Fluid Resistance
• The transmission of energy from an object
passing through a fluid to the fluid is known
as fluid resistance.
• The resistance of an object passing through
a fluid increases as the speed of the object
increases and as the viscosity of the fluid
increases.
Contact Forces
Surface and Form Drag
• Surface drag is a result of the friction
between the surface and the fluid.
• The fluid closest to the object (boundary
layer) rubs against the object creating
friction.
• Kyle (1989) reported that wearing loose
clothing can increase surface drag from 2%
to 8%.
Contact Forces
Surface Drag
Van Ingen Schenau
(1982) reported a
10% reduction in
surface drag when a
speed skater wears a
smooth body suit.
Contact Forces
Form Drag
Form drag occurs when air is driven past an object
and is diverted outward creating a low pressure
region behind the object.
low pressure
high pressure
Contact Forces
Form Drag
Low form drag
The orientation of the object
will affect the frontal area
and will play an important
role in the amount of form
drag.
High form drag
Contact Forces
.5m2 (upright)
.42m2 (touring)
.34m2 (racing)
The second cyclist can ride within the low
pressure zone of the first cyclist and thus
lower the pressure difference and the drag.
This is called drafting.
frontal area
Contact Forces
Flow Type
laminar
At low velocities laminar
flow occurs. The
separated
boundary layer remains
attached to the surface.
During separated
flow the boundary layer
fully turbulent
separates toward the
back of the object and a
low pressure region
is formed. During fully turbulent flow the
boundary layer becomes turbulent and the
size of the pocket is decreased.
Contact Forces
Factors Affecting Flow Type
•
•
•
•
•
size
shape
surface roughness
viscosity of the fluid
flow velocity
Contact Forces
Airfoil
The particles following the path from D1 to D2
will be more spread out than particles following
the path from C1 to C2 because of the greater
distance. This creates a low pressure region
above the airfoil.
Bernoulli’s Principle, 1738
Contact Forces
Lift
Fair resistance
Flift
Flift = 1/2(ClArv2)
Fdrag = 1/2(CdArv2)
Fdrag
direction of
movement
Lift always acts perpendicular to drag.
Contact Forces
• The lift-to-drag ratio is critical (i.e. the larger the
ratio, the more effective the airfoil is in flight).
• L/D ratio is dependent on the angle that the airfoil
makes with the incoming air (this is called the
ANGLE OF ATTACK).
• Increasing the angle of attack increases the L/D
ratio to a point; beyond that point the angle
becomes too steep and the airfoil stalls
• typical angles of attack:
airfoil - below 15o
javelin - 10o
Contact Forces
Angle of
Attack
(degrees)
0
10
20
25
27
28
29
30
35
40
45
50
60
70
80
90
Lift
(N)
0.00
4.33
10.64
12.83
13.80
13.80
11.01
11.21
10.12
8.50
8.90
8.62
6.88
4.77
2.55
0.00
Drag
(N)
1.17
1.50
4.13
5.79
6.88
7.41
7.94
8.18
8.74
9.55
11.13
12.22
14.98
16.43
16.88
17.73
Lift
Drag
0.00
2.89
2.58
2.22
2.01
1.86
1.39
1.37
1.16
0.89
0.80
0.71
0.46
0.29
0.15
0.00
Lift to drag ratios
for the discus.
Adapted from Aerodynamic
Factors Which Influence
Discus Flight, Ganslen.
Contact Forces
Rotating Objects
Direction of air flow
low pressure
zone
Rotating objects can also
create a pressure difference.
high pressure
zone
Magnus Effect,Contact
1852
Forces
intended direction
of flight
actual direction
of flight
low pressure
zone
high pressure
zone
Contact Forces
The golf club imparts backspin on the golf ball and
increases the length of the drive.
Contact Forces
Dimples on a golf ball increase the velocity of the boundary
layer and can dramatically influence the length of a drive.
Depth of Dimple
(mm)
0.05
0.10
0.15
0.20
0.25
0.30
Carry
(m)
107
171
194
204
218
206
Length of Drive
(m)
134
194
212
218
239
219
From The Mechanics of Sport, E. Bade.
Contact Forces
Terminal Speed
An object falling through a fluid reaches its
terminal speed when the drag force is equal to its
weight. This results in a net force of zero and thus
no further acceleration takes place.
weight
drag force
Contact Forces
Estimated Terminal Speeds of
Selected Spheres
Weight Diameter
K
Terminal V
Ball
(N)
(cm) (Drag Factor)
(m/s)
16-lb shot
71.27
1.86
0.00014
145.28
Baseball
1.43
1.14
0.0016
42.47
Golf ball
0.45
0.66
0.0018
40.23
Basketball
5.84
3.73
0.007
20.12
Ping-Pong ball
0.03
0.58
0.04
8.94
Adapted from Sport
Science by Peter J.
Brancazio.
C DrD 2 g
K
8W
VT 
g
K
CD:
r:
D:
W:
VT:
coefficient of drag
fluid density
sphere diameter
weight of sphere
terminal speed
Contact Forces