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86 Aeronautical Engineer’s Data Book
5.2.4 Sources and sinks
A source is an arrangement where a volume of
fluid (+q) flows out evenly from an origin
toward the periphery of an (imaginary) circle
around it. If q is negative, such a point is termed
a sink (see Figure 5.7). If a source and sink of
equal strength have their extremities infinitesi­
mally close to each other, whilst increasing the
strength, this is termed a doublet.
5.3 Flow regimes
5.3.1 General descriptions
Flow regimes can be generally described as
follows (see Figure 5.8):
Steady Flow parameters at any point do
flow not vary with time (even though
they may differ between points)
Unsteady Flow parameters at any point vary
flow with time
Laminar Flow which is generally considered
flow smooth, i.e. not broken up by eddies
Turbulent Non-smooth flow in which any
flow small disturbance is magnified,
causing eddies and turbulence
Transition The condition lying between
flow laminar and turbulent flow regimes

5.3.2 Reynolds number
Reynolds number is a dimensionless quantity
which determines the nature of flow of fluid
over a surface.
Inertia forces
Reynolds number (Re) =
Viscous forces
VD VD
= =
µ
where = density
µ = dynamic viscosity
= kinematic viscosity
V = velocity
D = effective diameter