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Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at rest (fluid
statics) and the subsequent effects of the fluid upon the boundaries, which may be either
solid surfaces or interfaces with other fluids. Both gases and liquids are classified
as fluids, and the number of fluids engineering applications is enormous: breathing,
blood flow, swimming, pumps, fans, turbines, airplanes, ships, rivers, windmills, pipes,
missiles, icebergs, engines, filters, jets, and sprinklers, to name a few. When you think
about it, almost everything on this planet either is a fluid or moves within or near a
fluid.
The essence of the subject of fluid flow is a judicious compromise between theory
and experiment. Since fluid flow is a branch of mechanics, it satisfies a set of welldocumented
basic laws, and thus a great deal of theoretical treatment is available. However,
the theory is often frustrating, because it applies mainly to idealized situations
which may be invalid in practical problems. The two chief obstacles to a workable theory
are geometry and viscosity. The basic equations of fluid motion (Chap. 4) are too
difficult to enable the analyst to attack arbitrary geometric configurations. Thus most
textbooks concentrate on flat plates, circular pipes, and other easy geometries. It is possible
to apply numerical computer techniques to complex geometries, and specialized
textbooks are now available to explain the new computational fluid dynamics (CFD)
approximations and methods [1, 2, 29].1 This book will present many theoretical results
while keeping their limitations in mind.
The second obstacle to a workable theory is the action of viscosity, which can be
neglected only in certain idealized flows (Chap. 8). First, viscosity increases the difficulty
of the basic equations, although the boundary-layer approximation found by Ludwig
Prandtl in 1904 (Chap. 7) has greatly simplified viscous-flow analyses. Second,
viscosity has a destabilizing effect on all fluids, giving rise, at frustratingly small velocities,
to a disorderly, random phenomenon called turbulence. The theory of turbulent
flow is crude and heavily backed up by experiment (Chap. 6), yet it can be quite
serviceable as an engineering estimate. Textbooks now present digital-computer techniques
for turbulent-flow analysis [32], but they are based strictly upon empirical assumptions
regarding the time mean of the turbulent stress field.
Chapter 1
Introduction
3
1Numbered references appear at the end of each chapter.