By:
R.Ananthakrishnan
3rd year
Energy
Aerodynamics plays a prominent role in the flight of a cricket ball released by a bowler. The main interest is in the fact that the ball can follow a curved flight path that is not always under the control of the bowler.
INTRODUCTION
On each delivery, the ball can have a different trajectory, varied by changing the pace, the length, the line or, most subtly of all, by moving or "swinging" the ball through the air so that it drifts sideways.
A cricket hall has six rows of preeminent stitching along its equator, with typically 60-80 stitches m each row. Which makes up the "primary" seam. The better quality cricket balls used are in fact made out of four pieces of leather, so that each hemisphere has a line of internal stitching forming the "'quarter" or "secondary" seam. The two quarter seams are traditionally set at right angles to each other. These primary and quarter seams play a critical role in the aerodynamics of a swinging cricket ball.
AERODYNAMICS OF CONVENTIONAL SWING
Suppose you suspend two balls a few centimeters apart and blow between them. Because the air between them is disturbed and the air outside is stable and has a greater pressure, the balls will come together.
Fast bowlers in cricket make the ball swing by a judicious use of the primary seam. The ball is released with the seam at an angle to the initial line of flight. Over a certain Reynolds number range, the seam trips the laminar boundary layer into turbulence on one side of the ball whereas that on the other (non seam) side remains laminar (Fig. 1).
[The Reynolds number is defined as, Re = Ud/v, where U is the ball velocity, d is its diameter, and v is the air kinematic viscosity.]
[The Reynolds number is defined as, Re = Ud/v, where U is the ball velocity, d is its diameter, and v is the air kinematic viscosity.]
By virtue of its increased energy, the turbulent boundary layer, separates later (further back along the ball surface) compared to the laminar layer and so a pressure differential, which results in a side force, is generated on the ball .
When a cricket ball is bowled, with a round arm action as the laws insist, there will always be some backspin imparted to it. In simple terms, the ball rolls-off the fingers as it is released. In scientific terms, the spin is necessarily imparted to conserve angular momentum. The ball is usually held along the seam so that the
backspin is also imparted along the seam (the ball spins about an axis perpendicular to the seam plane). At least this is what should be attempted, since a "wobbling" seam will not be very efficient at producing the necessary asymmetric orientation, and hence separation. This type of release is obviously not very easy to master and it is the main reason why not every bowler can swing even a brand new cricket ball effectively.
The maximum side force is obtained at a bowling speed of about 30 m/s (67 mph) with the seam angled at 20 ° and the ball spinning backwards at a rate of 11.4 revs/s. At a seam angle of 20 °, the Re based on trip (seam) height is about right for effective tripping of the laminar boundary layer. At lower speeds, a bowler should select a larger seam angle so that by the time the flow accelerates around to the seam, the critical speed for efficient tripping has been reached.
Of course, releasing a ball spinning along the seam (without much wobble) becomes more difficult as the seam angle is increased. Spin on the ball helps to stabilize the seam orientation. Basically, for stability, the angular
momentum associated with the spin should be greater than that caused by the torque about the vertical axis due to the flow asymmetry. Too much spin is of course also detrimental, since the effect of the ball’s surface roughness is increased and the critical Re is reached sooner.
AERODYNAMICS OF REVERSE SWING
As discussed above, for conventional swing it is essential to have a smooth polished surface on the non seam side facing the batsman so that a laminar boundary layer is maintained. At the critical Re, the laminar boundary layer on the non seam side undergo transition and the flow symmetry, and hence side force, starts to decrease. A further increase in Re results in the transition point moving upstream, towards the front of the ball. A zero side force is obtained when the flow field on the two sides of the ball becomes completely symmetric. In terms of reverse swing, the really interesting flow events start to occur when the Reynolds number is increased beyond that for zero side force. Amongst other factors, transition is strongly dependent on the condition (or roughness) of the ball’s surface. Of course, the negative sideways deflection will not be as much as the positive deflection since the ball spends less time in the air at the higher velocity. So it seems as though
reverse swing can be obtained at realistic , but relatively high bowling velocities. In particular, reverse swing can be clearly obtained even on a new ball, without any tampering of the surface. Some of the fastest bowlers, such as Jeff Thomson (Australia) and Shaun Tait in present times have been measured bowling in the 90+ mph range and so reverse swing would certainly be achievable by them. Alas, not every bowler can bowl at 90 mph, so what about the mere mortals who would still like to employ this new art? The "old" ball, with an estimated use of about 60 overs, gives less positive side force compared to the new balls, but it also produces reverse swing at a lower velocity of about 65-75 mph.
