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While watching last winter's Super Bowl, the annual American football
bonanza, some engineers from Fluent began to ponder the aerodynamics of
a football in flight. They hunted down a regulation football, learning
that the Canadian version is quite different from the American one, and
modeled it in motion after a perfect throw. The perfect throw was assumed
to give the ball forward motion only, with spin but without wobble. Their
goal was to compute the forces and moments on the ball and understand
the mechanics of the laces in disrupting the boundary layer.
Contours of velocity magnitude for a kicked ball at one instant during
the ball's rotation (normal to the direction of travel)
A sliding mesh model in FLUENT 5 was used for the transient
simulation. One sphere was used for the outermost (free stream)
boundary and another for the sliding mesh interface. A forward
speed of 40 miles/ hour and a rotation speed of 300 rpm were
assigned to the ball. Turbulence was incorporated through the use
of the RNG model.
Contours of turbulence kinetic energy for a thrown ball show the disrupting
effect of the laces
Results show a high pressure region in front of the ball with a long
wake region behind it. The laces clearly cause the boundary layer to separate,
and the separation region rotates with the ball. The predicted moments
on the ball suggest that even though the ball is thrown straight, the
rotation of the laces destabilizes the motion and causes the ball to begin
wobbling. A second simulation of a kicked ball used an axis of rotation
normal to the direction of travel. The fluctuating velocity field in front
of and behind the kicked ball illustrate the inefficiency of this mode
of transport.
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