At low velocities or in tubes of small diameter, flow is streamlined. This means that a given particle of fluid moves straight forward without bumping into other particles and without

crossing their paths. Streamline flow is often referred to as laminar flow, which is defined as a flow situation in which fluid moves in parallel lamina or layers. As an example of stream­ line flow, consider an open stream flowing at a slow, uniform rate with logs floating on its surface. The logs represent particles of fluid. As long as the stream flows at a slow, uniform rate, each log floats downstream in its own path, without crossing or bumping into the others.

If the stream narrows and the volume of flow remains the same, the velocity will increase. If the velocity increases sufficiently, the water becomes turbulent. Swirls, eddies, and cross-motions are set up in the water. As this happens, the logs are thrown against each other and against the banks of the stream, and the paths followed by dif­ferent logs will cross and recross.

Particles of fluid flowing in pipes act in the same manner. The flow is streamline if the fluid flows slowly enough, and remains streamline at greater velocities if the diameter of the pipe is small. If the velocity of flow or size of pipe is increased sufficiently, the flow becomes turbulent.

Although a high velocity of flow will produce turbulence in any pipe, other factors con­ tribute to turbulence. Among these are the roughness of the inside of the pipe, obstructions, and the number and degree of curvature of bends in the pipe. In setting up or maintaining fluid power systems, care should be taken to eliminate or minimize as many causes of turbulence as possible, since energy consumed by turbulence is wasted.

Although designers of fluid power equipment do what they can to minimize turbu­lence, it cannot be avoided. For example, in a 4-inch pipe at 680°F, flow becomes tur­ bulent at velocities over approximately 6 inches per second (ips) or about 3 ips in a 6- inch pipe. These velocities are far below those commonly encountered in fluid power systems, where velocities of 5 feet per second (fps) and above are common. In lami­nar flow, losses due to friction increase directly with velocity. With turbulent flow, these losses increase much more rapidly.


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 make it resist being moved when it is at rest and resist any change in its rate of motion when it is moving.

The basic statement covering inertia is Newton’s first law of motion. 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, the engine to make an automobile begin to roll must perform considerable work-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.


To 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 balanced must act on the object. Some unbalanced force must be applied whenever fluids are set in motion or increased in velocity; conversely, forces are made to do work elsewhere whenever flu­ ids 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 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 (fps) 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 relationship of these five factors to each other:

I. 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 may or may not be present, but which 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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