CLASSIFICATION OF FLUID FLOWS
Earlier we defined fluid mechanics as the science that deals with the behavior of fluids at rest or in motion, and the interaction of fluids with solids or
other fluids at the boundaries. There is a wide variety of fluid flow problems encountered in practice, and it is usually convenient to classify them
on the basis of some common characteristics to make it feasible to study
them in groups. There are many ways to classify fluid flow problems, and
here we present some general categorie
Viscous versus Inviscid Regions of Flow
When two fluid layers move relative to each other, a friction force develops between them and the slower layer tries to slow down the faster layer.
This internal resistance to flow is quantified by the fluid property viscosity,
which is a measure of internal stickiness of the fluid. Viscosity is caused by
cohesive forces between the molecules in liquids and by molecular collisions in gases. There is no fluid with zero viscosity, and thus all fluid flows
involve viscous effects to some degree. Flows in which the frictional effects
are significant are called viscous flows. However, in many flows of practical interest, there are regions (typically regions not close to solid surfaces)
where viscous forces are negligibly small compared to inertial or pressure
forces. Neglecting the viscous terms in such inviscid flow regions greatly
simplifies the analysis without much loss in accuracy.
The development of viscous and inviscid regions of flow as a result of
inserting a flat plate parallel into a fluid stream of uniform velocity is shown
in Fig. 1–17. The fluid sticks to the plate on both sides because of the no-slip
condition, and the thin boundary layer in which the viscous effects are significant near the plate surface is the viscous flow region. The region of flow on
both sides away from the plate and largely unaffected by the presence of the
plate is the inviscid flow region.
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