By Mike Sondalini
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do not all behave the same. Blood
has different flow characteristics than water.
Paint flows differently to gasoline petrol.
Liquids are categorized by their behaviors when undergoing shear.
Those liquids that have a constant shear rate with change of
velocity (like water) are called Newtonian (
first developed the
mathematical explanation for the phenomenon).
Those with shear rates that vary with changing velocity (like paint
and blood) are Non-Newtonian. The
shear rate is a measure of a fluid�s viscosity or slipperiness.
The density of a fluid affects its viscosity.
Fluids with more mass per unit volume are heavier and require more
energy to move them and shear less easily.
A temperature rise decreases the viscosity and density of liquids.
The more viscous, or less slippery, a fluid the harder it is to get
shearing between layers. The
high viscosity prevents rapid velocity changes occurring between layers.
The sub layer in viscous fluids is thicker than in low viscosity
low speeds the whole flow across a pipe is laminar and the fluid slides
over itself. As the speed
becomes faster eddies start to form and cross the fluid layers.
A transition from laminar to turbulent flow develops.
At still higher velocities the flow in the core of the pipe becomes
turbulent with swirling eddies throughout.
Figure 2 shows where the various flow regions occur at a tank
Figure 2 Flow regimes at a tank nozzle.
The laminar sub
layer is always present against the pipe wall.
But as the velocity rises the energetic swirling eddies begin to
impact more deeply and the sub layer begins to thin.
At still higher velocities the sub layer thins further and the
taller roughness peaks stick into the turbulent region.
Where the sub layer covers the roughness projections the wall is
considered �smooth�. When
the wall roughness pokes out of the sub layer the wall is considered
�rough�. This means the
same wall can be both smooth and rough depending on the fluid�s
have proven the pressure loss along a pipe with laminar flow is
proportional to the velocity (p �
V) where as for turbulent flow the pressure loss is proportional to the
square of the velocity (p �
V2). A slower flow
permits a thicker sub layer and creates a �smooth� pipe wall.
This minimizes the losses along the pipe.
There is a very much greater loss of pressure in turbulent flow.
pipe system designer has to strike a practical balance between increasing
the pipe diameter to reduce energy loss and keeping the diameter small to
lower installation costs.
Continued in Pumping Basics
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