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Swing bowling is a technique used for bowling in the sport of cricket. Practitioners are known as swing bowlers. Swing bowling is generally classed as a subtype of fast bowling.
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The essence of swing bowling is to get the cricket ball to deviate sideways as it moves through the air towards or away from the batsman. In order to do this, the bowler makes use of five factors:
The asymmetry of the ball is encouraged by the constant polishing of one side of the ball by members of the fielding team, while allowing the opposite side to deteriorate through wear and tear. Over time, this produces a marked difference in the aerodynamic properties of the two sides.
The main theory of swing bowling surrounded turbulent and laminar airflow. Turbulent air separates from the surface of the ball earlier than laminar flow air, so that the separation point moves toward the front of the ball on the turbulent side. On the laminar flow side it remains towards the back; inducing a greater lift force on the laminar airflow side of the ball. When calculated the net lift force doesn't provide enough to force for the amount of swing observed, the additional force is provided by the pressure-gradient force.
To induce the pressure-gradient force the bowler must create regions of high and low static pressure on opposing sides of the ball. The ball is then "sucked" from the region of high static pressure towards the region of low static pressure. The Magnus effect utilises the same force but by manipulating spin across the direction of motion. A layer of fluid, in this case air, will have a greater velocity when moving over another layer of fluid than a solid, in this case the surface of the ball, and the greater the velocity of the fluid, the lower its static pressure. When the ball is new the seam is used to create a layer of turbulent air on one side of the ball - by angling it to one side and spinning the ball along the seam. This changes the separation points of the air with the ball, this turbulent air creates a greater coverage of air; providing lift. The next layer of air will have a greater velocity over the side with the turbulent air due to the greater air coverage and as there is a difference in air velocity, the static pressure of both sides of the ball are different and the ball is both 'lifted' and 'sucked' towards the turbulent airflow side of the ball.
When the ball is older and there is an asymmetry in roughness the seam no longer causes the pressure difference, and can actually reduce the swing of the ball. Air turbulence is no longer used to create separation point differences and therefore the lift and pressure differences. The rough side of the ball is a collection of tears from the surface of the ball, these tears act in the same manner as the dimples of a golf ball. The tears hold the air, creating a layer of air over the rough side of the ball, the shiny side doesn't have the tears and therefore no layer of air, thereby providing net lift force. The next air layer will have a greater velocity over the rough side, due to the layer of air, thereby having a lower static pressure than the shiny side, swinging the ball. If the tears cover the rough side of the ball, the separation point on the rough side will move to the back of the ball, further than that of the turbulent air, thereby creating more lift and faster air flow. This is why a slightly older ball will swing more than a new ball. If the seam is used to create the turbulent air on the rough side, the tears won't fill as quickly as they would with laminar flow, dampening the lift and pressure differences.
Reverse swing occurs in exactly the same manner as conventional swing, despite popular misconception. Over time the rough side becomes too rough and the tears become too deep - this is why golf ball dimples are never below a certain depth, and so "conventional" swing weakens over time; the separation point moves toward the front of the ball on the rough side. When polishing the shiny side of the ball, numerous liquids are used; sweat, saliva, sunscreen, hair gel (which is why bowlers may do their hair before a game) and other illegal substances like Vaseline (applied to the clothing where the ball is polished). This doesn't disappear; it penetrates the porous surface of the leather ball. Over time this expands and stretches the surface of the ball (which increases the surface area meaning more lift) and creates raised bumps on the polished side, due to the non-uniform nature of the expansion. These raised bumps invariably creates valleys, which hold the air in the same manner as the tears on the rough side. Thereby creating a layer of air over the shiny side, moving the separation point towards the back of the ball on the shiny side. The greater air coverage is now on the shiny side meaning more lift and faster secondary airflow, and therefore lower static pressure on the shiny side - swinging the ball towards it instead.
The rough side tears hold the air more easily than the shiny side valleys and so to maintain the air within the valleys the initial air layer must have a very high velocity, which is why 'reverse' swing is primarily, but not necessarily, achieved by fast bowlers. Due to the less static nature of the initial air layer it takes longer for the swing to occur, which is why it occurs later in the delivery, this is also why conventional and reverse swing can occur in the same delivery.
All other things being equal, cold and humid weather enhances swing, due to the lift force increasing with the density of the air. Colder air is more dense than warm air. The humidity is a measure of the water content in the air, and although water vapour does not increase the mean density of the air since water molecules weigh less than nitrogen or oxygen molecules, tiny suspended droplets of condensed liquid water will certainly do so since water is a fluid one-thousand times more dense than air.
