(March 2012) It is understood among pilots that laminar flow is something good. But exactly what laminar flow looks and feels like eludes us, because air is invisible and so we never see the difference or the transition between laminar and turbulent.
One common example of laminar flow in everyday experience is smoke rising from a just-extinguished candle. (Candles surpassed cigarettes in popularity as journalistic laminar-flow analogues late in 2008.) If the air is still, the smoke rises for a few inches in a perfectly smooth column. This is the laminar portion: The paths of all of the tiny “packets” of smoke — packets being imaginary volumes of very small size, but significantly larger than individual molecules — are parallel.
Then, suddenly, the flow breaks apart and the column becomes unsteady and several times wider than before. This is the turbulent state: The paths of packets are no longer parallel, but eddy and swirl from side to side within the rising column.
The problem with the rising-smoke example is that it does not involve a surface, and so it is difficult to intuitively relate it to flow over the wing or fuselage of an airplane. Nor does it tell us anything about why turbulent flow is draggier than laminar.
The other day I noticed an instance of laminar transition that bears a bit more resemblance to flow on an airplane. I was rinsing the bottom of a pot, which, because the pot was quite tall, was within half an inch of the faucet, so that water flowed smoothly onto it without splashing and spread out radially in all directions. A few inches from the faucet was a plainly visible line beyond which the water’s appearance changed. Inside that arc, water spread smoothly outward, like combed hair; beyond it, it appeared bumpy, tremulous and, in short, turbulent.
You might expect the transition line to move in or out as you open or close the faucet. It doesn’t, or at any rate it moves very little, until you reduce the flow almost to a trickle. At that point, the transition line disappears and the flow becomes entirely laminar.
The reason the drag is greater in the turbulent region has to do with the nature of drag itself. (It’s convenient to think of the airplane as standing still, like the pot in the sink, and of the air as blowing past it.) The airplane’s surface slows down air close to it, forming a thin layer, called the boundary layer, at the bottom of which is an infinitesimally thin layer that adheres to the surface and does not move at all.
The word laminar means layered, because a laminar fluid resembles a stack of thin lubricated films. At greater and greater distances from the skin, the layers slide faster and faster, until you arrive at the “free stream” outside the boundary layer. The laminar boundary layer is paper-thin at the leading edge of the wing, and grows to perhaps an eighth of an inch in thickness on, say, a smooth sailplane wing.