What Causes the 'Knuckle' Effect in Sports Balls?

[adam7]

The 'Knuckle'(ball) Effect..?

I've always been intrigued... how does the 'knuckle' effect (the ball wobbling randomly in the air) - seen in baseball with knuckleballs and oddly enough at the recent soccer world cup with the Jabulani football - work?

And why does fewer seams on the ball make the ball 'wobble' more in the air?

 

[ZapperZ]

And why does fewer seams on the ball make the ball 'wobble' more in the air?

You might want to read this:

http://www.npr.org/blogs/showmeyourcleats/2010/07/09/128411155/what-science-says-about-smooth-balls

Zz.

 

[adam7]

Thanks Zz, that helps explain why the knuckling is more noticable, i.e. this ball knuckles at a speed which most soccer players kick the ball at. But I still don't understand why the ball behaves so randomly? Do the seams on the ball trip turbulence and cause random boundary layer separation? Or am I on the wrong track here?

 

[ZapperZ]

Thanks Zz, that helps explain why the knuckling is more noticable, i.e. this ball knuckles at a speed which most soccer players kick the ball at. But I still don't understand why the ball behaves so randomly? Do the seams on the ball trip turbulence and cause random boundary layer separation? Or am I on the wrong track here?

Read this, then:

http://news.cnet.com/8301-13772_3-9914179-52.html

Zz.

 

[adam7]

Ok ta Zz definite progress here, but I'm still confused as in my original question about why the ball wobbles randomly. Surely if the ball is spinning even if slowly, the boundary layer will only become turbulent on one side of the ball, which would just give regular curve, not erratic wobbles?

 

[ZapperZ]

Ok ta Zz definite progress here, but I'm still confused as in my original question about why the ball wobbles randomly. Surely if the ball is spinning even if slowly, the boundary layer will only become turbulent on one side of the ball, which would just give regular curve, not erratic wobbles?

The ball is spinning slow enough that the side of the ball with the seam and causing these air turbulence keeps changing during the trajectory of the ball.

Zz.

 

[adam7]

Ok that's great, just what I needed thanks

 

[adam7]

In my view it seems that the ball is not any lighter as some may say (it is actually on the heavier side of the strictly regulated limits for ball weight), but just moving faster through the air. I understand that this is due to all the dimples and ridges on the surface of the ball (tripping a turbulent boundary layer, less pressure drag), but would this not be negated by fewer seams (i.e. less seams, less places to trip turbulent boundary layer, more pressure drag) ?

 

[ZapperZ]

Here, try this:

http://www.riken.jp/lab-www/library/publication/review/pdf/No_30/30_020.pdf

Zz.