Supposing the discharge into the air, so that p1 = pa,
h + pa/G = v12/2g + pa/G + (v12/2g) (1/cc − 1)2;
(v1/2g) {1 + (1/cc − 1)2} = h;
∴ v1 = √(2gh) / √ {1 + (1/cc − 1)2 };
(1)
| Fig. 58. |
where the coefficient on the right is evidently the coefficient of velocity for the cylindrical mouthpiece in terms of the coefficient of contraction at EF. Let cc = 0.64, the value for simple orifices, then the coefficient of velocity is
cv = 1/√ {1 + (1/cc − 1)2 } = 0.87
(2)
The actual value of cv, found by experiment is 0.82, which does not differ more from the theoretical value than might be expected if the friction of the mouthpiece is allowed for. Hence, for mouthpieces of this kind, and for the section at GH,