Proceedings of the 13th Asian Congress of Fluid Mechanics
17-21 December 2010, Dhaka, Bangladesh
Aerodynamics of Modern Footballs
Firoz Alam , Harun Chowdhury, Hazim Moria, Himani Mazumdar and Aleksandar Subic
*
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Melbourne, Australia
*E-mail: [email protected]
Abstract Ball sports are becoming faster and more
demanding than ever before, pushing traditional ball designs to
their limits. In order to meet the increasing requirements of
performance and marketing edge, the ball manufacturers are
producing new designs progressively. A traditional spherical
football made of 32 leather panels stitched together in 1970s
has become only 14 synthetic curved panels thermally bonded
(without stitches) and more recently 8 panels football used in
2010 FIFA World Cup. The primary objectives of this study
were to evaluate aerodynamic performances of footballs made
of 32 panels, 14 panels and 8 panels. The aerodynamic forces
and moments were measured experimentally for a range of
wind speeds (20 km/h to 120 km/h) and the non-dimensional
drag coefficient was determined and compared.
Keywords Aerodynamics, football, wind tunnel, drag
coefficient.
1. Introduction
The flight trajectories of sports balls largely depend on the
aerodynamic behaviour of the balls. Depending on
aerodynamic behaviour, the ball can be deviated
significantly from its anticipated flight path resulting in a
curved and unpredictable flight trajectory. Lateral
deflection in flight, commonly known as swing, swerve,
curve or knuckle, is well recognized in cricket, baseball,
golf, tennis, volleyball and football (soccer). In most of
these sports, the lateral deflection is produced by spinning
the ball about an axis perpendicular to the line of flight.
Therefore, the aerodynamic properties of a sport ball is
considered to be the fundamental for the players, coaches
(trainers), regulatory bodies, ball manufacturers and even
the spectators. It is no doubt that football is the most
popular game in the world. No other game is so much
loved, played and excited spectators than the football. It is
played in every corner by every nation in the world. It is
also perhaps the only game that can be played by everyone
regardless of player’s socio-cultural and economic
background. It can also be played in all climatic
conditions. Although, the football among all sport balls
traditionally has better aerodynamic properties and
balance, however, over the years, the design of football has
undergone a series of technological changes, in which the
ball has been made to be more spherical and
aerodynamically efficient (claimed by the manufacturers)
by utilizing new design and manufacturing processes.
Adidas, the official supplier (based on commercial contract
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between FIFA and Adidas) and manufacturer of footballs
to FIFA (Federation Internationale de Football
Association), has applied thermal bonding to replace
conventional stitching to make a seamless surface design
and carcass shape by using 14 and more recently 8 curved
panels (making the ball topologically equivalent to a
truncated octahedron) instead of 32 panels previously used
in the ball since 1970.
The new way to manufacture the football was applied to
14 panels ‘Teamgeist’ (team spirit) ball just before the
1996 FIFA World Cup in Germany. Adidas again
introduced 8 panels ‘Jabulani’ (to celebrate) ball using
almost the similar technology just before 2010 FIFA
World Cup in South Africa. It is claimed that the ball is
more spherical and performs more uniformly regardless of
where it is hit. However, no independent studies have been
reported in support of this statement. Although the
aerodynamic behaviours of other sports balls have been
studied by Alam et al. [1, 2], Mehta [4], and Smits and
Ogg [5], scant information is available to the public
domain about the aerodynamic behaviour of new seamless
footballs except the experiential studies by Alam et al.
[10], Asai et al. [6, 7] and computational study by Barber
et al. [8]. Therefore, the primary objective of this work is
to experimentally study the aerodynamic properties of the
8 and 14 panels seamless balls and also a traditional 32panel ball.
2. Description of Balls
Three new balls have been selected for this study. These
balls are: a) a 32 leather panels ball manufactured by Nike
b) 14 panels Teamgeist ball manufactured by Adidas, and c)
8 panels Jabulani ball manufactured by Adidas. All three
balls are FIFA approved. Both 14 panels Teamgeist and 8
panels Jabulani balls are made of synthetic panels and their
panels are thermally bonded. The Nike ball is made of 32
pentagon and hexagon leather panels stitched together. All
three balls are shown in Figure 1. The physical dimensions
of these 3 balls are illustrated in Table 1.
