Study On Radiation Efficiency Of Rib-stiffened Cylindrical Shell Under Multi-forces

Real time monitoring and predicting the sound radiation from underwater objects are very helpful for mastering their sound radiation properties,judging the safety distance,and promoting the underwater survivor ability.Generally,the typical structure of underwater objects is cylindrical shell.Therefore,the sound radiation efficiency of cylindrical shell excited by multi-forces is researched in this thesis.There are many methods for predicting the sound radiation from submerged rib-stiffened cylindrical shell,such as analytical method,numerical method,experimental method,etc.The analytical method is more adaptable for simple structures than for the complex ones.The numerical method has good adaptability for any shape of structures,and can be used to calculate and analyze the vibration and sound radiation from submerged complex structures,and its predicting precision is also very high.The models in this thesis are rib-stiffened single and double shell structures.For such structures,FEM or FEM+BEM are often used to predict the structural vibration and sound radiation.For large complex structures,the most difficulty of sound field prediction by FEM is the fluid volume dimension surrounding the structures is too big in order to satisfy the far field condition,and therefore,node number will be very large.The correspondent convergence speed of numerical calculation and calculating efficiency will be very low.The FEM+BEM can decrease the node number greatly,however,it coefficient matrix is the function of frequency,which is not suitable for frequency response calculation,and void at eigen frequency.The FEM+AML,for the same calculation precision,can increase the calculation efficiency greatly without the eigen frequency problem.This method is adopted here to research the sound radiation efficiency for complex models theoretically and experimentally.The key problem here is the affection of exciting force position and exciting type to the sound radiation efficiency.The sound radiation efficiency variation due to the exciting force type,such as point force,line force,area force and the different composition of multi-force,has been researched.Based on the comparison between the numerical calculation and experiment,it is shown that sound radiation prediction by using line force or area force model agrees better with the experimental results than that by using point force model.The main reason is that there is no ideal point force in real experiment model,instead there often exists a platform on which the exciter is mounted,which equals to a line or area force excited on the experiment structure model.More modes can be excited up by point force than line or area force,which mainly excites up the lower order modes.That is why numerical result by using point force model is more different from the experimental result than that by using line or area force model.Therefore,force load distribution should be considered carefully in engineering application in the future.Besides,the space position of different composition of exciting force should also be paid more attention to,which is also one of the key points inducing the variation of structure sound radiation efficiency.