Turbulence

Turbulence

TDC

KEVIN CAMERON

TURBULENCE IS DEFINED AS A Motion of fluids in which local velocities and pressures fluctuate irregularly. When I was a small boy, I stared at smoke rising from a parent’s cigarette. At first, the stream of smoke rose straight up, accelerating. Then, having risen smoothly through some distance, the stream went crazy, oscillating from side-to-side, mixing itself with the surrounding air.

The straight stream was laminar flow. A “lamina” is a layer, and in the straight stream, the fast-moving inner stream was surrounded by slower and slower moving layers of air being pulled along by friction with that inner stream.

Then something happened to disturb all that smoothness, and once disturbed, there was no restoring force to push it back toward smoothness and stability. The result was turbulence.

I worked in a lab that filmed classic fluid-flow experiments for teaching use. One of these was the work of Osborne Reynolds (1842-1912). A horizontal, straight cylindrical tube of glass is provided with a smooth bellmouth opening, and a clear fluid, such as a glycerin-water mixture, is caused to flow steadily through it at various velocities. A much smaller tube, terminating at the center of this bellmouth, delivers a red liquid at the same velocity. As with the cigarette smoke, the red fluid at lower speeds travels down the center of the larger tube in straight-line fashion. This is laminar flow. At some higher velocity, the dyed fluid flows straight for a distance and then suddenly appears to fill the larger tube. When Reynolds observed this flow with spark photography, he could see that the initially smooth flow in the tube had broken up into rapid, complex oscillations.

Turbulent flows lie behind several processes of importance to motorcyclists. Foremost is combustion in the cylinders of engines. A motionless, chemically correct mixture of gasoline and air, ignited by a spark, burns at about one foot per second. If that were how fast it burned in a lOOOcc superbike engine turning 12,000 rpm, the engine would make 25 revolutions in the time it took the flame to travel from the central sparkplug to the cylinder wall an

inch-and-a-half away. That’s obviously not practical. What makes these engines practical is turbulent flame propagation—the kind of violent mixing motion I saw in the cigarette smoke and that Reynolds saw in his glycerin experiment in the 1870s.

When fuel sprays out of a 12-hole fuel injector, the tiny jets of fuel drag layers of the surrounding air along with them. Just as in rising cigarette smoke, tiny irregularities lead to instability, and the smooth, fast-moving fuel column oscillates, stretches and breaks into “sausages,” which whirl and break again into fragments whose surface tension pulls them into droplet form. The droplets evaporate fuel vapor that is stripped away by the vigorously turbulent flow in the intake port. The instability of turbulent motion is thus essential to the mixture formation process.

Turbulence creates a great amount of the aerodynamic drag that consumes so much power at high speed. As the vehicle moves through the air, the flow over its surfaces is at first streamline, almost laminar. Then something happens, and the flow oscillates locally and separates from the surface. An ideal streamlined body, like that of a trout, has low drag because the smooth attached flow exerts nearly as much pressure on its after surfaces as it does on its forward ones. If fore-and-aft pressures were equal, drag would be zero—an attractive idea. But in practice, it’s very hard to keep the flow attached to those after surfaces (and a motorcycle, as short as it is, usually has no tapering after surfaces of the kind that fish do). Oscillations develop in the air flowing over the vehicle, the flow separates from the surface and its

pressure energy (which was helping to “push” on the vehicle’s after surfaces) is converted into the random velocities of turbulence. It is, I am tempted to say, a drag.

Engines of every kind (yes, that includes electric motors!) generate waste heat, which must be actively removed to keep the temperatures of the hottest parts under control. As coolant moves through water passages, low velocity and smooth surfaces can result in a situation in which the flow is close to laminar. This means that the slow-moving layer close to the hot metal surface gets very hot, the next layer out from it, moving a bit faster, is less hot and so on until, in the fast-flowing main body of liquid, little heat is present. The laminar flow acts as effective insulation by preventing mixing between the layers.

This situation occurred in aircraft oil coolers during operation at high altitude where the air is very cold. Hot oil flowed along the metal surfaces inside the cooler, giving up its heat quickly. In the next layer of hot oil, less heat was transmitted until, in the middle of the flowing stream, hot oil flowed without being cooled at all. The equipment being cooled—a hard-working piston engine, for example—overheated and alarms were triggered. Engineers investigating this found that oil passages in the cooler worked better if corrugated. This caused the flow to oscillate, mixing the cooled layers near the cooler’s metal surfaces with the hotter core flow until this mixing produced a uniformly cooled stream of oil ready to be recirculated through the machinery.

Early liquid-cooling systems had large water passages, through which flow was sluggish with near-stagnant regions. This seemed intuitively right, for it was easy to assume that putting plenty of water around hot parts just had to do a good cooling job. But without the vigorous mixing of high turbulence, the water next to the hot surfaces glided along without mixing with the layer next to it. It therefore got hotter and hotter until it boiled into steam, pushing coolant out through the pressure relief valve. For many years, coolant passages in engines were afterthoughts, designed by the foundry department. Large passages were easier to core and locate in molds than small, high-velocity passages. Today, thinner coolant passages create the turbulence that carries heat away most effectively.

Turbulence makes me think of daily life, but that’s another subject entirely. O