A sting mount was developed to hold the ball on a force
sensor in the wind tunnel, and the experimental set up with
the sting mount in the wind tunnel test section is shown in
Figure 2. The aerodynamic effect of the sting on the ball
was measured and found to be negligible. The distance
between the bottom edge of the ball and the tunnel floor
was 420 mm, which is well above the tunnel boundary
layer and considered to be out of significant ground effect.
Table 1: Ball physical parameters
Manufacturer
Number of Panels
Panel Joints
Surface Materials
External Diameter
Size
External Shape
Jabulani Ball
Teamgeist Ball
Adidas
Adidas
8
14
Thermal Bonding Thermal Bonding
Synthetic
Synthetic
~ 220 mm
~ 220 mm
5
5
Relatively Smooth Relatively Smooth
Sphere
Sphere
Nike Ball
Nike
32
Stitched
Leather
~ 220 mm
5
Relatively Less
Smooth Sphere
3. Experimental Set Up
In order to measure the aerodynamic properties of two
footballs experimentally, the RMIT Industrial Wind Tunnel
was used. The tunnel is a closed return circuit wind tunnel
with a maximum speed of approximately 150 km/h. Two
mounting studs (stings) holding the ball with a six
component force sensor (type JR-3) in the wind tunnel were
manufactured and purpose made computer software was
used to digitise and record all 3 forces (drag, side and lift
forces) and 3 moments (yaw, pitch and roll moments)
simultaneously. More details about the tunnel can be found
in Alam et al. [3]. The experimental set up of both balls in
the wind tunnel is shown in Figure 2.
(a) Nike made 32-panels Football (with seams and
stitches)
a) 32-panel football
(b) Adidas made 14 panels ball (Teamgeist)
b) 14-panel football
Fig. 2: Experimental setup in the test section of RMIT
Industrial Wind Tunnel
c) Adidas made 8 panels ball (Jabulani)
Fig. 1: Photographs of footballs
Each ball was fixed to the sting with an adhesive in order
to make it very rigid. Three forces (drag, lift and side
force) and their corresponding moments were measured
simultaneously under a range of speeds (20 km/h to 130
km/h within an increment of 20 km/h). The aerodynamic
forces are defined as drag (D) acting in the opposite
direction to the wind, lift (L) acting perpendicular to the
wind direction, and the side force acting (S) sideways
based on a frontal view. The measured aerodynamic forces
were converted to non-dimensional drag coefficient (CD),
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the lift coefficient (CL) and the lateral-force coefficient
(CS), using the formula as defined in Eqs 1 to 3.
D
(1)
2
2 ρV A
L
(2)
CL =
1 ρV 2 A
2
S
(3)
CS =
1 ρV 2 A
2
CD =
1
However, only the drag forces (D) and its non dimensional
CD values are presented in this study.
4. Results
In order to understand the flow structure around the 32
panels ball, a 14 panels and 8 panels balls, airflow was
visualized using smoke which are shown in Figure 3. The
flow visualisation around the 8 panels ball is not shown
here.
subsequently generated favourable pressure gradient and
delayed flow separation as shown in Figure 3(a). The
airflow appears to be separated at around 100° from
horizontal direction. Generally, the flow separates at
around 90º from the horizontal for a smooth surfaced
sphere. For the 14 and 8 panels balls, the surface is more
spherical and smooth. The ball behaviour is very similar to
a smooth sphere. As shown in Figure 3(b), the airflow
separates at around 90º from the horizontal as in the case
of a smooth sphere. Therefore, the 14-panel ball can
potentially generate more aerodynamic drag at low speeds.
A similar pattern (like 14 panels ball) was also observed
around the 8 panels ball (not shown here).
The aerodynamic drag forces for all three balls: 32 panels
Nike ball, 14 panels Teamgeist Adidas ball and 8 panels
Jabulani as a function of speeds are shown in Figure 4. In
addition, a smooth sphere with 200 mm diameter has also
been tested to compare and validate our findings against
the smooth sphere data published by Achenbach [11]. As
the diameter of the smooth sphere is not the same as the 3
balls used in this study, the drag force for the smooth
sphere is not shown in Figure 4. A significant variation in
drag forces between the 32 panels Nike ball and 14 panels
and 8 panels balls was noted (see Figure 4). A sudden
reduction of forces was noted at a relatively low speed for
the 32 panels ball, followed by 14 panels ball and 8 panels
ball respectively. The Nike ball displayed more
aerodynamic drag compared to the Adidas balls in the
range of 30 km/h to 120 km/h.