The critical Reynolds number on the used ball is lower because of the
rougher surface. The key to reverse swing is early transition of the boundary layers on
the ball's surface and the exact velocity beyond which reverse swing is obtained in
practice will decrease with increasing roughness.
SWINGING AN OLD BALL
There is another distinct advantage in maintaining a sharp contrast in surface roughness on the two sides or hemispheres of the ball. The primary seam plays a crucial role in both types of swing. It trips the laminar boundary layer into a turbulent state for conventional swing and thickens the turbulent boundary layer for reverse swing. During the course of play, the primary seam becomes worn and less pronounced and not much can be done about it unless illegal procedures are invoked to restore it, as discussed above. However, a hall with a worn seam can still be swung, as long as there is a sharp contrast in surface roughness between the two sides. In this case, the difference in roughness, rather than the seam, can be used to produce the asymmetric flow. The seam is oriented lacing the batsman (straight down the pitch) at zero degrees incidence. The critical Re is lower for the rough side and so, in a certain Re range, the boundary layer on the rough side will become turbulent, while that on the smooth side remains laminar. The laminar boundary layer separates early compared to the turbulent boundary layer, in the same way as for conventional swing, and an asymmetric flow, and hence side force, is produced. The ball in this case will swing towards the rough side. At very high bowling speeds, the boundary layers on both surfaces will be turbulent and the ball will swing towards the smooth side, much like in the case of reverse swing.
EFFECTS OF METEOROLOGICAL CONDITIONS
When the ground is soft with green wet grass, the new ball will retain its shine for a longer time, thus helping to maintain a laminar boundary layer on the non-seam side. However, the real question is whether a given
ball will swing more on a damp or humid day. The only property of air that may conceivably be influenced by a change in meteorological conditions is the Re through a change in the air viscosity or density. However, Bentley et al. (1982) showed that the average changes in temperature and pressure encountered in a whole day would not change the air density and viscosity, and hence Re. by more than about 2%. Incidentally, although humid or damp air is often referred to as constituting a "heavy" atmosphere by cricket commentators, humid air is in fact less dense than dry air. There is no (positive) scientific evidence which supports the view that humid conditions are more conducive to swing. There is a possibility that the amount of spin imparted to the ball may be affected. It was although found that the varnish painted on all new balls reacts with moisture to produce a somewhat tacky surface.The tacky surface would ensure a better grip and thus result in more spin as the ball rolls-off the fingers.
MYTHS AND MISCONCEPTIONS
The notion that this makes the ball heavier on this side and the ball would therefore swing in that direction has no aerodynamic basis to it whatsoever.
CONCLUSIONS
While it is generally believed (with some justification) that tampering with the ball's surface helps in achieving reverse swing, the exact form of the advantage is still not generally understood. It is shown here that the critical bowling speed at which reverse swing can be achieved is lowered as the ball's surface roughness increases. Perhaps the biggest misconception is that one must tamper with the ball to achieve reverse swing and this is certainly not the case. Reverse swing can be obtained with a brand new (red) tour-piece ball, but only at bowling speeds of more than 90 mph. It is now known how late swing is actually built into the flight path of a swinging cricket ball and it is this, rather than some special phenomenon, that is often observed on the cricket field.
The introduction of the new white ball with its unique outer cover finish has started a new controversy on how its swing properties differ from those of a conventional red ball. Needless to say, cricket ball aerodynamics would not be such a fascinating subject if all the mysteries and
controversies could be readily answered and settled.
