minimize as many causes of turbulence as
possible, since the energy consumed by turbulence
is wasted. Limitations related to the degree
and number of bends of pipe are discussed in
chapter 5.
While designers of fluid power equipment do
what they can to minimize turbulence, it cannot
be avoided. For example, in a 4-inch pipe at 68°F,
flow becomes turbulent at velocities over approxi-
mately 6 inches per second or about 3 inches per
second in a 6-inch pipe. These velocities are far
below those commonly encountered in fluid power
systems, where velocities of 5 feet per second and
above are common. In streamlined flow, losses
due to friction increase directly with velocity. With
turbulent flow these losses increase much more
rapidly.
FACTORS INVOLVED IN FLOW
An understanding of the behavior of fluids in
motion, or solids for that matter, requires an
understanding of the term inertia. Inertia is the
term used by scientists to describe the property
possessed by all forms of matter that makes the
matter resist being moved if it is at rest, and
likewise, resist any change in its rate of motion
if it is moving.
The basic statement covering inertia is
Newton’s first law of motion—inertia. Sir Isaac
Newton was a British philosopher and mathe-
matician. His first law states: A body at rest tends
to remain at rest, and a body in motion tends to
remain in motion at the same speed and direction,
unless acted on by some unbalanced force.
This simply says what you have learned by
experience—that you must push an object to start
it moving and push it in the opposite direction
to stop it again.
A familiar illustration is the effort a pitcher
must exert to make a fast pitch and the opposition
the catcher must put forth to stop the ball.
Similarly, considerable work must be performed
by the engine to make an automobile begin
to roll; although, after it has attained a certain
velocity, it will roll along the road at uniform
speed if just enough effort is expended to
overcome friction, while brakes are necessary to
stop its motion. Inertia also explains the kick or
recoil of guns and the tremendous striking force
of projectiles.
Inertia
To
and Force
overcome the tendency of an object to
resist any change in its state of rest or motion,
some force that is not otherwise canceled or
unbalanced must act on the object. Some
unbalanced force must be applied whenever fluids
are set in motion or increased in velocity; while
conversely, forces are made to do work elsewhere
whenever fluids in motion are retarded or
stopped.
There is a direct relationship between the
magnitude of the force exerted and the inertia
against which it acts. This force is dependent
on two factors: (1) the mass of the object
(which is proportional to its weight), and (2)
the rate at which the velocity of the object
is changed.
The rule is that the force in
pounds required to overcome inertia is equal
to the weight of the object multiplied by the
change in velocity, measured in feet per second,
and divided by 32 times the time in seconds
required to accomplish the change. Thus, the rate
of change in velocity of an object is proportional
to the force applied. The number 32 appears
because it is the conversion factor between weight
and mass.
There are five physical factors that can act on
a fluid to affect its behavior. All of the physical
actions of fluids in all systems are determined by
the relationships of these five factors to each
other. Summarizing, these five factors are as
follows:
1. Gravity, which acts at all times on all
bodies, regardless of other forces
2. Atmospheric pressure, which acts on
any part of a system exposed to the open
air
3. Specific applied forces, which mayor may
not be present, but which, in any event, are
entirely independent of the presence or absence
of motion
4. Inertia, which comes into play whenever
there is a change from rest to motion or the
opposite, or whenever there is a change in
direction or in rate of motion
5. Friction, which is always present whenever
there is motion
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