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<br />\ <br />---I, <br />~ <br />==-, <br />\ <br />~ <br />" <br /> <br />only on boundary roughness. However, when the state of flow is lami,nar <br />or transitional, viscosity of the fluid becomes an important factor <br />in determining the value of the friction loss coefficient. The func- <br />tional relationship between Reynolds number and friction loss coef- <br /> <br />ficients is available for flow in pipes. and this relationship can <br /> <br /> <br />be converted for use in open channels by relating the characteristic <br /> <br /> <br />length terms in Reynolds number as follows: <br /> <br /> 2 <br />R = AlP = 4/1TD = DI4 (3-2) <br />where: <br />A = area of flow cross section <br />0 = diameter of pipe <br />P = wetted perimeter <br />R = hydraulic radius (area/wetted perimeter) <br /> <br />Effect of Gravity <br /> <br />In pipe flow, the force of gravity is not a factor. but in open <br />channel flow it plays a major role. It is the force of gravity that <br />converts pressure energy into kinetic energy. This force is counter- <br />acted by the inertia of the water. The following figure illustrates <br />the relationship between pressure head and velocity head as the depth <br />of flow is changed by the force of gravity. <br /> <br />3.03 <br />