How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 527

Why are there dimples on golf balls? - DM
If there were no turbulence around a golf ball as it moved through the air, there would be regions of slow-moving high-pressure air in front of it and behind it, and regions of fast-moving low-pressure air around its sides. Because of their symmetry, these pressures wouldn't exert any overall force on the golf ball and it would fly through the air without experiencing any air resistance. But there is turbulence behind a moving golf ball and this turbulence spoils the high-pressure region behind the ball. Since there is less high-pressure behind the golf ball to push it forward, the ball experiences a backward force—the slowing force of pressure drag. The size of this pressure drag force is roughly proportional to the size of the turbulent wake.

The size of the turbulent wake depends on the airflow behind the ball. On a smooth ball, air flowing into the rising pressure behind the ball experiences friction with the ball's surface and loses energy. This surface air soon reverses its direction of flow, triggering a large turbulent wake. A golf ball's dimples complicate the airflow very near the ball's surface so that new, rapidly moving air is able to flow in close to the ball's rear surface, where it can delay the onset of the flow reversal. The turbulent wake that eventually forms is relatively small, so that the golf ball experiences less pressure drag than a smooth ball. That's why a golf ball can travel so far before slowing down.

         

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