For flying, airplanes usually need as smooth a surface and an aerodynamic fuselage as possible, while golf balls are just the opposite, with more than 300 small dimples on their surface. Why are golf balls designed this way?
The "rough surface" allows golf balls to fly higher and farther. Among the various ball sports popular in the world today, golf can be considered a "veteran", with a history of 500 to 600 years. The earliest golf balls were made of wood, and later they were sewn with leather and filled with feathers. However, this type of ball had a major drawback—when it was hit into water or wet by dew, its weight would change significantly. Starting from 1845, smooth round balls made of rubber or plastic began to appear, which had the advantage of not increasing in weight when wet by water. However, another major problem was discovered—the distance the ball flew after each hit was greatly reduced. Although the original leather balls were relatively rough, they flew farther. Why did the surface becoming smoother result in a shorter flight distance? It was later found that the design of the ball's surface played a crucial role!
Initially, air was considered to be non-viscous, or frictionless. Scientists call this non-viscous fluid an ideal fluid. Theoretically, objects flying in an ideal fluid should not encounter resistance. However, this completely contradicts the observed phenomenon—the resistance objects encounter when flying in the air is absolutely not negligible. The French scholar D'Alembert was the first to recognize this contradiction, so this phenomenon is also known as the "D'Alembert Paradox". The study of golf ball flight distance can be said to have started from this point.
When the golf ball flies at a low speed in the air, the fluid molecules near the ball basically flow along the ball's surface. At this time, the resistance the ball encounters is mainly due to the viscosity of the fluid. However, when the ball flies at a high speed, a symmetrical vortex is formed behind the ball. After the fluid molecules near the ball travel a certain distance, they detach from the ball's surface, a phenomenon also known as boundary layer separation. Boundary layer separation always occurs simultaneously with vortices. At the center of the vortex, the fluid pressure is lower. If an object is located around a large ocean vortex, it will be sucked into the vortex. This has been depicted in many disaster movies. Behind the ball is a vortex with very low central pressure, while the pressure in front of the ball is higher. Due to this pressure difference, the ball experiences a large amount of resistance. Generally speaking, the higher the ball's flight speed, the earlier the boundary layer separation occurs, and the larger the vortex area formed behind the ball, resulting in a greater amount of resistance caused by this pressure difference. Fundamentally, the phenomenon of boundary layer separation in flying balls is also due to the viscosity of the fluid.
On the smooth surface of the ball, due to the early occurrence of boundary layer separation and the resulting large pressure difference between the front and back of the ball, the resistance is very large. Therefore, after the golf ball was changed from leather to plastic, its flight distance was greatly reduced. For this reason, people had to make the golf ball "rough", that is, covered with small round dimples. When the golf ball flies, small vortices will be generated near the dimples. Due to the attraction of the fluid molecules near the golf ball by the vortices, the boundary layer separation point is postponed for a long time. Therefore, the vortex area formed behind the ball is much smaller than that formed behind the smooth ball, greatly reducing the resistance caused by the pressure difference between the front and back of the ball. The maximum flight distance of a smooth golf ball is only a few dozen meters, while the maximum flight distance of a "rough" golf ball can reach more than 200 meters. It can be seen that the drag reduction effect of the "rough" golf ball is very obvious.