10.1 Introduction

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Numerous examples can be drawn from engineering applications for vibrating (dynamic) systems.

A steam generator of a nuclear power plant that undergoes flow-induced vibration; a high-rise building

subjected to seismic motions at its foundation; an incinerator tower subjected to aerodynamic

disturbances; an airplane excited by atmospheric turbulence; a gate valve under manual operation; and a

heating, ventilating, and air conditioning (HVAC) control panel stressed due to vibrations in its support

structure are such examples.

10-1

© 2005 by Taylor & Francis Group, LLC

Consider an aircraft in flight, as schematically shown in Figure 10.1. There are many excitations on this

dynamic system. For example, jet engine forces and control surface movements are intentional

excitations, whereas aerodynamic disturbances are unintentional (and unwanted) excitations.

The primary response of the aircraft to these excitations will be the motions in various degrees of

freedom (DoF), including rigid-body and flexible (vibratory) mode motions.

Even though the inputs and outputs (excitations and responses) are functions of time, they can also be

represented as functions of frequency, through Fourier transformation. The resulting Fourier spectrum of a

signal can be interpreted as the set of frequency components which the original signal contains. This

frequency-domain representation of a signal can highlight many salient characteristics of the signal and

also those of the corresponding system. For this reason, frequency-domain methods, particularly Fourier

analysis, are used in a wide variety of applications such as data acquisition and interpretation,

experimental modeling and modal analysis, diagnostic techniques, signal/image processing and pattern

recognition, acoustics and speech research, signal detection, telecommunications, and dynamic testing

for design development, quality control, and qualification of products. Many such applications involve

the study of mechanical vibrations.

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