By Chris Burgess - 1982
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Why a Cricket Ball Swings
Bowling can be divided into:
- Fast Bowling, and
- Slow or Spin Bowling
The physical principles involved in the two types of movements are vastly different.
The aerodynamics of a cricket ball depend on two possible states of the outer layer (boundary layer). These are:
- Laminar - in which the flow is regular, smooth and bearly parallel to the surface.
- Turbulent - where the general average motion is roughly parallel to the surface but there are rapid random fluctuations in velocity, direction and magnitude.
The transition from laminar to turbulent flow depends on a scientific parameter know as the "Reynolds number" which is directly related to the flow speed and dimension of the ball.
The definition of Reynolds number is:
Udv-1
Where U is the velocity of the ball
d
is the diameter of the ball, and
v
is the velocity of the medium in which the ball is travelling, that is
air!
The transition in a flow takes place if the Reynolds number exceeds a critical value.
Laminar boundary layers are not very good at withstanding sharp rises
in pressure and they will separate from the surface leaving a region of
highly disturbed and irregular at the back of the ball; this is called the
WAKE. This separation gives a pressure distribution:
the pressure exerted at the front of the ball is no longer the same as at
the back which produces a result that tends to swing the ball.
Fast bowling
TO obtain swing a fast bowler grips the ball with the seam
angled across the line POINTS of flight. At point 'A' (see fig 2) the
flow speed is low enough for the boundary layer All on both sides of 'A'
to be Laminar. On surface 'AD. the boundary layer is "tripped"
by the seam, becomes turbulent and
therefore remains attached for longer OF: distance. The surface 'AB',
provided the Number is greater than the c Reynolds critical value, the
boundary layer would remain, re
Laminar and separate early and therefore the pressure on surface 'AD'
is less than PRESSURE ?R that on surface 'AB'. The pressure differ? ?,E
ential results in a side force which makes the ball deviate from its line
of path, ie. swing. The exact value of "The critical T Reynolds Number
depends on parameters such. Ell! "Jik 10 as air turbulence and surface
roughness of the ball, but is approximately equal to 5 W 0 VA 1.5 X 10
which is about 70 miles per hour o(or 113 kilometres per h ur).
At bow ling speeds above this the boundary layer on side 1ABI would become turbulent. This leads to pressure distribution of 'AB', 'AD' to be nearly equal. The pressure differential which caused the side force (swing) is reduced and therefore the swing is reduced.
The concept of 'late swing' can also be related to the critical value of the Reynolds Number. If the ball is released at a speed just above the critical value then it may slow down enough to change the flow and hence swing relatively late in flight. This also explains why a ball bowled very fast, ie. greater than 75 mph, swings only after bouncing, when it has slowed down past the critical value.
| Fast Bowling |Slow Bowling |
SLOW BOWLING (SPIN)
A spin bowler can make the ball deviate from trajectory. To achieve this movement a different set of physical principals are needed relative to the fast bowler.
The movement through the air occurs at low Reynolds Numbers. This movement through the air is entirely due to spin caused by what is known as the "MAGNUS EFFECT".
The magnus Effect is greatest when spin about an axis perpendicular to the line of flight 'squashes' the air on one side, resulting in a pressure differential which pushes the ball in the direction of spin.
Basically the ball can be spun about any one 0 E three axes. See figure 4.i.
About the axis in direction of flight, like a drill. Fig 4.ii About the horizontal axis 7 perpendicular to the direction of flight like a wheel.
Fig 4.iii About a vertical HIGH SPEED axis perpendicular to flight like a spinning top.
FIG 3 ? THE MAGNUS EFFECT
ON po sine W R EeA, TOP
If the ball is drilling through the air the spin trips the boundary layer symetric and flow of air is the same as all sides, therefore no pressure differential a and therefore no sideways movement through the air.
The Magnus Effect operates most successfully with the axis of spin perpendicular to the flight path, with the ball spinning like a wheel or top rather than like a drill. This causes a conflict of ambitions for the spinner ? the spin that gives the maximum break will give least movement through the air and vice-verca.
| Fast Bowling |Slow Bowling |