Viscous flows: Flows in which the frictional effects are significant. Inviscid flow regions: 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. The flow of an originally uniform fluid stream over a flat plate, and the regions of viscous flow (next to the plate on both sides) and inviscid flow (away from the plate).

Compressible versus Incompressible Flow
Incompressible flow: If the
density of flowing fluid remains
nearly constant throughout (e.g.,
liquid flow).
Compressible flow: If the density
of fluid changes during flow (e.g.,
high-speed gas flow)
When analyzing rockets, spacecraft,
and other systems that involve high-
speed gas flows, the flow speed is
often expressed by Mach number
Schlieren image of a small model of
the space shuttle orbiter being tested at
Mach 3 in the supersonic wind tunnel
of the Penn State Gas Dynamics Lab.
Several oblique shocks are seen in the
air surrounding the spacecraft.
Ma = 1 Sonic flow
Ma < 1 Subsonic flow
Ma > 1 Supersonic flow
Ma >> 1 Hypersonic flow
The speed of sound changes with temperature and varies with the fluid.
Gas Constant is R = 0.287 [kJ/kgK] for air
Compressibility and the speed of sound
For an ideal gas – air is almost ideal
the ratio of specific heats
T is absolute temperature in [K]; T = t0C +273
t is in Centigrade degrees while K is Kelvin
Laminar versus Turbulent Flow
Laminar flow: The highly
ordered fluid motion
characterized by smooth
layers of fluid. The flow of
high-viscosity fluids such as
oils at low velocities is
typically laminar.
Turbulent flow: The highly
disordered fluid motion that
typically occurs at high
velocities and is
characterized by velocity
fluctuations. The flow of low-
viscosity fluids such as air at
high velocities is typically
turbulent.
Transitional flow: A flow
that alternates between
being laminar and turbulent.
Laminar, transitional, and turbulent flows.
Natural (or Unforced)
versus Forced Flow
Forced flow: A fluid is forced
to flow over a surface or in a
pipe by external means such
as a pump or a fan.
Natural flow: Fluid motion is
due to natural means such as
the buoyancy effect, which
manifests itself as the rise of
warmer (and thus lighter) fluid
and the fall of cooler (and thus
denser) fluid. In this Schlieren image of a girl in a swimming suit, the
rise of lighter, warmer air adjacent to her body
indicates that humans and warm-blooded animals are
surrounded by thermal plumes of rising warm air.
Steady versus Unsteady Flow
• The term steady implies no change at
a point with time.
• The opposite of steady is unsteady.
• The term uniform implies no change
with location over a specified region.
• The term periodic refers to the kind of
unsteady flow in which the flow
oscillates about a steady mean.
• Many devices such as turbines,
compressors, boilers, condensers,
and heat exchangers operate for long
periods of time under the same
conditions, and they are classified as
steady-flow devices.

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