Fluid mechanics is the study of fluids either in motion (fluid dynamics) or at rest (fluidstatics) and the subsequent effects of the fluid upon the boundaries, which may be eithersolid surfaces or interfaces with other fluids. Both gases and liquids are classifiedas 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 thinkabout it, almost everything on this planet either is a fluid or moves within or near afluid.The essence of the subject of fluid flow is a judicious compromise between theoryand experiment. Since fluid flow is a branch of mechanics, it satisfies a set of welldocumentedbasic laws, and thus a great deal of theoretical treatment is available. However,the theory is often frustrating, because it applies mainly to idealized situationswhich may be invalid in practical problems. The two chief obstacles to a workable theoryare geometry and viscosity. The basic equations of fluid motion (Chap. 4) are toodifficult to enable the analyst to attack arbitrary geometric configurations. Thus mosttextbooks concentrate on flat plates, circular pipes, and other easy geometries. It is possibleto apply numerical computer techniques to complex geometries, and specializedtextbooks are now available to explain the new computational fluid dynamics (CFD)approximations and methods [1, 2, 29].1 This book will present many theoretical resultswhile keeping their limitations in mind.The second obstacle to a workable theory is the action of viscosity, which can beneglected only in certain idealized flows (Chap. 8). First, viscosity increases the difficultyof the basic equations, although the boundary-layer approximation found by LudwigPrandtl 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 turbulentflow is crude and heavily backed up by experiment (Chap. 6), yet it can be quiteserviceable as an engineering estimate. Textbooks now present digital-computer techniquesfor turbulent-flow analysis [32], but they are based strictly upon empirical assumptionsregarding the time mean of the turbulent stress field.Chapter 1Introduction31Numbered references appear at the end of each chapter.