Acoustic Design Method And Noise Reduction Performance Analysis Of Space Truss Raft System

Taking the interactions and vibration transmission characteristics between floating raftsystem and shell structures as research objective, the primary purpose of this thesis is topresent a qualitative and quantitative account of the complex floating raft system model,whilst simultaneously designing a new truss like floating raft structure to support vibratingmachinery. A method that can be employed to calculate vibration transmissibility ofmulti-input multi-output complex floating raft system is presented and a design model ofspace truss raft system and its installation method are proposed. An approximative calculationmethod for acoustic radiation of floating raft system and hull structure is established. Usingthe radiated noise as the quantitative index of floating raft system is of evident engineeringdemands and great importance of applications. The detailed content lists as follows:Base on vectorial four pole parameter model, a method of analyzing the dynamicalbehaviors of the floating raft structure is established, which takes into account the interactionof series and parallel coupled substructures, such as equipments, upper mounts, raft structure,lower mounts and base foundation. This prediction method can be used to calculate thevibration transmission in complicated structures and investigate the vibration isolationefficiency of the floating raft isolator system. The experiment results show that the calculatedvalue of vibration response of base and raft matches the measured value well in the frequencyband of16～1000Hz. The difference between calculated and measured of vibrationacceleration levels with raft output installation points and base input installation pointsm isless than5dB. With freedom velocity modified by equipments fixing side's input impedancein low frequency range, the predicted results' precision has been increased. The calculationerror obtained by vectorial four pole parameter model has been reduced to3dB. Thecalculated results exhibit that the transmission vibration of raft structure has been magnifiedin low frequency range. The power level difference of vibration isolation performance is10dB less than acceleration level difference. The dynamical behavior of both two-dimensional truss girder and quadrate girder isinvestigated, and the truss raft vibration characteristics of scattering of wave types and a stopband analysis are revealed. The calculated example shows that compared with quadrate girder,the low frequency rigidity of truss structure is comparatively higher and the displacement ofvibration is relatively smaller. With the frequency increasing, compressional wave, torsionalwave and flexural wave energy multiply scatter until a balance of wave types is reached.Wave scattering occurs in the unobstructed struts and the joints. The losses of vibrationenergy due to scattering at joints are significant, that can be used to maximize the decrease ofvibration transmission energy.One new cradle type truss raft structure and the foundation setup system are developedusing the truss structure's vibration attenuating mechanism. The shape of raft structure ischanged from plane to space, and the two-dimensional raft structure is extended to thethree-dimensional raft. The electromagnetic actuator and pump are selected to be the sourceforce, and the vibration of floating raft system is tested. The characteristic of the truss raft'svibration transmission is obtained, by comparing with the plate raft, the acceleration vibrationlevel difference of the truss raft system is3～9dB higher. The anti-vibration performance oftruss raft structure is improved in the200～400Hz frequency range.One high impedance crescent foundation model is developed for the truss raft's setupneed. The foundation is distributed along the circumferential direction,and the vibration ofradial direction is not directly forced. Comparing the plate foundation, the crescentfoundation has the advantage of increasing the performance of raft's isolation effect anddecreasing the vibration of shell. The numerical calculation shows the surface inputimpedance of foundation increases by12.6dB, and the acceleration vibration level differenceof the raft system is9dB higher than the plate foundation. When the impedance of foundationis10times bigger than that of lower mounts, the force input the base is not changing withfoundation impedance.The vibration and acoustic radiation of cylindrical shell in the fluid induced by themechanical vibration is developed. The basic design index of floating raft structure isextended from the vibration isolation effect to the hull's radiation noise. The calculation results shows that the radiating noise of the crescent foundation coupled with the truss raftsystem is6dB less than the plate foundation condition and plate raft system, the vibration ofcylindrical shell in the fluid is reduced by7.6dB. The truss raft structure has the advantage ofmultiple distribution space setup method comparing with the plate raft structure. Using the32points installation model instead of original16points method, the vibration level of shell isdecreased by4.7dB, and the radiation noise is decreased by5.8dB.