352 HYDRAULICS 



in which w is the weight of a column of water 1 sq. in. in cross- 



P 

 section and 12 in. high, usually taken as .434 Ib. The term - 



represents the head necessary to produce the pressure p and is 

 called the pressure head. 



The pressure head in a water pipe can be measured by the 

 height to which the water will rise in a tube inserted in the 

 pipe. Such a tube or gauge is called a piezometer or piezometric 

 tube. 



Velocity Head. When water in a pipe or a channel is flow- 

 ing to a level h ft. lower than the starting point, if factional and 

 other resistances are not considered, the velocity attained 

 during the flow is v = ^2gh, which is the same as the velocity 

 attained by a body falling through a height h. Solving for A; 



The expression is called the velocity head. 

 2g 



Loss of Head. Owing to frictional and other resistances, a 

 loss of energy occurs in flowing water, thus reducing the theo- 

 retical velocity of the flow, and, consequently, the discharge. 

 This loss is usually expressed as a fractional factor of the theo- 



V 2 



retical velocity head , the factor being called the coefficient 



of hydraulic resistance. 



Flow of Water Through a Standard Orifice. When the 

 water in flowing through a hole touches the opening on the 

 inside edges only, the hole is a standard orifice. The theoretical 

 discharge is 



Q = Av 



in which A is the area of the orifice, and v = "\2g/t, h being the 

 head or the distance of the center of the orifice from the level 

 surface of the water. The actual discharge is reduced on 

 account of frictional resistances and contraction of the jet. 

 The friction reduces the velocity to 98% of the theoretical 

 velocity and the contraction reduces the cross-section of the 

 issuing jet to 62% of the area of the orifice. The actual dis- 

 charge is, therefore, Q a = .98X.62 Q = .61 A 



