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Reference
Gas vs. Liquid
Unlike liquids, gases cannot be poured from one open
container into another, but they deform under shear stress just
the same. Because shear stresses result from relative motion,
stresses are equivalent whether the fluid flows past a
stationary object or the object moves through the fluid.
Although a fluid can deform easily under an applied force, the
fluid’s viscosity creates resistance to this force. The viscosity
of gases, which is much less than that of liquids, increases
slightly as the temperature increases, whereas that of liquids
decreases when the temperature increases. Fluid mechanics is
mostly concerned with Newtonian fluids, or those in which
stress, viscosity, and rate of strain are linearly related.
Pressure and Density
Pressure and density are considered mechanical properties of
the fluid, although they are also thermodynamic properties
related to the temperature and entropy of the fluid. For a small
change in pressure, the density of a gas is essentially
unaffected. For this reason, gas and all liquids can be
considered incompressible. If density changes are significant
in flow problems, however, then the flow must be considered
compressible. Compressibility affects results when the speed
of the flow approaches the speed of sound.
Fluid Flow-Real Fluids
Equations concerning the flow of real fluids are complex. In
turbulent flow, the equations are not completely known.
Laminar flow is described by the Navier-Stokes equations, for
which answers can be derived only in simple cases. Only by
using large computers can answers be derived in more
complex flow situations. Experimentation is still important
for fully correlating theory with actual flow.
