Page 20 - Fluid-Structure Interactions Slender Structure and Axial Flow (Volume 1)

INTRODUCTION 3

The second case also occurred in a nuclear power station, this time a gas-cooled system
(PaYdoussis 1980; Case 35). It involved excessive vibration of the piping - so excessive
that the sound associated with this vibration could be heard 3km away! The excitation
source was not local; it was a vortex-induced vibration within the steam generator, quite
some distance away. A similar but less spectacular such case involved the perplexing
vibration of control piping in the basement of the Macdonald Engineering Building at
McGill University, which occurred intermittently. The source was eventually, and quite
by chance, discovered to be a small experiment involving a plunger pump (to study
parametric oscillations of piping, Chapter 4) three floors up!
Another case involved a boiler (Pdidoussis 1980; Case 23), and the report from the
field stated that ‘There is severe vibration on this unit. The forced draft duct, gas duct
and superheater-economizer sections all vibrate. The frequency I would guess to be
60-100 cps. It feels about like one of those ‘ease tired feet’ vibration machines’. A
very colourful description, but lacking in the kind of detail and quantitative information
one would wish for. The difficulty of instrumenting the troublesome operating system a
posteriori should also be remarked upon.
To be able to deal with practical problems involving flow-induced vibration or insta-
bility, one needs first of all a certain breadth of perspective to be able to recognize in
what class of phenomena it belongs, or at least in what class it definitely does not belong.
Here experience is a great asset; reference to books with a broader scope would also
be recommended [e.g. Naudascher & Rockwell (1994), Blevins (1990)l. Once the field
has been narrowed, however, to be able to solve and to redesign properly the system, a
thorough familiarity with the topic is indispensable. If the problem is one of axial flow,
then here is where this book becomes useful.
A final point, before embarking on more specific items, should also be made: despite
what was said at the beginning of the discussion on practical concerns - that applica-
tions and problems are often synonymous - flow-induced vibrations are not necessarily
bad. First of all, they are omnipresent; a fact of life, one might say. They occur when-
ever a structure is in contact with flowing fluid, no matter how small the flow velocity.
Admittedly, in many cases the amplitudes of vibration are very small and hence the
vibration may be quite inconsequential. Secondly, even if the vibration is substantial, it
may have desirable features, e.g. in promoting mixing, dispersing of plant seeds, making
music by reed-type wind instruments; as well as for wave-generated energy conversion,
or for the enhancement of marine propulsion (Chapter 4). Recently, attempts have been
made ‘to harness’ vibration in heat-exchange equipment so as to augment heat transfer,
so far without spectacular success, however. Even chaotic oscillation, usually a term with
negative connotations, can be useful, e.g. in enhancing mixing (Aref 1995).

1.2 CLASSIFICATION OF FLOW-INDUCED VIBRATIONS

A number of ways of classifying flow-induced vibrations have been proposed. A very
systematic and logical classification is due to Naudascher & Rockwell (1980, 1994), in
terms of the sources of excitation of flow-induced vibration, namely, (i) extraneously
induced excitation, (ii) instability-induced excitation, and (iii) movement-induced excita-
tion. Naudascher & Rockwell consider flow-induced excitation of both body and fluid
oscillators, which leads to a 3 x 2 tabular matrix within which any given situation can
be accommodated; in this book, however, we are mainly concerned with flow-induced