Experimental and computational determination of global resonances in ship structures. PDF
Preview Experimental and computational determination of global resonances in ship structures.
' SSaSm not** ••'• 989091 1H 1 HrBHBS vfiflHfl Di JSnGKsfllEH&aEzK ras iflHiraisS hBHHS 1MMMMBU EKSnSI Sa MNNM8 ! « rmcdmcs HH WHS 3P ' SDGBDGCCBBBCPOI "*— BCCBMC BHM H BflQBDBB3QCG>9BKBB WET am KftTOQn OK Jilli EXPERIMENTAL AND COMPUTATIONAL DETERMINATION OF GLOBAL RESONANCES IN SHIP STRUCTURES by MATTHEW EDWARD BARBER B. S. Chemistry, University ofNew Mexico (1978) Submitted to the Department of OCEAN ENGINEERING In partial fulfillment ofthe requirements for the degrees of NAVAL ENGINEER and MASTER OF SCIENCE IN MECHANICAL ENGINEERING at the MASSACHUSETTS INSTITUTE OFTECHNOLOGY May 1991 © Matthew E. Barber The author hereby grants to M.I.T. and to the U.S. Government permission to reproduce and to distribute copies ofthis thesis document in whole or inpart. *las Carmichael, Chairman Department Graduate Committee, Department ofOcean Engineering 54140 3zt7/f 6.1 J EXPERIMENTAL AND COMPUTATIONAL DETERMINATION OF GLOBAL RESONANCES IN SHIP STRUCTURES by MATTHEW EDWARD BARBER Submitted to the Department ofOcean Engineering on 10 May 1991 in partial fulfillment ofthe requirements ofthe degrees ofNaval Engineer and Master ofScience in Mechanical Engineering ABSTRACT How machinery vibrations are transmitted through a foundation, into a hull and ultimately into the water is ofgreat concern to ship designers. Global modes result from strong coupling between the vibration modes ofa machine's foundation and the ship's hull, and can cause peaks in the radiated sound power spectrum. In simple structures the global modes can be identified and altered by making drive point mobility measurements and adding mass to the structure. Radiated soundpower is altered because the added mass shifts the modal frequencies and modifies the coupling. In a more realistic structure with a higher impedance foundation there is more interaction with the hull and a greater density ofmodes making global mode identification harder, so additional methods are needed for identification. A finite element method is used to determine the vibration modes ofa fluid loaded model foundation and hull. The forced response ofthe hull model is calculated and used in a boundary element method to predict the radiated sound power ofthe structure in water. The results are compared to experimental measurements from the actual model in a reverberant sound tank. Numerical methods are shown to be very useful in classifying the modes of a structure and predicting frequency shifts resulting in modal coupling to form global modes. Thesis Supervisor: Richard H. Lyon Title: Professor ofMechanical Engineering ACKNOWLEDGEMENTS Thank you to Professor Richard Lyon for his interest in me and guidance throughout this research. Thank you to Bolt Beraneck and Newman Inc. especially Jeff Doughty for allowing me to use their acoustic facilities. Thank you to Dr. David Goldsmith at DTRC Annapolis for taking interest in my project and allowing me access to the engineers and researchers at DTRC. I owe my greatest thanks to my family; Matthew and Emma, who always brighten even my worst days, and mostly to my wife Bettye without whose support I could never have completed these three years at MTT or reached this point in my Naval career.
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