Page 19 - Fluid-Structure Interactions Slender Structure and Axial Flow (Volume 1)

2 SLENDER STRUCTURES AND AXIAL FLOW

(ii) to serve as a vehicle in the search for new phenomena or new dynamical features,
and in the development of new mathematical techniques. More of this will be discussed
in Chapters 3-5. However, the foregoing serves to make the point that the curiosity-
driven work on the dynamics of pipes conveying fluid has yielded rich rewards, among
them (i) the development of theory for certain classes of dynamical systems, and of new
analytical methods for such systems, (ii) the understanding of the dynamics of more
complex systems (covered in Chapters 6-11 of this book), and (iii) the direct use of
this work in some a priori unforeseen practical applications, some 10 or 20 years after
the original work was done (Paidoussis 1993). These points also justify why so much
attention, and space, is devoted in the book to this topic, indeed Chapters 3-6.
Other topics covered in the book (e.g. shells containing flow, cylindrical structures
in axial or annular flow) have more direct application to engineering and physiological
systems; one will therefore find sections in Chapters 7- 11 entirely devoted to applications.
In fact, since ‘applications’ and ‘problems’ are often synonymous, it may be of interest to
note that, in a survey of flow-induced vibration problems in heat exchangers and nuclear
reactors (Paidoussis 1980), out of the 52 cases tracked down and analysed, 36% were
associated with axial flow situations. Some of them, notably when related to annular
configurations, were very serious indeed - in one case the repairs taking three years, at
a total cost, including ‘replacement power’ costs, in the hundreds of millions of dollars,
as described in Chapter 11.
The stress in this book is on the fundamentals as opposed to techniques and on physical
understanding whenever possible. Thus, the treatment of each sub-topic proceeds from
the very simple, ‘stripped down’ version of the system, to the more complex or realistic
systems. The analysis of the latter invariably benefits from a sound understanding of the
behaviour of the simpler system. There are probably two broad classes of readers of a
book such as this: those who are interested in the subject matter per se, and those who
skim through it in the hope of finding here the solution to some specific engineering
problem. For the benefit of the latter, but also to enliven the book for the former group,
a few ‘practical experiences’ have been added.
It must be stressed, however, for those with limited practical experience of flow-induced
vibrations, that these problems can be very difficult. Some of the reasons for this are:
(i) the system as a whole may be very complex, involving a multitude of components,
any one of which could be the real culprit; (ii) the source of the problem may be far
away from the point of its manifestation; (iii) the information available from the field,
where the problem has arisen, may not contain what the engineers would really hope to
know in order to determine its cause. These three aspects of practical difficulties will be
illustrated briefly by three examples.
The first case involved a certain type of boiling-water nuclear reactor (BWR) in which
the so-called ‘poison curtains’, a type of neutron-absorbing device, vibrated excessively,
impacting on the fuel channels and causing damage (Paidoussis 1980; Case 40). It was
decided to remove them. However, this did not solve the problem, because it was then
found that the in-core instrument tubes, used to monitor reactivity and located behind
thc curtains, vibrated sufficiently to impact on the fuel channels - ‘a problem that was
“hidden behind the curtains” for the first two years’ ! Although this may sound amusing
at this point, neither the power-station operator nor the team of engineers engaged in the
solution of the problem can have found it so at the time.