# Fluid mechanics & pitching question.

In NY Times “Play” magazine from this week, they have an interview with Mike Mussina. He talks about getting a scuffed ball.

Three statements.

All true?

In that scenario, I’m imagining a smooth flow over the right side, and a turbulent flow over the left side. But, it’s not immediately obvious to me that that would push the ball right. It almost seems like the ball would want to move “into the turbulence”, like that would be where there is less resistance.

As if the scuff were acting like “drag”.

Correct effect, wrong explaination.

On a smooth round body, laminar flow occurs at the front. At some point the air flow suddenly detaches from the body, and large scale tubulence occurs. If small scale turbulence is induced into the laminar flow prior to the break-away location, then the air maintains near laminar flow for a much longer distance. This is known as “tripping the boundary layer”

The scuffing trips the boundary layer, causing the flow to stay attached to that side of the ball longer, thus the air flows farther around the left side of the ball, and ends up moving rightward after the balls passage. The reaction to this rightward moving air causes the ball to be deflected leftward.

Another way of looking at this is to say that the left side is more streamlined than the right side. Thus the right side throws a wider wake than the left, pulling the ball leftward. (because more air is being deflected to the right). YES, it is counter-intuative that a slightly rough surface has less drag than a smooth surface, but even navel archetects are finally learning this.

I assume the ball is thrown with topspin or backspin, yes? If it were rotating on a vertical axis wouldn’t the scuff be on either side the same amount of time, more or less?

A golf ball provides an example (flies further because of its dimples).

New developments in plastic surgery?

So, Kevbo – are you saying that I’m correct, and Mussina is wrong. . .at least in the effects?

Contrapuntal – that’s what he made it sound like. Like he was just throwing the ball with the scuff on the left. He didn’t mention imparting any spin around the vertical axis. I’m not sure what effect that would have, the scuff moving “into the wind” on one side, and “with the wind” on the other side. Perhaps that could change things.

Golfers learned this back in the day of the gutta-percha ball. One that had been nicked by use went furtner than a new ball.

The drag curve for smooth and roughened balls track pretty close as a function of Reynolds number out to some critical value of the Reynolds number. At that point the drag of the roughened surgace drops suddenly, sometimes by a factor of three.

If you can hit a golf ball hard enough to reach that Reynolds number (which I can no longer do, sob) your distasnce increases remarkably.