SMALL ORIFICES 



119 



The mean of three experiments 1 on a large rectangular converging 

 mouthpiece under a head of 9*6 feet gave a value of 6" ="981. In this 

 mouthpiece the length was 9*6 feet ; the large end 2'4 X 3*2 feet ; and the 

 small end '044 X "623 feet, opposite faces enclosing angles respectively of 

 ll-88' and 15' 18'. 



ART. 44. BELL-MOUTH ORIFICE. 



With a bell-mouth orifice (Fig. 64 a) having a curvature approximately 

 the same as that of the natural 

 stream lines, the pressure over 

 the walls of the mouthpiece will 

 be zero ; the coefficient of con- 

 traction, unity ; and the only 

 loss of head, that due to viscosity. 

 Here C e = l;C v = '975 (experi- 

 mental) ; 



. . C = '975 



FIG. 64. 



Since v = '975 



~ = "95 h 



. . loss of energy due to this type of mouthpiece = '05 h 



v 2 

 = *05 -~ ft. Ibs. per Ib. (approximately). 



Experiments by Weisbach indicate that the general proportions of 

 such a mouthpiece should be as shown in Fig. 65, the sides of the mouth- 

 piece making an angle of 67 with the plane of the orifice at a point 



distant about ^ from that plane. 



Bell-mouth Orifice with Diverging Outlet. If a diverging pipe be fitted 

 to the bell-mouth as shown in Fig. 64 b, 

 so long as this pipe runs full the 

 pressure at the throat is reduced just as 

 at the vena contracta in the previous 

 forms of mouthpiece. The effect of the 

 adjutage is therefore to increase the 

 velocity of flow through the throat, and 

 hence the discharge. The loss of energy 

 due to the divergence may be calculated as in the previous case. 



FIG. 65. 



1 " Practical Hydraulics," Downing, 1861. By Lespinasse et Languedoc. 



