Last Updated 20th August, 2002
In this project, it is to design a new ball flinger or bowling machine for cricket practice to develop reactions and techniques and test a low cost non-powered system to propel the balls automatically at different suitable adjustable speeds and rotation for the cricket practice.
Ball pitching devices or bowling devices has been used in sport practice for many years to develop techniques and reactions. Typically balls are propelled from a device using motors, discs, pneumatics, or springs and speed spin that can be set by the operator. The Bola is the world¡¦s no. 1 cricket-bowling machine. The following shows background information or design criteria to develop a professional cricket-bowling machine.
The cricket ball flight can be described in different types of delivery.
- Movement through the air (Normal Swing or Reverse Swing)
- Movement through the ground (Dip)
- Movement off the ground (Spin)
- Movement off the ground (Seam)
1. The Movement through the Air (Swing)
A new, smooth-surfaced ball will tend to maintain laminar layers up to separate even on the seam side, whilst a badly worn ball will tend to induce turbulence on the side remote from the seam.
The aerodynamics of cricket ball depends upon the behaviour of the two possible states of the boundary layer.
- Laminar
- Turbulent
In a laminate boundary layer the flow is regular, steady, smooth and nearly parallel to the surface. In the turbulent boundary layers, the motion is still roughly parallel to the surface, but in addition there are rapid random fluctuations in velocity, direction and magnitude.
This delivery requires a different action to the out-swinger of in-swinger. The ball can then be swung both into and away from the batsman depending solely on, which side of the ball is delivered at the front ¡V generating either normal or reverse swing.
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Figure 1: Normal swing
The leading part of the ball is covered by a film of fast moving air, called the boundary layer. About halfway round the ball, the boundary layer separates from the surface. On the non-seam side the boundary layer peels away before the halfway mark. But on the seam side the flow is disrupted by the protuberance of the seam, the boundary layer is tripped into a chaotic turbulence and peels away after the halfway mark. The effect is to make the air pressure on the seam side of the ball lower and this pushes the ball towards the seam side, away from the batsman.
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The figure shows the standard out-swinger; for the in-swinging delivery the ball is reversed, with the seam pointing to the leg-side. This delivery requires a different action to the out-swinger, allowing the (good) batsman to recognise the deliveries as they are delivered.
Contrary to intuition and popular belief, the swing is not primarily due to the difference in friction caused by the rough and smooth sides of the ball - although it is this difference that helps generate turbulent flow on one side and laminar flow on the other. For a blunt object like a sphere the main contributor to aerodynamic drag is the point of separation of the boundary layer, and turbulent flow holds the layer on to the surface longer, reducing the pressure on that side of the ball.
{This is the same principal that allows a dimpled golf ball to fly further than a smooth one. In this case the dimples promote turbulent flow which reduces the pressure drop behind the golf-ball, thus reducing drag.}
Figure 2: Photo of cricket ball in a wind tunnel experiment
The ideal ball for normal swing is highly polished on one side with a prominent seam delivered at an angle of about 20 degrees to the direction of flight, and with about 11 revs per second spin about an axis perpendicular to the seam
Smoke has been injected into the boundary layer to reveal its structure. Wind is blowing from left to right. (from Mehta and Wood, 1980)
There is no simple linear relationship between the speed of delivery and the amount of sideways movement for conventional swing. Up to a certain limit - dependent of the atmospheric conditions and the condition of the ball - the amount of swing increases with the speed of delivery. As the ball's speed increases past this limit however turbulence will start to develop on the shiny side reducing the net side force. This is why medium pace bowlers can often generate more swing than fast bowlers. If the ball is bowled even faster still, the turbulence may begin before the seam causing reverse swing! (Turbulence is initiated at the back of the ball and moves forward as the speed increases).
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Figure 3: Reverse swing
With a turbulent boundary layer on both sides of the ball, the effect of the seam is reversed. It now acts as a ramp, pushing the turbulent air away from the ball and causing the boundary layer to peel away sooner. That makes the pressure on that side higher, forcing the ball to swing towards the batsman.
To get reverse swing with a new ball, smooth on both sides, experiments show that the bowler has to reach 80-90 miles per hour to get appreciable movement. This kind of speed has only ever been achieved consistently by a few bowlers. A scuffed ball however can generate substantial reverse swing at speeds well within the capabilities of any medium-paced bowler.
The ideal ball for reverse swing has one side rough, the other smooth, with a prominent seam in between. The seam should be angled at about 15 degrees to the direction of flight, pointing away from the desired direction of movement. The ball can then be swung both into and away from the batsman depending solely on which side of the ball is delivered at the front ¡V generating either normal or reverse swing. Because the bowler does not need to change either his grip or his action, the batsman will have no clue which way the ball is likely to move.
