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Reference
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Laminar vs. Turbulent Flow
When flow velocity increases, the flow becomes unstable, and
changes from laminar to turbulent flow. In turbulent flow, gas
particles start moving in highly irregular and difficult-to-
predict paths. Eddies form and transfer momentum over
distances varying from a few millimeters, as in controlled
laboratory experiments, to several meters, as in a large room
or other structure. Equations for turbulent flow are more
complex than the formulas for laminar flow. For most
answers, they require empirical relations derived from
controlled experiments.
Whether a flow is laminar or turbulent generally can be
determined by calculating the Reynolds number (Re) of the
flow. The Reynolds number is the product of the density
(designated by the Greek lower-case letter rho {
D
}), a
characteristic length L, and a characteristic velocity v, all
divided by the coefficient of viscosity (designated by the
Greek lower-case letter mu {
:
}): Re = (
D
)LV/
:
Reynolds Number (Re)
The Reynolds number is dimensionless, a pure number. As
long as Re is small, the flow remains laminar. When the
Reynolds number becomes greater than a critical value, the
flow becomes turbulent. With rho, L, and mu constant, Re
varies simply as velocity changes. For flow in smooth round
pipes, critical value is about 2,000, with L equal to the
diameter of the pipe.
Pascal’s Law
In 1653, Blaise Pascal came up with the idea that in a fluid at
rest, the pressure on any surface exerts a force perpendicular
to the surface and independent of the direction or orientation
of the surface. Any added pressure applied to the fluid is
transmitted equally to every point in the fluid. Pascal used his