The boundary layer is the region of fluid whose velocity is less than 99% of unobstructed fluid flow. It separates laminar and turbulent flow zones, with laminar flow being smooth and predictable, and turbulent flow being random and violent. Golf balls have dimples to reduce air friction in high-speed flight.
A boundary layer occurs when a fluid flows past a fixed surface. It is typically defined as the region of fluid whose velocity is less than 99% of unobstructed fluid flow. In other words, it is the area of a moving fluid that is slowed down by more than 1% from a stationary surface. The boundary layer was defined to better understand fluid mechanics by dividing the flow into two regions that exhibit different behavior. Regions inside and outside the boundary layer also generate friction in different ways.
One of the first problems in aerodynamics research was solving the complex Navier-Stokes equations, which are believed to govern fluid flow. There are many cases where the solutions of the Navier-Stokes equations are not known. It was noted, however, that the fluid flow exhibited two general modes of behavior: laminar and turbulent. Laminar flow is a smooth and predictable flow, like that of a pellet falling into honey. The turbulent flow is random and violent, like that coming out of a fire hose.
The boundary layer separates these two fluid flow zones. Within the boundary layer, the flow is mainly laminar. In this region, the flow behavior is dominated by viscous stresses. Viscous stress is directly proportional to the speed of a passing object; a highly viscous fluid, such as honey, imposes a lot of friction on objects moving rapidly through it. Laminar flow is characterized by fluid flowing in parallel lines without irregularities.
Outside the boundary layer, fluid flow is predominantly turbulent. Turbulent flow, whether in a liquid or a gas, exhibits similar behavior. The chaotic variations in particle speed and direction make accurate predictions impossible with current knowledge. The effect of friction in turbulent flow is also different from laminar flow. Friction is generally no longer proportional to the fluid velocity in the turbulent regime.
The reason golf balls have dimples is related to the boundary layer of air. At low speeds, such as when putting, a perfectly spherical golf ball would have no major problems with air friction. During high-speed flight, however, spherical golf balls would have a larger boundary layer than dimpled balls, which would mean more air flowing laminar. This laminar flow would actually cause more air friction than a turbulent flow. Dimpled golf balls fly farther than their spherical counterparts because they have a smaller boundary layer and don’t experience as much friction with the air.
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