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 well documented 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 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.

The second obstacle to a workable theory is the action of viscosity, which can be neglected only in certain idealized flowS). First, viscosity increases the difficulty of the basic equations, although the boundary-layer approximation found by Ludwig Prandtl in 1904 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. Thus there is theory available for fluid-flow problems, but in all cases it should be
backed up by experiment. Often the experimental data provide the main source of information about specific flows, such as the drag and lift of immersed bodies.

Fortunately, fluid mechanics is a highly visual subject, with good instrumentation and the use of dimensional analysis and modeling concepts is widespread.

Thus experimentation provides a natural and easy complement to the theory. You should keep in mind that theory and experiment should go hand in hand in all studies of fluid mechanics.

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