will increase rapidly.For the relatively high frequency band in the 0-600 hz calculation frequency range calculated in this paper,the amplitude of the sound power level-frequency curve changes much less,but at this time,the formant is also more disorderly and dense.So according to the structural damping ratio and different fluid on vibration modal damping composite structure features and acoustic radiation response characteristic analysis of the conclusions,the researchers in the design of engineering structure vibration noise,not blindly pursue high damping ratio of damping layer,laying or easily affected by the low-frequency excitation and produce resonance.As far as possible,the frequency range of the "stable mode" of the structure should be designed so as to reduce the possibility of resonance caused by excitation.At the same time,the design also takes into account the characteristics of the structure's daily working environment,structural strength,stability;For the structural response part,the measures of vibration reduction and noise reduction are mainly applied to the internal excitation source region,so as to prevent the generation of directional long-distance propagation of sound waves and destroy the need of acoustic stealth.