Analysis of troubled water

Turbulence: Computer models help scientists to examine the secret life of rushing fluids.

Medicine & Science

September 13, 2004|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Have you ever noticed, when you're shaving, how the water comes out of the tap in a nice, smooth stream at first? But when you turn the water up to rinse your razor, it comes out in a turbulent jumble.

Do you ever wonder why?

Well, scientists do, and it's more than idle curiosity. They say turbulence also happens inside oil pipelines, and along the skins of airplanes, and in ships and cars, too. It slows things down and costs us plenty.

So engineers, physicists and mathematicians in England, the Netherlands and Germany have taken another look at pipe turbulence. They peered inside pipes and reported their findings in the journal Science.

This isn't a new question. In 1839, a German physicist, Gotthilf Heinrich Ludwig Hagen, noticed that fluids came out of pipes in two ways - one smooth and one turbulent.

Nearly 50 years after that, an Irishman named Osbourne Reynolds worked out a formula to predict when the flow of water - or beer or chocolate syrup - would switch from smooth (or "laminar") to turbulent.

The formula ties the turbulence to the diameter of the pipe and the thickness and velocity of the fluid. Reynolds also noticed that once the turbulence started, the fluid dragged. It took more force to get it through the pipe.

If you're an oil company moving crude in pipelines, turbulence means you need more pumping stations and pay more to run them. If you're an airline, air turbulence on wings means you spend more on jet fuel.

Scientists know that turbulence happens. "But we don't understand in detail why it happens," said BjM-vrn Hof, a research associate at the Delft University of Technology in the Netherlands. He led the European team that tackled the problem - one of the oldest riddles in fluid dynamics.

Other researchers had already made some progress simulating turbulence with powerful computer models. "You can actually show mathematically where it becomes unstable," said Hof. "It goes in steps and becomes more and more complicated, and eventually goes turbulent."

But in real-world experiments with pipe flow, he said, "you don't have well-defined, nice steps. Rather, it happens suddenly or all at once, and you don't really know when it's going to happen."

Scientists needed to see things happening for themselves, inside the pipes. So, the group led by Hof figured out how to watch water move inside a 1.5-inch pipe. Rigging up a laser, a mirror and really fast cameras, they took pictures of tiny particles in the water as they flowed down the tube.

The pictures showed that when the water reached a certain speed, it began to break into eddies, waves and cross-currents. Some were fast, and some were slow. They moved down the pipe and shifted in mostly unstable, unpredictable ways. And that slowed things down inside the pipe, something like New York City traffic.

The important thing is that the pictures looked a lot like the computer simulations. And that means the hydraulic engineers and computer modelers are getting the math right. They're starting to understand turbulence. Someday, they might find new ways to reduce it.

That could make oil, and a lot of other things, flow faster and cheaper.

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