minimize as many causes of turbulence aspossible, since the energy consumed by turbulenceis wasted. Limitations related to the degreeand number of bends of pipe are discussed inchapter 5.While designers of fluid power equipment dowhat they can to minimize turbulence, it cannotbe 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 persecond in a 6-inch pipe. These velocities are farbelow those commonly encountered in fluid powersystems, where velocities of 5 feet per second andabove are common. In streamlined flow, lossesdue to friction increase directly with velocity. Withturbulent flow these losses increase much morerapidly.FACTORS INVOLVED IN FLOWAn understanding of the behavior of fluids inmotion, or solids for that matter, requires anunderstanding of the term inertia. Inertia is theterm used by scientists to describe the propertypossessed by all forms of matter that makes thematter resist being moved if it is at rest, andlikewise, resist any change in its rate of motionif it is moving.The basic statement covering inertia isNewton’s first law of motion—inertia. Sir IsaacNewton was a British philosopher and mathe-matician. His first law states: A body at rest tendsto remain at rest, and a body in motion tends toremain in motion at the same speed and direction,unless acted on by some unbalanced force.This simply says what you have learned byexperience—that you must push an object to startit moving and push it in the opposite directionto stop it again.A familiar illustration is the effort a pitchermust exert to make a fast pitch and the oppositionthe catcher must put forth to stop the ball.Similarly, considerable work must be performedby the engine to make an automobile beginto roll; although, after it has attained a certainvelocity, it will roll along the road at uniformspeed if just enough effort is expended toovercome friction, while brakes are necessary tostop its motion. Inertia also explains the kick orrecoil of guns and the tremendous striking forceof projectiles.InertiaToand Forceovercome the tendency of an object toresist any change in its state of rest or motion,some force that is not otherwise canceled orunbalanced must act on the object. Someunbalanced force must be applied whenever fluidsare set in motion or increased in velocity; whileconversely, forces are made to do work elsewherewhenever fluids in motion are retarded orstopped.There is a direct relationship between themagnitude of the force exerted and the inertiaagainst which it acts. This force is dependenton two factors: (1) the mass of the object(which is proportional to its weight), and (2)the rate at which the velocity of the objectis changed.The rule is that the force inpounds required to overcome inertia is equalto the weight of the object multiplied by thechange in velocity, measured in feet per second,and divided by 32 times the time in secondsrequired to accomplish the change. Thus, the rateof change in velocity of an object is proportionalto the force applied. The number 32 appearsbecause it is the conversion factor between weightand mass.There are five physical factors that can act ona fluid to affect its behavior. All of the physicalactions of fluids in all systems are determined bythe relationships of these five factors to eachother. Summarizing, these five factors are asfollows:1. Gravity, which acts at all times on allbodies, regardless of other forces2. Atmospheric pressure, which acts onany part of a system exposed to the openair3. Specific applied forces, which mayor maynot be present, but which, in any event, areentirely independent of the presence or absenceof motion4. Inertia, which comes into play wheneverthere is a change from rest to motion or theopposite, or whenever there is a change indirection or in rate of motion5. Friction, which is always present wheneverthere is motion2-11
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