(5)
and from 255 experiments obtained for the constants the values
α = 0.007409; β = 0.1920.
This gives the following values at different velocities:—
| v = | 0.3 | 0.5 | 0.7 | 1 | 11⁄2 | 2 | 3 | 5 | 7 | 10 | 15 |
| ζ = | 0.01215 | 0.01025 | 0.00944 | 0.00883 | 0.00836 | 0.00812 | 0.90788 | 0.00769 | 0.00761 | 0.00755 | 0.00750 |
In using this value of ζ when v is not known, it is best to proceed by approximation.
§ 98. Darcy and Bazin’s Expression for the Coefficient of Friction.—Darcy and Bazin’s researches have shown that ζ varies very greatly for different degrees of roughness of the channel bed, and that it also varies with the dimensions of the channel. They give for ζ an empirical expression (similar to that for pipes) of the form
ζ = α (1 + β / m);
(6)
where m is the hydraulic mean depth. For different kinds of channels they give the following values of the coefficient of friction:—