Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 26

where

density of fluid (kg/m 3 ) ρ =

V = mean fluid velocity (m/s)

d = diameter of the pipe (m)

μ = coefficient of viscosity of the fluid (kg/m/s)

2,100. A transition between laminar and tur-
bulent flow occurs for R N between 2,100 and 4,000 (transition flow). Above 4,000,
the flow is turbulent. At turbulence range, the flow becomes unstable, and there is
increased mixing that results in viscous losses which are generally much higher than
those of laminar flow.
The Reynolds number can be considered in another way, as

Generally a flow is laminar if R N ≤

Inertia forces
Viscous forces

R N = (1.18)

The inertia forces represent the fluid's natural resistance to acceleration. The vis-
cous forces arise because of the internal friction of the fluid. In a low Reynolds
number flow, the inertia forces are small and negligible compared to the viscous
forces. Whereas, in a high Reynolds number flow, the viscous forces are small
compared to the inertia forces.

1.3.5 Velocity Profile of Pipe Flow

Typical velocity profile of a pipe flow is shown in Fig. 1.11 . The velocity is zero at
the surface, increases thereafter and reaches its maximum at the center of the pipe.

Fig. 1.11 Diagram showing
velocity distribution in pipe
flow V max

1.3.6 Head Loss in Pipe Flow and Its Calculation

1.3.6.1 Causes and Components of Head Loss

When fluid flows through the pipe, the internal roughness of the pipe wall can create
local eddy currents vicinity to the surface, thus adding a resistance to the flow of
the pipe. Pipes with smooth walls have only a small resistance to flow (frictional
resistance). Smooth glass, copper, and polyethylene have small frictional resistance,
whereas cast iron, concrete, and steel pipe, etc. create larger eddy currents and effect
on frictional resistance.