My previous article on golf ball and the reason for dimples on it, made me to investigate more about the aero-dynamics of the cricket ball. Now lets understand the science behind the swing of the cricket ball..!
As we all know, a cricket ball is made of either 2 pieces or 4 pieces, stitched to form a seam. The seam has 6 rows of stitches with 60 to 80 stitches each row. This is called as the Primary seam. In a 4 piece ball, each hemispheres are stitched together by an internal stitching. This forms the secondary seam.
Flow Dynamics:
Lets first understand the basic flow dynamics of a sphere. When a ball is flying through air, it forms a layer around the ball surface called "The Boundary Layer". This layer cannot travel all over along with the ball. So it has to separate at a point where it is called as "Flow separation". This flow separation determines the pressure. The early flow separation means more pressure and late separation means less pressure. This difference in pressure makes the ball to swing...!
So it is clear that there must be a difference in pressure on either side of the ball to make it swing..!
Now lets see the Boundary Layer more closely...!
The boundary layer has two states. One is laminar flow and other is turbulent flow. The laminar flow is smooth flow with out any disturbance or turbidity. When a cricket ball flies in air, if it does not has any deviation in the position of seam ,then both sides of the ball may have a laminar air flow pattern.
Let us understand the swing of the cricket ball with the seam position being straight...!
This kind of swing does not require expertise bowling skills but of course practice is essential. Observing the above picture, by positioning the seam straight, the ball being rough on one side and smooth on other side, with the average bowling speed less than 70 miles per hour, the ball starts to swing automatically. When we look at the figure, the rough side creates turbulence and thus delays the flow separation. This causes the pressure to reduce on the rough side. When looking at the smooth side, the laminar flow exists, and the flow separation point is seen early than the rough side. This imparts more pressure on the smooth side. Thus the ball swings from high pressure side to low pressure side.
Now lets say the bowler positions the seam towards the slips or away from the right hand batsman, as shown in the above figure. The rough side and the seam produces turbulence, thus delaying the flow separation, causing lesser pressure. On the other side, the smooth surface, renders a laminar flow, with out any disturbance. This creates early flow separation and thus high pressure. So the ball is being pushed from high pressure to low pressure side, i.e , the ball swings away from the batsman, called "OUT SWINGER". When the seam in positioned towards the right hand batsman, then the ball swings into the batsman, called as "IN SWINGER".
So now wondering what is a Reverse Swing...?
The reverse swing is mostly produced only by few bowlers who bowl at a speed of around 85 to 90 miles/hr. This happens because, at such high speeds, the laminar flow becomes turbulent before reaching the ball surface. Thus when the flow hits the seam, the boundary layer becomes more turbulent, making the layer thick and cause early flow separation when compared to bottom side. Thus this again causes the pressure difference, which makes the ball to swing. But to be observed in depths, that the swinging direction is opposite to the seam position...! Thus it is called as reverse swing...!
Thus the science(aero-dynamics) behind the cricket ball is clear and appreciated. Comments and discussions are welcome..!
11 comments:
Hi, nice one there mate breaking things down for newbies although i must add i found something which i thought was fundamentally wrong about your explanation of reverse wing.
Check this out mate: http://www.youtube.com/watch?v=075O6fRJesQ&feature=search
Here is my understanding of swing bowling. I have used the example of bowling an OUTSWINGER to a Right Handed Batsman (RHB):
NORMAL CONVENTIONAL swing.
1, Fast-Medium or Medium-Fast
2, Bowled around 70-80mph
3, New ball only
4, Seam position towards 1st/2nd slip
5, 'Rough' side facing slip (although this isn't really applicable)
6, Ball moves away from RHB
REVERSE swing:
1, Fast or (a few) Fast-medium bowlers
2, Bowled over 85-90mph
3, Generally bowled using an old ball
4, Seam position towards 1st/2nd slip
5, Shiny side of the ball facing slip
6, Ball moves away from RHB
This is my understanding but feel free to point out if there are any flaws. I'll check back in a few days time.
conventional swing is well known in all books
reverse swing :
1, any bowler at any speed, but is noticed more at higher speeds, due to greater inertia effect of reverse rotation of the ball.
2. generally bowled using a ball which has a weight difference between both sides, i.e shiny side has moisture (swet / spit / water) to make it heavier then when new. And the, rough side is dry hence lighter. The rough side is also made lighter then when new, as the leather may have worn down more and lost weight due to this wear and tear.
3. ball seam position towards fine leg, but ball is also angled downwards. The seam position to help the ball move in that direction and the tilt of th ball to eventually help the ball dip in flight.
4 here is the science. the reverse rotation of the ball, causes inertia of that ball on the weighted side, this causes force on this side hence moves the ball in that direction. This is aided by the seam angle and tilt. to maximise reverse swing.
5. the round arm action by the right handed bowler i.e WAQAR YOUNIS , will aid reverse rotation more, so increase the inertia effect, and the round arm will help the seam position and tilt more. waqar swung the ball more then anyone, watch his bowling in super slow motion and you will see seam position is to wards fine leg, and the ball is titled downwards and he actually spins the ball back at the point of release.
the above article is on reverse swing is false. and inaccurate, as in both cases the ball swings in the same direction as according to the diagrams.
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