Typically, a swing bowler aligns the seam and the sides of the ball to reinforce the swing effect. This can be done in two ways:
The curvature of swing deliveries can make them difficult for a batsman to hit with his bat. Typically, bowlers more commonly bowl outswingers, as they tend to move away from the batsman, meaning he has to "chase" the ball in order to hit it. Hitting away from the batsman's body is dangerous, as it leaves a gap between the bat and body through which the ball may travel to hit the wicket. Also, if the batsman misjudges the amount of swing, he can hit the ball with an edge of the bat. An inside edge can ricochet on to the wicket, resulting in him being out bowled, while an outside edge can fly to the wicket-keeper or slip fielders for a catch.
An inswinger presents relatively fewer dangers to the batsman, but can result in bowled or leg before wicket dismissals if the batsman misjudges the swing on the ball.
An inswinger combined with a yorker can be especially difficult for the batsman to defend against, especially if used as a surprise delivery after a sequence of outswingers.
There has been a distinct lack of left-arm swing bowlers in the game. Four of the most famous left-arm bowlers were Pakistan's Wasim Akram, Australia's Alan Davidson, Sri Lanka's Chaminda Vaas and India's Zaheer Khan.
Pioneers and notable practitioners of reverse swing have mostly been Pakistani fast bowlers. In the early days of reverse swing, Pakistani bowlers were suspected of ball tampering to achieve the conditions of the ball that allow reverse swing, and today, it has been unproven so far. Former Pakistan international Sarfraz Nawaz and Sikander Bakht were the founders of reverse swing during the late 1970s, and they passed the knowledge on to former team-mate Imran Khan,[1] who in turn taught the duo of Wasim Akram and Waqar Younis. The English pair of Andrew Flintoff and Simon Jones are also well known for the ability to reverse swing the ball having been taught by Troy Cooley.[2] More recently, a destructive display of swing bowling was demonstrated by Dale Steyn and Zaheer Khan during the test match between India and South Africa at Nagpur. Normal swing occurs mostly when the ball is fairly new. As it wears more, the aerodynamics of the asymmetry change and it is more difficult to extract a large amount of swing. When the ball becomes very old—around 40 or more overs old—it begins to swing towards the shine. This is known as reverse swing—meaning a natural outswinger will become an inswinger and vice versa.[3] In essence, both sides have turbulent flow, but here the seam causes the airflow to separate earlier on one side. The result is always a swing to the side with the later separation, so the swing is away from the seam.
Reverse swing tends to be stronger than normal swing, and to occur late in the ball's trajectory. This gives it a very different character from normal swing, and because batsmen experience it less often, they generally find it much more difficult to defend against. It is also possible for a ball to swing normally in its early flight, and then to reverse as it approaches the batsman. This can be done in two ways [4] one for the ball to reverse in the opposite direction to the original swing, giving it an "S" trajectory; and the other for it to reverse in the same direction making the swing even more pronounced. Either way it can be devastating for the batsman: in the first instance, he is already committed to playing one way, which is often the wrong way to play swing in the opposite direction; and in the second instance, his stance will have conformed to dealing with the degree of expected swing and could leave him vulnerable to being caught behind, LBW or bowled. Two back to back deliveries from Wasim Akram, one of each type, were considered to be the turning point of the 1992 World Cup Final.[4]
Playing swing bowling is considered to be a hallmark of a batsman's skill. While often a batsman will be encouraged to play defensively, in many instances he may be skilled enough to attack.
A batsman often needs to anticipate beforehand what the ball will do and adjust accordingly to play swing bowling. This can be done by observing the bowler's grip and action (which may have a marked difference depending on which type of swinger is to be delivered), by observing the field set, which may depend on the types of deliveries expected (as a rule outswingers will have more slips assigned) or by means of prior knowledge of the bowler; many can bowl or are proficient in only one type of swing. Traditional methods include the batsmen playing the ball as late as possible, and not playing away from the body.
Since reverse occurs at faster speeds, later in the trajectory of the ball and with no real obvious change in action and grip (Waqar Younis from Pakistan for example had the same action and grip for nearly all his deliveries[5]), batsmen with a quick eye and reflexes will do well. In his autobiography Wasim Akram mentions four batsmen— Rahul Dravid, Brian Lara, Aravinda De Silva, and Martin Crowe —who had such reflexes and who were exceedingly difficult to bowl to.[6]
Controversy regarding reverse swing has never left modern cricket, as the Pakistani team was accused of ball tampering by the controversial Australian umpire Darrell Hair during the fourth test against England in 2006 when the ball began to reverse swing after the 50th over.[7] His co-umpire Billy Doctrove fully supported him in this action. A hearing subsequently found that there was insufficient evidence to convict anyone of ball tampering.[8]