For reverse swing the amount of sideways movement is related to the speed of delivery, making this a particularly effective delivery for fast bowlers.
Humidity: Despite being widely observed in practice, there is currently no theoretical, or experimental, evidence for humidity having any affect on the amount of swing. Humid air is less dense than dry air ¡V although the difference is minimal ¡V and so would be expected to induce less swing. Experiments in wind tunnels show no noticeable difference in the amount of swing between dry and humid air, and there is no measurable aerodynamic difference in the state of the ball due to moisture.
Late Swing: There are several possible explanations for late swing ¡V where sideways movement occurs only late in the ball¡¦s flight.
i). It is an illusion. The flight path of a ball with a constant sideways-acting force applied to it is parabolic: the amount of the sideways movement naturally increases along the flight path.
ii). The ball is initially above the transition speed for turbulent flow on the shiny, non-seam side, but drops below this threshold as it decelerates in flight, particularly after bouncing, initiating late swing.
iii). The ball rotates slightly in flight, with the seam becoming angled and thus initiating late swing.
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2 The Movement through the air (Dig)
Any object that rotates will develop a force normal to the direction of motion. This transverse force is a consequence of the Magnus Effect. A cricket ball given top spin has a smaller relative velocity to the undisturbed air at the top than at the bottom and so there is a net downward force on the ball tending to keep it¡¦s trajectory flat.
A spinning ball also experiences aerodynamic lift, which acts perpendicular to its trajectory and causes it to curve in flight. In 1852, Gustav Magnus discovered that the airflow around a spinning ball changes the direction of its wake and generates a sideways force on the ball. The direction of the Magnus force depends upon the spin direction - whatever direction the front most point of the ball is turning is the direction of the force. Also, the more spin imparted to the ball, the more it will curve.
Figure 4: Magnus force acted on the ball
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3 The Movement off the ground (Spin)
It has many types of spin. Spin is a ball sideways after pitching. There are five types of spin.
1. All round spin is that a bowler who can spin the ball both ways with equal ease.
2. Bottom spin is that a ball spun such that it slows abruptly on pitching, with a backward oriented spin motion
3. Off spin means that a ball moving from left to right of the screen.
4. Left-spin is that the ball is gripped across the seam, and as it is bowled the wrist helps the fingers spin the ball from leg to off.
Figure 5: Motion of spin (Left spinner)
The ball is released out of the front of the hand. A good rhythm is necessary to land the ball in the right spot.
5. Topspin is a ball spun with a forward momentum spin. The position of the wrist changes, pointing to the side. The ball is gripped in the same way, but this time fingers and wrist spin the ball in a straight line towards the batsman.
Figure 6: Top-spinner
Figure 6 shows that the result is a ball, which goes straight on. But bounces more, with over-spin.
4. The Movement off the Ground (Seam)
The art of the seam bowler in cricket is boundary layer transition and separation. The bowling technique is aligning the seam at a small angle to the flight path. A cricket ball has a single, circumferential seam, which looks remarkably like a trip wire. If a cricket ball is launched without spin so that the seam is tilted forward on the top of the ball, then the boundary layer over the top surface becomes turbulent, whereas the boundary layer on the bottom surface remains laminar. The wake becomes asymmetric, and a downward force is produced so that the ball dips sharply. A similar effect can be obtained with a baseball if the seam is held correctly.
The addition of spin complicates this picture enormously. Spin can produce a side force on a ball due to the Magnus effect. The effect of spinning the ball can depend strongly on the orientation of the seam. Knuckle ball pitchers typically pitch a baseball with very little spin, and they rely largely on the uneven tripping of the boundary layer. However, even a little spin will make the direction of the side force change dramatically, and it is no wonder that the behaviour of a knuckle ball is highly unpredictable.
The seam is along the "equator" of the two-hemisphere ball. Better quality balls are made of 4 pieces of leather so that each hemisphere has a line of internal stitching forming the "secondary seam". The secondary seams of the two hemispheres are at right angles to each other.
Fast bowlers make judicious use of the primary seam to swing the ball. The ball is released into the airflow with the seam at a slight angle. (When bowled right) The seam trips the laminar boundary layer into turbulence on one side of the ball. This turbulent boundary layer by virtue of its increased energy, separates relatively late compared to the boundary layer on the non-seam side, which separates in a laminar state. The asymmetric boundary layer separation results in an asymmetric pressure distribution that produces the side force responsible for the swing. When a ball is bowled, with a round arm action as rules insist, there will always be some backspin imparted to it. The ball being held along the seam, the backspin is also imparted along the seam. So the asymmetry of the boundary layer separation is maintained. A prominent seam obviously helps the laminar to turbulence transition process, whereas a smooth and polished surface on the non-seam side helps maintain a laminar boundary layer.
Figure 7: Aerodynamic of
ball (Seam occurred)
