128 



HYDRAULICS AND ITS APPLICATIONS 



ART. 49. LARGE VERTICAL RECTANGULAR ORIFICE, NEGLECTING 

 VELOCITY OF APPROACH. 



Let //' = height of free surface above top of vena contracta. 

 b = breadth of orifice. 

 d = depth ,, 



b' = breadth of vena contract*} wumeA ^ be ft rect le . 

 d f = depth 



FIG. 70. 



Consider the flow over any element of the vena contracta at a depth y 

 below the free surface, and having an area I' B y (Fig. 70). 

 The velocity at this depth = C *J 2 g y feet per second. 



/. Flow over this area per second = C v b' B y */ 2 g y, c.f. 

 Integrating this expression, and giving y the limits 77' -j- d' and H f 

 we get the flow over the whole area. 



//' + d' 



/H' + d' 

 ^Td y = - 



H' 



H' 



= C b' 



(1) 



The difficulty of obtaining accurate measurements of I', H' and d' now 

 arises, since these vary with, but do not bear a fixed ratio to b, H, and d, 



