82 



HYDKAULICS AND ITS APPLICATIONS 



V.A.P. 



v a p 



plate is forced away from the orifice, so that an intermittent action is set 

 up, the plate vibrating to and from the orifice, but never being driven far 

 away. 



The ball-nozzle (Fig. 39), designed to give a thin evenly distributed 

 discharge from a fire hydrant, acts on exactly the same principle. 



Here a cage is fitted to prevent the loss of the ball on the first impact 

 of the jet. 



ART. 33. Loss DUE TO THE SUDDEN ENLARGEMENT OF CROSS 

 SECTION OF A STREAM. 



Whenever the cross section of a pipe or channel increases abruptly, the 

 water, on passing the enlargement, is thrown into a state of unsteady 



motion, with a consequent loss of available 

 energy. If the mean velocities before and 

 after passing the enlargement are known, 

 the equations of momentum may be applied 

 to determine the magnitude of this loss. 



For, consider the case of flow through 

 a horizontal pipe (Fig. 40), suffering a 

 sudden change of area from a to A. 



Let p and v be the pressure and mean 

 velocity immediately before the change of 

 section. 



Let P and V be the pressure and mean velocity in the large pipe when 

 the motion has steadied after impact. 



Let p' be the mean pressure on the end of the pipe at EE. 

 Neglecting frictional effects, which will be small, the forces tending to 

 produce a change of momentum in the direction of flow between the 

 sections at which p and P are measured are : 



(1) the pressure over the area at p = pa. 1 



(2) EE =p'(A - a). 



(3) at P = PA. 



/. The force producing change of momentum in the direction of 

 motion = {p a + p' (A a) P A} Ibs. (1) 



This must equal the change of momentum per second in the direction 

 of this force 



W A V* Wav* W , , T72 



= - {A V 2 a v 

 9 99 



(2) 



1 The pressure p being measured at the mean depth of the section. 



