Small density changes of liquids corresponding to large pressure changes
can still have important consequences. The irritating “water hammer” in a
water pipe, for example, is caused by the vibrations of the pipe generated by
the reflection of pressure waves following the sudden closing of the valves.
Laminar versus Turbulent Flow
Some flows are smooth and orderly while others are rather chaotic. The
highly ordered fluid motion characterized by smooth layers of fluid is called
laminar. The word laminar comes from the movement of adjacent fluid
particles together in “laminates.” The flow of high-viscosity fluids such as
oils at low velocities is typically laminar. The highly disordered fluid
motion that typically occurs at high velocities and is characterized by velocity
fluctuations is called turbulent (Fig. 1–17). The flow of low-viscosity
fluids such as air at high velocities is typically turbulent. The flow regime
greatly influences the required power for pumping. A flow that alternates
between being laminar and turbulent is called transitional. The experiments
conducted by Osborn Reynolds in the 1880s resulted in the establishment of
the dimensionless Reynolds number, Re, as the key parameter for the
determination of the flow regime in pipes (Chap. 8).
Natural (or Unforced) versus Forced Flow
A fluid flow is said to be natural or forced, depending on how the fluid
motion is initiated. In 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. In natural flows, any
fluid motion is due to natural means such as the buoyancy effect, which
manifests itself as the rise of the warmer (and thus lighter) fluid and the fall
of cooler (and thus denser) fluid (Fig. 1–18). In solar hot-water systems, for
example, the thermosiphoning effect is commonly used to replace pumps by
placing the water tank sufficiently above the solar collectors.
Steady versus Unsteady Flow
The terms steady and uniform are used frequently in engineering, and thus it
is important to have a clear understanding of their meanings. 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. These meanings are consistent with their everyday use (steady
girlfriend, uniform distribution, etc.).
The terms unsteady and transient are often used interchangeably, but
these terms are not synonyms. In fluid mechanics, unsteady is the most general
term that applies to any flow that is not steady, but transient is typically
used for developing flows. When a rocket engine is fired up, for example,
there are transient effects (the pressure builds up inside the rocket
engine, the flow accelerates, etc.) until the engine settles down and operates
steadily. 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. (Note that the flow field near the
rotating blades of a turbomachine is of course unsteady, but we consider the
overall flow field rather than the details at some localities when we classify