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Abstract C, a numerical coefficient , defined, as Fd/8μv representing the increase of resistance due to curvature, d, diameter of pipe. D, mean diameter of coil. d/D, curvature ratio. F, intensity of frictional drag on wall of pipe, i. e., shear stress at boundary, g, gravitational acceleration, m, hydraulic mean depth, = area/perimeter, v, mean velocity of flow, = volume flowing per unit time/cross sectional area of pipe, ρ, density of fluid, μ viscosity of fluid. All the values given in the paper are dimensionless, except in those cases in which some dimension is specifically stated.
Christopher White (Sat,) studied this question.