View Single Post
Old 03-31-2010, 11:07 PM   #24 (permalink)
user removed
Master EcoModder
 
Join Date: Sep 2009
Posts: 5,927
Thanks: 877
Thanked 2,020 Times in 1,303 Posts
The dimples on golf balls were discovered almost by accident about 200 years ago. Smooth golf balls fly erratically, something you will see on the driving range when they get worn down.

The first golf balls were like little baseballs and very time consuming to produce ( called featheries) they were very similar in construction to tiny baseballs but packed with feathers, with a leather cover stitched together.

Then they started casting them out of gutta percha, a primitive type of plastic. At first they were smooth, but it was soon discovered that when the surface got scratched up they were more controllable, so they eventually became dimpled as the were cast.

The spin imparted to a golf ball and the dimples create pressure areas below in front and behind at the top of the ball. When the axis of that spin is truly horizontal the flight of the golf ball is perfectly straight on a windless day. When the axes is tilted relative to the ground the ball will turn, in flight, in the direction of the lower side of the axis. This is a slice or a hook depending on the angle of the axis.

I wonder if anyone here has measured the fuel economy of their car after it was nailed in a hail storm, which would (under certain circumstances) create a consistent dimpling of the surface of the sheet metal, at least on the top of the vehicle.

The turbulent wake behind smooth round or tubular object oscillates behind the object and creates more drag than the same wake when it is disturbed by dimpling.

I remember the skin of most sharks is so rough it will literally shred your flesh if you rub up against them.

Another thing to consider is the steps in a planing hull that reduce the surface friction in high speed watercraft.

Not my specialty (aerodynamics) so don't be too hard on my statements if they contain significant flaws.

regards
Mech
  Reply With Quote