In Physics, what is a Boundary Layer?

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 the unimpeded fluid flow. In other words, it is the zone of a moving fluid that is slowed down more than 1 percent by a stationary surface. The boundary layer has been defined to better understand fluid mechanics by dividing flow into two regions that display different behavior. Regions inside and outside of the boundary layer also generate friction in different ways.

An early problem in aerodynamics research was solving the complex Navier-Stokes equations, which are believed to govern the flow of fluid. There are many cases where the solutions to the Navier-Stokes equations are not known. It was noticed, however, that fluid flow exhibited two general modes of behavior: laminar and turbulent. Laminar flow is smooth and predictable flow, like that of a ball falling through honey. Turbulent flow is random and violent, like that coming out of a fire hose.

The boundary layer separates these two zones of fluid flow. Inside the boundary layer, the flow is primarily laminar. In this region, flow behavior is dominated by viscous stresses. Viscous stress is directly proportional to the velocity of a passing object; a highly viscous fluid, like honey, imposes much friction on objects moving quickly through it. Laminar flow is characterized by fluid flowing in parallel lines without irregularities.

Outside of the boundary layer, fluid flow is dominantly turbulent. Turbulent flow, whether in a liquid or gas, shows similar behavior. Chaotic variations in speed and direction of particles make precise 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 fluid velocity in the turbulent regimen.

The reason golf balls have dimples in them is related to the boundary layer of air. At low velocities, such as during putting, a perfectly spherical golf ball would not have much of a problem with air friction. During high-velocity flight, however, spherical golf balls would have a larger boundary layer than dimpled balls—which would mean more air is flowing by in the laminar way. This laminar flow would actually cause more air friction than a turbulent flow would. Dimpled golf balls fly further than their spherical counterparts because they have a smaller boundary layer and do not experience as much air friction.