32 pos1,Drag
14 pos1, Drag
8 pos1, Drag
5.0
4.5
4.0
(a) Airflow around the 32-panels football
Drag [N]
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
120
140
Speed [km/h]
Fig. 4: Aerodynamic drag of 3 balls
(b) Airflow around the 14-panels football
Fig. 3: Airflow structure of football with smoke flow
visualization
Due to the roughness created by the seams in 32 panels
ball, the airflow over ball became turbulent and
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The drag coefficient (CD) as a function of Reynolds
number ( Re = ρ V d ) for the 14 panels Adidas ball, 8
µ
panels Adidas balls, 32 panels Nike ball and a sphere is
shown in Figure 5. The CD value of the smooth sphere has
clearly demonstrated notable variation and flow transition
from laminar to turbulent flow. According to Achenbach
[11], the airflow undergoes from laminar to fully turbulent
between Re = 1.0 ×105 to Re = 4.0 × 10 5 , which is the case
for the CD value of the smooth sphere used in this study
(see Figure 5). The airflow around the 32 panels Nike ball
undergoes laminar to turbulent much earlier Reynolds
number compared to the smooth sphere. Due to the
pentagon and hexagon shapes, seams and stitches, the
surface of the 32 panels ball is significantly rougher
compared to the smooth sphere and lesser extend the 14
and 8 panels Adidas balls. The surface roughness triggers
the flow transition much earlier for the 32 panels ball. This
observation agrees well with the findings by Asai et al. [7].
The airflow becomes fully turbulent for the 32 panels ball
at 30 km/h ( Re = 1.2 ×105 ), 14 panels ball at 40 km/h (
Re = 1.6 × 105 ), 8 panels ball at 60 km/h ( Re = 2.4 × 105 )
and the smooth sphere at 100 km/h ( Re = 3.7 × 105 ).
The average CD values after the transition for the 32 panels
and 14 panels ball are similar. However, a slightly lower
CD value was noted after the transition for the 8 panels ball
expect at 120 km/h speed.
Cd 32 panel ball
0.60
Cd 14 panel ball
Cd Sphere
Drag Coefficient, Cd
0.50
•
The 8 panels ball (Jabulani) possesses lower CD value
at speeds over 60 km/h compared to other two balls.
•
The CD value for 14 panels ball (Teamgeist) is lower
between 30 to 60 km/h.
Acknowledgements
The authors are highly grateful to the Australian Football
Federation for providing the balls for this study and their
strong support.
The authors express their sincere gratitude and thanks to
Mr Dinh Le and Mr Mustafa Salehi, School of Aerospace,
Mechanical and Manufacturing Engineering, RMIT
University for their assistance with the data collection and
wind tunnel testing.
References
0.30
0.20
0.10
1.E+05
2.E+05
3.E+05
4.E+05
5.E+05
Reynolds number
Fig. 5: Drag coefficients of balls and a smooth sphere
5. Discussion
The CD value largely depends on the roughness of the ball
exterior surface and the seams. They can cause additional
drag due to the boundary-layer separation. The results
indicate that the CD value for a football is in between a
smooth sphere and a golf ball. The golf ball data was not
shown here, however for more details, see [6]. As the
speeds of the football are generally in the range of 90 km/h
to 130 km/h during a free kick or long shot, the CD value
of 32-panel and 14-panel balls are expected to be the same.
The 8 panels ball will travel faster at speeds between 60 to
120 km/h due to its lower CD value. Therefore, players
need to develop better strategy for long and short pass as
the airflow transition can have significant effects on drag
thereby on distance and flight trajectory.
6. Conclusions
The following conclusions were made from the work
presented here:
•
The 32 panels ball undergoes through flow transition
much earlier compared to the 14 panels ball, 8 panels
ball and smooth sphere.
Cd 8 panel ball
0.40
0.00
0.E+00
•
The average CD values for 32 panels, 14 panels and 8
panels balls are: 0.15, 0.19 & 0.21 respectively
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