Figure 1. (Flow from right to left)
Seeing the spate of recent posts on rec.sport.cricket on this topic, gave me the motivation to dig up all my old references and start writing something that will be more than just speculation. This page will focus on cricket ball aerodynamics(at least in the beginning) and will soon include discussions of the "other" sports.
Disclaimer: This contains no original work of mine. All information is from the references listed at the end.(Some wordings and phrases are borrowed literally with no fear of Copyright violations :-) Heck, I am not making any money out of this and I am certainly not asking for glory!!
I am continuing to work on writing to provide more information and grow this section to include more related topics (baseball and golf balls). I am not very happy with the figures I have created. I will hopefully have more meaningful and better looking diagrams/graphs up soon. I welcome all comments/corrections/suggestions from you. Send them to mak@b-bop.com or krishna.achutarao@laitram.com
The commonly accepted explanation is that a spinning object creates a sort of whirlpool of rotating air about itself. On the side where the motion of the whirlpool is in the same direction as that of the windstream to which the object is exposed, the velocity will be enhanced. On the opposite side, where the motions are opposed, the velocity will be decreased. According to Bernoulli's principle, the pressure is lower on the side where the velocity is greater, and consequently there is an unbalanced force at right angles to the wind. This is the magnus force.
The more recent studies agree that the magnus force results from the asymmetric distortion of the boundary layer displacement thickness caused by the combined spinning and flow past the spherer. In the case of a sphere(or cylinder), the so-called whirlpool, or more accurately the circulation, does not consist of air set into rotation by friction with a spinning object. Actually an object such as a sphere or a cylinder can impart a spinning motion to only a very thin layer next to the surface. The motion imparted to this layer affects the manner in which the flow separates from the surface in the rear. Boundary layer separation is delayed on the side of the spinning object that is moving in the same direction as the free stream flow, while the separation occurs prematurely on the side moving against the free stream flow. The wake then shifts toward the side moving against the free stream flow. As a result, flow past the object is deflected, and the resulting change in momentum flux causes a force in the opposite direction(upwards in the case shown in figure 1).
Figure 1. (Flow from right to left)
This phenomenon is influenced by the conditions in the thin layer next to the body, known as the boundary layer, and there may arise certain anomalies in the force if the spin of the body introduces anomalies in the layer, such as making the flow turbulent on one side and not the other. One such is the reverse Magnus effect which may occur for smooth spheres. Rough balls such as cricket balls, baseballs, golf balls and tennis balls, do not show this anomalous effect.
Fast bowlers make judicious use of the primary seam to swing the ball. The ball is released into the air flow 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 nonseam 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 alung 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 nonseam side helps maintain a laminar boundary layer. Mehta has a really nice Wind tunnel Smoke photograph of flow over a cricket ball. A schematic is in figure 2.
Figure 2.
Mehta goes on to report a lot of results from wind-tunnel tests and also compares them with results obtained by Barton(1982) and Bentley et al.(1982)
To come to the crux of their observations:
The two piece ball is in general found to have better swing properties than the four piece ball. The secondary seam serves as an effective roughness that helps to cause transition of the laminar boundary layer on the nonseam side.
Barton concluded that the ball with a more pronounced seam than average (> 1mm) swung more. Bentley et al. could not corroborate that as they found that there was no correlation of swing with seam size or shape.
Bentley et al. found that the seam on all new balls is efficient at tripping the boundary layer in the speed range 15 < U < 30 m/s (i.e 54 < U < 108 Km/hr). The swing properties obviously deteriorate with age as the seam is worn and the surface scarred. For that matter the spin or rotation of the ball is not theoretically necessary for swing! [Note to non-cricket savvy people: the ball in cricket is only replaced after a minimum fixed number of overs(deliveries if you will) have been bowled. So the wear and tear is a natural consequence of the game. Only "natural" sources like spit and saliva may be used in maintaining the shine on the ball - usually achieved by polishing the ball on clothing . No means of scuffing the ball may be used. Rolling the ball on the ground is legal.]
The two parameters that a bowler can control to some extent are the
ball seam angle and the spin rate.
The optimum seam angle for U = 30 m/s is about 20 degrees. At lower
speeds (especially for U < 15 m/s) a bowler should select a larger seam
angle than 30 degrees, so that by the time the flow accelerates around
the seam, the critical speed has been reached. It is better not to trip
the boundary layer too early(low angle), since the turbulent boundary layer
grows at a faster rate and will therefore separate relaively early(compared
with a later tripping). At the same time, the seam angle should not be
so large that the boundary layer separates before reaching the seam, since
this would result in symmetrical separation on the ball and hence zero
size force. In a case like this, if transition occurs in the boundary layer
upstream of the seam, then the effect of the seam will be to act as a boundary-layer
"fence" that thickens the boundary layer even further. This asymmetry would
lead to a negative side force for postcritical Reynolds numbers. This effect
can be produced even at low seam angles by inducing early transition of
the laminar boundary layer through an increase of the free-stream turbulence.
(Note: Could this be an explanaion for the reverse swing achieved by Wasim
Akhram and Waquar Younis? It seems unlikely that they get up in the Reynolds
number range required for that.........)
Spin on the ball helps stabilize the ball's seam orientation. Too much spin is detrimental since the effective roughness on the ball's surface is increased. This is more relevant at higher speeds (U > 25m/s). Barton's results indicate that the optimal spin rate is 5 rev/s, whereas Bentley et al.'s results indicate a much higher rate of 11 rev/s. These anomalies are considered to be due to differences in experimental setups. In practice, a bowler can impart upto 14 rev/s though it is not very easy to control.
The flow pattern around the cricket ball depends on the properties of the air and the ball itself. The only properties of air affected by weather conditions are density and viscosity. These will influence the Reynolds number. However, Bentley et al. found that the average changes in temperature and humidity encountered in a day only affect the Reynolds number to the tune of 2%. Several measurements have been made of the effect of humidity, and even wetness of the ball (due to condensation etc.) and no significant changes have been measured under laboratory conditions. Does the dampness make the ball tackier and hence enable the bowler to impart a better spin rate? This was the untested hypothesis of Bentley et al. This aspect of cricket ball aerodynamics remains a mystery.