Streaking. The observation of streaky swirling waves bolsters a theory of turbulence in pipes.

Tackling Tubular Turbulence

For more than a century, scientists have puzzled over how the smooth flow of a liquid through a pipe suddenly erupts into a chaotic tangle of eddies and swirls. Now, a team of physicists, engineers, and mathematicians has spotted the building blocks of turbulence in tubes.

Researchers know how turbulence develops in a few special cases. For example, the flow of fluid trapped between two concentric cylinders grows more complicated in well ordered steps as the outer cylinder rotates about the inner one faster and faster. But they've struggled to explain turbulence in a simple pipe, in which all hell breaks loose as soon as the speed passes a critical value.

Since 1990, however, theorists have used computer simulations to piece together a novel explanation of turbulence in pipes. According to the simulations, a fluid in a pipe can flow in various complicated swirling waves. These waves are unstable, so at low flow speeds they quickly die out and the liquid resumes its smooth, uniform flow. But once the flow exceeds a certain speed, it no longer returns to the smooth state when a wave dies out. Instead, the flow bounces from one swirling state to another, producing turbulence.

Now, researchers have spotted the type of waves the models predict. Physicist Bjorn Hof and engineer Casimir van Doorne of Delft University of Technology in the Netherlands and colleagues sent water through a long pipe, 4 centimeters in diameter, that was designed to suppress turbulence. A jet mounted on the side of the pipe disturbed the flow and a laser and cameras 6 meters downstream tracked the motion of beads in the water.

As the flow speed notched up, the researchers looked for signs that the jolt from the jet had created one of the fleeting waves. They observed vortices that shoved faster water toward the sides of the pipe and slower water toward the center, creating long streaks of slower and faster moving water stretching along the pipe. The patterns of streaks closely resembled those predicted by the simulations for various flow speeds, they report in the 10 September Science.

The results are a clear step toward explaining turbulence in pipes, says Alessandro Bottaro, an engineer at the University of Genoa in Italy. “They are justifying the theoretical work that has been done in the past 15 years,” he says. But Tom Mullin, a physicist at the University of Manchester in the United Kingdom, notes that the patterns seen in the experiments and the simulations aren't exactly the same: “It's a long way yet to show that there's a complete connection.”

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