78 



HYDKAULICS AND ITS APPLICATIONS 



provided by an occasional eddying of such colour bands as get below the 

 horizontal through the lowest point of the throat. 



The velocity at which this occurs depends on the angle of divergence 

 of the sides of the passage. As the velocity is increased this unsteady 

 motion extends to the whole of the water in the diverging portion of the 

 passage, the motion in the converging part being uniformly steady. 

 The pressure curve now appears as shown in (b), and is discontinuous at 

 the throat. 



On plotting the energy curve, 



~ -f z = y, the 



curve shown in 



Fig. 36 is obtained, and shows very clearly the magnitude of the 

 various losses. 



Drawing smooth curves between the plotted points from A to P and 

 from P to B, it will be noted that the observed point E at P lies on 

 neither of these curves. In the figure, HI represents loss of head, due to 

 viscous resistance between A and P : H 2 , the loss due to eddy formation 

 at the throat ; and H 3 , the loss due to viscous resistance and to eddy 

 formation between P and B. Evidently from the position of E and the 

 form of the curve P B, almost the whole energy of eddy formation is 

 absorbed at and within a very small distance of the throat. With speeds 

 below the critical velocity H 2 = and HI = H 3 (approx.). 



With speeds exceeding the critical, H 3 becomes less or greater than 

 HI, depending on the velocity, as, with fairly low velocities, the eddies 

 formed at the throat die out to some extent before reaching B, their 

 kinetic energy being transformed into available pressure energy and thus 

 reducing II 3 . At suitable velocities indeed H 3 may be negative, indicating 



