Experimental and computational determination of global resonances in ship structures.

368 Pages·2.8 MB·en_US

by  Barber, Matthew Edward.

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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
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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.