Research On Vibro-acoustic Coupling Mechanism Of Submerged Viscoelastic Composite Cylindrical Shell

At the background of vibration and noise reduction and radiated noise prediction of submerged structure,the researches about vibration and sound radiation modeling and coupling mechanism of submerged viscoelastic composite cylindrical shell are carried out.Based on describing the coating movement by using viscoelastic theory,a calculated model of vibration and sound radiation from submerged cylindrical shell with homogeneous viscoelastic coating is established,and the vibration reducing and decoupling characteristics of the coating are analyzed.The ill-conditioned matrix problem for coating modeling is analyzed.It is found that the matrix element distribution unbalance induced by specific function and its derivative is the main reason of matrix ill-condition that can induce singularity of calculation.For that,the matrix balance processing and removing-zero processing is proposed,by which the matrix ill-condition is improved significantly.Lastly,coating modeling of Cartesian coordinate transform is proposed,by which the matrix ill-condition is improved remarkably and the consuming time is reduced significantly.The actual coating structure is complex,and it's parameters under coupling state are difficult to obtain,that bring difficulty to modeling and noise prediction.For that,an equivalent parameter method is proposed,in which the real complex coating is equivalent to a homogeneous viscoelastic coating.In order to get the equivalent parameters of the coating,a matching method of theoretical and measured mean quadratic velocity of coating is proposed.By simulation,the coating equivalent parameters retrieval research is carried out for multilayer composite coating and embedded cavity coating respectively.For the case of equivalent parameters varying sharply with frequency,a method of frequency band error average processing and smooth processing is proposed.The simulation results indicate that,in the low frequency band,the coating modeling based on the equivalent parameter method is feasible in the calculation of sound radiation from a complex cylindrical shell with coating.Considering ring ribs,circular plates and coating on inner and outer shell,a calculated model of vibration and sound radiation from submerged circumferential stiffened cylindrical shells is established,and then is verified by numerical method.The sound insulation and decoupling effect of coating to double cylindrical shells is researched,the results show that the noise reduction effect of coating covered on outer shell is obviously better than the effect of coating covered on inner shell.The contribution and effect of outer shell to sound radiation from double cylindrical shell is researched.Then a sound radiation calculation method based on vibration of inner shell is proposed,and then a modified method is presented when outer shell is covered with coating.Examples show that the average error is less than 3 dB.The vibration and sound radiation coupling characteristic between inner and outer shell is researched.It shows that the sound radiation of double cylindrical shells can be reduced well when the two coupling channels,linked water layer and circular plates,are both decoupled.As an improvement of traditional longitudinal stiffener modeling,considering radial bending,circumferential bending,longitudinal vibration and torsional vibration of stiffeners,a vibration and sound radiation model of submerged longitudinal stiffened cylindrical shell is established.The theoretical results have a good agreement with numerical results.The effect of longitudinal stiffeners to vibration and sound radiation of cylindrical shell is researched.It shows that the circumferential modal coupling induced by longitudinal stiffeners leads to many low-frequency radiated peaks,that is detrimental to the low-frequency noise reduction of cylindrical shell.Lastly the vibration and sound radiation characteristic of circumferential stiffened cylindrical shell and longitudinal stiffened case are compared.The results show that the longitudinal stiffening under circumferential stiffening can help to reduce the vibration and sound radiation of cylindrical shell.The vibration and sound radiation experiments of single and double cylindrical shells are carried out on lake.A concept of vibration-sound transfer efficiency from mean quadratic velocity of inner shell(or single shell)to sound radiated power is proposed.Combining the mean quadratic velocity measurement of inner shell(or single shell),the sound radiated power is predicted.The predicted result agrees well with the measured one.That solves the problem of cylindrical shell sound radiation prediction when exciting position is known but the amplitude is unknown.Based on matching theoretical value of shell's mean quadratic velocity and measuring value,the equivalent parameters of coating are retrieved,and then sound radiated power of single cylindrical shell with coating is predicted.The predicted result's variation trend with frequency has a good agreement with the measured one.The vibration reduction and decoupling characteristic of coating is researched by experiment.The results show that the decoupling characteristic is mainly embodied in medium-high frequency.The effect of noise reduction is better when the coating is covered on the outer shell.Those are consistent with theory research.