The value of the coefficient of contraction for this case is, however, not well ascertained, and the result is somewhat modified by friction. For water entering a cylindrical, not bell-mouthed, pipe from a reservoir of indefinitely large size, experiment gives
ɧa = 0.505 v2/2g.
(3)
If there is a diaphragm at the mouth of the pipe as in fig. 89, let ω1 be the area of this orifice. Then the area of the contracted stream is ccω1, and the head lost is
| ɧc = {(ω/ccω1) − 1}2 v2/2g = ζcv2 / 2g |
(4)
if ζ, is put for {(ω/ccω1) − 1}2. Weisbach has found experimentally the following values of the coefficient, when the stream approaching the orifice was considerably larger than the orifice:—
| ω1/ω = | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 |
| cc = | .616 | .614 | .612 | .610 | .617 | .605 | .603 | .601 | .598 | .596 |
| ζc = | 231.7 | 50.99 | 19.78 | 9.612 | 5.256 | 3.077 | 1.876 | 1.169 | 0.734 | 0.480 |
| Fig. 90. |
When a diaphragm was placed in a tube of uniform section (fig. 90) the following values were obtained, ω1 being the area of the orifice and ω that of the pipe:—