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 categories.
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. 1–15. The fluid sticks to the plate on both sides because of
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 categories.
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. 1–15. The fluid sticks to the plate on both sides because of
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