Fluid Resistance
Objectives
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Define drag
Explain the difference between laminar and
turbulent flow.
Explain the difference between frictional drag and
pressure drag.
Define viscosity and explain how it can be
measured.
Use Stoke’s law and Poiseuille’s Law to solve
problems involving fluid resistance.
Review
Frictional force – the opposing force
resulting when one solid slides against
another.
 Drag – the opposing force of an object
in a fluid.
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Factors effecting drag
Speed of the object (or fluid)
 Size and shape of the object
 The physical properties of the fluid
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Speed of Fluid
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Laminar (streamlined) - results in
frictional drag.
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Turbulent – results in
pressure drag
Speed of Fluid
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Drag forces increase linearly with
velocity until turbulence takes place.
Viscosity ( - eta)

The property of fluid that describes its
internal frictional forces (like the density
of a fluid).
Syrup and tar have high viscosity whereas
air and water have less viscosity
lb
N
s
*
s
*
 Viscosity units
2
ft
m2

=
F y
Av
(Pa • s)
(psi • s)
Viscosity (cont.)
A Liquid's viscosity decreases with
temperature increase
 A Gas’ viscosity increases with increases in
temperature
 Motor oil rated by the Society of
Automotive Engineers (SAE) – SAE10 has
less viscosity than SAE 40. What does
10W40 mean?
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Stokes’ Law

The drag force on an object in fluid flow
(or an object moving in a fluid).
Fdrag = 6 r v 
Viscosity
Product of a
constant (sphere)
Radius
Speed
Terminal Velocity

The constant speed that occurs when
the drag force equals the gravitational
force acting on an object.
Poiseuille’s Law
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Gives the volume flow rate of a fluid
flowing through a tube or pipe.
Pipe radius
•
V= Volume
flow rate
 r 4 P
8
L
Viscosity of fluid
Change in
pressure for length
‘L’
Factors effecting flow through
a Pipe
Radius (cross-sectional area) of pipe
 Length of pipe
 Viscosity of fluid
 Bends or “T’s” in the pipe

P
R=

•
V
Summary
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Drag is the force that opposes the motion of an
object moving through a fluid or the force a moving
fluid exerts on a stationary object.
Laminar flow is slow, smooth flow over a surface,
where particles follow streamlines. The streamlines
define theoretical layers of fluid that do not mix.
The friction between the successive layers of fluid
is called frictional drag.
Turbulent flow is irregular flow with eddies and
whorls that mix the fluid. Turbulence causes a
wake behind a moving object. The pressure
difference between the fluid outside the wake and
the fluid inside the wake causes pressure drag.
Summary (cont.)
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Drag increases with speed. When turbulence is created,
pressure drag increases more rapidly than friction drag.
Viscosity is the property of a fluid that describes its
internal friction. The SI units of viscosity are Pa • s.
Stokes’ law can be used to calculate the drag force on a
sphere moving at constant speed in a viscous fluid.
When drag equals the gravitational force acting on a
falling body, the body falls at a constant speed, called
the terminal speed of the body.
Poiseuille’s law can be used to calculate the volume flow
rate of pressure drop of viscous fluid flowing through a
tube or pipe.