In this chapter some basic concepts of fluid mechanics are
introduced and discussed. A substance in the liquid or gas
phase is referred to as a fluid. Fluid mechanics is 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.
The flow of an unbounded fluid over a surface is external
flow, and the flow in a pipe or duct is internal flow if the
fluid is completely bounded by solid surfaces. A fluid flow is
classified as being compressible or incompressible, depending
on the density variation of the fluid during flow. The densities
of liquids are essentially constant, and thus the flow of
liquids is typically incompressible. The term steady implies
no change with time. The opposite of steady is unsteady, or
transient. The term uniform implies no change with location
over a specified region. A flow is said to be one-dimensional
when the velocity changes in one dimension only. A fluid in
direct contact with a solid surface sticks to the surface and
there is no slip. This is known as the no-slip condition, which
leads to the formation of boundary layers along solid surfaces.
A system of fixed mass is called a closed system, and a
system that involves mass transfer across its boundaries is
called an open system or control volume. A large number of
engineering problems involve mass flow in and out of a system
and are therefore modeled as control volumes.
In engineering calculations, it is important to pay particular
attention to the units of the quantities to avoid errors caused
by inconsistent units, and to follow a systematic approach. It
is also important to recognize that the information given is
not known to more than a certain number of significant digits,
and the results obtained cannot possibly be accurate to
more significant digits. The information given on dimensions
and units; problem-solving technique; and accuracy, precision,
and significant digits will be used throughout the entire
text.
