Re: .Bernoulli and curve balls.

[Marlyn Jakub <mjakub@physics.otago.ac.nz>]

Leigh states:

> This problem is difficult. It is also complicated. The answer is
> well known, however, and that should be a guide to tell us whether we
> are modelling it reasonably or not. The glib "explanations" in terms
> of the Bernoulli effect are unsatisfying and unedifying in addition
> to being incorrect. I can't find the Hecht explanation of the curve
> ball which was cited as 98% correct and I'd like to have that page
> number again 

Physics ('94) by Hecht, Ch.11,Sect.12 & 13 (look at Figs.11.40 & Q17).
This text at least uses circulation ideas to explain the lateral force
developed in smooth-surfaced moving spheres. The text does NOT address the
American Baseball version of a sphere with turbulence-inducing stitches over
much of the surface. To address the new effects, the British have evidently
done work on the cricket ball, which has only a single band of stitching
which is about 1-inch wide. The ball is slightly larger than a baseball, so
the stitched band makes it possible to spin the ball so that airflow on one
side has a smooth surface, while the other side's air path becomes turbulent
as it crosses the band of stitching. Refer to an article by Mehta in New
Scientist, 21 Aug 93, p.21, which states that the direction of curvature now
depends on the speed of the ball and the condition of the smooth surface. It
seems to me that one could throw a baseball with a spin and orientation
which would also tend to have a smooth surface located on a critical region
on ONE side. 

The importance of surface turbulent layers is in determining where the
boundary layer will separate from the ball surface as air flows around the
circumference. With the dimpled golf ball, the induced turbulence keeps the
separation point farther along the circumference, thus lowering the diameter
of the wake behind the ball. However, this drag reduction is only for a
limited range of velocities.

With two types of boundary layers on the two sides of a cricket ball, the
separation points would have varying locations, depending on speed and other
factors of the two flow surfaces. Calculations modeling this are beyond my
ability...besides, cricket and baseball are not my cup of tea. 8-)