| With the development of modern society,people’s requirements for vibration and noise control are getting higher and higher based on considerations of safety,environmental protection,comfort and other aspects.In the shipping industry in recent years,higher standards of noise-related regulations and regulations have been issued both at home and abroad.Viscoelastic damping material is a kind of damping material with good performance.It can effectively reduce the generation of vibration and noise when laid on the surface of the structure.It is widely used in practical engineering fields such as ships,aerospace,and construction machinery structures.In this thesis,a frequency-dependent damping model is used to simulate viscoelastic materials,and the solution is solved by the finite element iteration method.Based on the frequency-dependent damping model,the complex modal vibration characteristics of the damping plate structure and the sensitivity analysis of the cylindrical shell structure are respectively analyzed.The optimization design of stiffened plate structure has been researched and analyzed.This thesis introduces the basic characteristics of viscoelastic damping materials and explains the characteristics of their material parameters changing with the change of the external environment.Based on this characteristic,a damping model that can consider the influence of frequency,that is,the frequency-dependent damping model,is used,which can be more accurate The simulation of viscoelastic damping materials.The finite element model of the constrained damping plate is established and its vibration characteristics are analyzed.The obtained natural frequency and loss factor are very close to the results of literature,and the relative error is small,indicating that the model built in this paper is correct.Based on the frequency-varying damping model in this thesis,models of free damping plate and constrained damping plate with different damping installation positions are established to study and analyze the influence of damping installation positions and boundary conditions on vibration characteristics.The analysis of the calculation example shows that the vibration mode of the damping plate with viscoelastic material is the compound vibration type,and the position of the damping material and the boundary conditions have an influence on the vibration characteristics:In terms of natural frequencies,the natural frequencies of the damping plates laid in a centralized manner are smaller than those laid in a distributed manner;for the damping plates with the same damping location and different boundary conditions,the natural frequencies of the concentrated free damping plates are greater than the natural frequencies of the constrained damping plates,and the natural frequencies of the decentralized free damping plate are all smaller than the natural frequencies of the constrained damping plate.The effect on the modal damping ratio is that the modal damping ratio of the constrained damping structure is significantly greater than that of the free damping structure,ranging from tens of times to a hundred times(this article only considers the influence of the damping layer,and the base layer and the constrained layer are both unresisted).In general,the damping ratio of each mode of the symmetrically laid constrained damping structure is the largest.This thesis derives the sensitivity calculation formulas for the natural frequency of vibration and the modal damping ratio.Taking the cylindrical shell with viscoelastic damping material as the research object,adopts the frequency-dependent damping model in Chapter 2,and takes the length of the damping layer,the radius of the cylindrical inner shell,and the damping The layer thickness is a design variable.The finite difference method is used to calculate the sensitivity of the natural frequency and modal damping ratio of the cylindrical shell.The results of the calculation example show that different design variables of the same objective function have different effects on different orders: for the natural frequency,the sensitivities of the three design variables are almost all negative,indicating that increasing the design variables will cause the natural frequency to decrease.Among them,the sensitivity of the 8th order natural frequency with respect to length,inner diameter,and thickness is relatively large,and the sensitivity of the 6th order natural frequency with respect to the length is relatively large,the absolute value of the sensitivity of the fifth-order natural frequency to inner diameter and thickness is larger,and the absolute value of the sensitivity of the third-order natural frequency to thickness is larger;for the modal damping ratio,the modal damping ratio is hardly affected by length,but greatly affected by the thickness.The third and fourth-order modal damping ratios are greatly affected by the inner diameter.Introduced the genetic algorithm in the optimization design,related indicators and theories of acoustics,and several common modal complexity indicators.Taking the common stiffened slab structure in engineering as the research object,laying viscoelastic damping materials and adopting the frequency-dependent damping model,taking the width of the stiffened slab’s panel,the height of the web and the size of the additional mass at a specific location as the design variables,the use of limited the difference method calculated the sensitivity of the natural frequency and the modal damping ratio,combined with the sound field cloud diagram and the calculated sound power to verify the correctness of the positive and negative directions of the sensitivity;the finite element method was used to analyze the dynamic characteristics of the structure,and the genetic algorithm was used to determine the size of the structure.The optimization analysis is carried out to minimize the mean square value of the normal vibration velocity on the surface of the structure,thereby achieving the purpose of reducing the sound power of the structure.And use the boundary element method to calculate the sound field of the structure before and after the optimization to verify the optimization effect.The modal changes of the structure before and after optimization are analyzed.The results show that the natural frequency and damping ratio of the structure before and after optimization have little changes,and the fourth-order and the fifth-order modal complexity MPD of the optimized mass structure have greater changes.By optimizing the design variables of the width of the stiffened panel,the height of the web and the size of the additional mass block at a specific position,the mean square value of the normal phase vibration velocity of the structure is reduced by 5.0 d B,6.65 d B and 7.68 d B,respectively.The structural sound power has been reduced by 5.7d B,7.4d B and 8.3d B,which proves that the structural optimization has achieved a certain effect. |
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