Most first courses in fluid mechanics concentrate on constant density
1
incompressible
2
flows.
In earlier chapters of this book, we mainly considered incompressible flow behavior. In a
few instances, variable density
1
compressible
2
flow effects were covered briefly. The notion
of an incompressible fluid is convenient because when constant density and constant
1
in-
cluding zero
2
viscosity are assumed, problem solutions are greatly simplified. Also, fluid in-
compressibility allows us to build on the Bernoulli equation as was done, for example, in
Chapter 5. Preceding examples should have convinced us that nearly incompressible flows
are common in everyday experiences.
Any study of fluid mechanics would, however, be incomplete without a brief intro-
duction to compressible flow behavior. Fluid compressibility is a very important consider-
ation in numerous engineering applications of fluid mechanics. For example, the measurement
of high-speed flow velocities requires compressible flow theory. The flows in gas turbine en-
gine components are generally compressible. Many aircraft fly fast enough to involve a com-
pressible flow field.
The variation of fluid density for compressible flows requires attention to density and
other fluid property relationships. The fluid equation of state, often unimportant for incom-
pressible flows, is vital in the analysis of compressible flows. Also, temperature variations
for compressible flows are usually significant and thus the energy equation is important. Cu-
rious phenomena can occur with compressible flows. For example, with compressible flows
we can have fluid acceleration because of friction, fluid deceleration in a converging duct,
fluid temperature decrease with heating, and the formation of abrupt discontinuities in flows
across which fluid properties change appreciably.
For simplicity, in this introductory study of compressibility effects we mainly consider
the steady, one-dimensional, constant
1
including zero
2
viscosity, compressible flow of an ideal
gas. In this chapter, one-dimensional flow refers to flow involving uniform distributions of
fluid properties over any flow cross-section area. Both frictionless
and frictional
compressible flows are considered. If the change in volume associated with a change
of pressure is considered a measure of compressibility, our experience suggests that gases
1
m
0
2
1
m
0
2
679
11
C
ompressible
Flow
Compressible flow
phenomena are
sometimes surpris-
ing; for example,
friction can accel-
erate fluid.