1. Basic Concepts of Fluid Flow
1.1 Introduction
Fluids are substances whose molecular structure offers no resistance to external shear forces: even the smallest force causes deformation of a fluid particle.
Although a significant distinction exists between liquids and gases, both types
of fluids obey the same laws of motion. In most cases of interest, a fluid can
be regarded as continuum, i.e. a continuous substance.
Fluid flow is caused by the action of externally applied forces. Common
driving forces include pressure differences, gravity, shear, rotation, and surface tension. They can be classified as surface forces (e.g. the shear force due
to wind blowing above the ocean or pressure and shear forces created by a
movement of a rigid wall relative to the fluid) and body forces (e.g. gravity
and forces induced by rotation).
While all fluids behave similarly under action of forces, their macroscopic
properties differ considerably. These properties must be known if one is to
study fluid motion; the most important properties of simple fluids are the
density and viscosity. Others, such as Prandtl number, specific h,eat, and surface tension affect fluid flows only under certain conditions, e.g. when there
are large temperature differences. Fluid properties are functions of other thermodynamic variables (e.g. temperature and pressure); although it is possible
to estimate some of them from statistical mechanics or kinetic theory, they
are usually obtained by laboratory measurement.
Fluid mechanics is a very broad field. A small library of books would be
required to cover all of the topics that could be included in it. In this book
we shall be interested mainly in flows of interest to mechanical engineers but
even that is a very broad area so we shall try to classify the types of problems
that may be encountered. A more mathematical, but less complete, version
of this scheme will be found in Sect. 1.8.
The speed of a flow affects its properties in a number of ways. At low
enough speeds, the inertia of the fluid may be ignored and we have creeping flow. This regime is of importance in flows containing small particles
(suspensions), in flows through porous media or in narrow passages (coating
techniques, micro-devices). As the speed is increased, inertia becomes important but each fluid particle follows a smooth trajectory; the flow is then
said to be laminar. Further increases in speed may lead to instability that
1.1 Introduction
Fluids are substances whose molecular structure offers no resistance to external shear forces: even the smallest force causes deformation of a fluid particle.
Although a significant distinction exists between liquids and gases, both types
of fluids obey the same laws of motion. In most cases of interest, a fluid can
be regarded as continuum, i.e. a continuous substance.
Fluid flow is caused by the action of externally applied forces. Common
driving forces include pressure differences, gravity, shear, rotation, and surface tension. They can be classified as surface forces (e.g. the shear force due
to wind blowing above the ocean or pressure and shear forces created by a
movement of a rigid wall relative to the fluid) and body forces (e.g. gravity
and forces induced by rotation).
While all fluids behave similarly under action of forces, their macroscopic
properties differ considerably. These properties must be known if one is to
study fluid motion; the most important properties of simple fluids are the
density and viscosity. Others, such as Prandtl number, specific h,eat, and surface tension affect fluid flows only under certain conditions, e.g. when there
are large temperature differences. Fluid properties are functions of other thermodynamic variables (e.g. temperature and pressure); although it is possible
to estimate some of them from statistical mechanics or kinetic theory, they
are usually obtained by laboratory measurement.
Fluid mechanics is a very broad field. A small library of books would be
required to cover all of the topics that could be included in it. In this book
we shall be interested mainly in flows of interest to mechanical engineers but
even that is a very broad area so we shall try to classify the types of problems
that may be encountered. A more mathematical, but less complete, version
of this scheme will be found in Sect. 1.8.
The speed of a flow affects its properties in a number of ways. At low
enough speeds, the inertia of the fluid may be ignored and we have creeping flow. This regime is of importance in flows containing small particles
(suspensions), in flows through porous media or in narrow passages (coating
techniques, micro-devices). As the speed is increased, inertia becomes important but each fluid particle follows a smooth trajectory; the flow is then
said to be laminar. Further increases in speed may lead to instability that