Turbulences under control

Researchers calm down turbulent flows while saving energy

A temporal sequence of cross sections in the tube, whereby the image section moves with the flow. The initially turbulent flow (colored markings) gradually becomes laminar. © MPI for Dynamics and Self-Organization
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Turbulent currents are energy eaters: they often consume more than ten times as much energy as a calm current of the same speed. Scientists have now succeeded in deliberately ending such turbulence at low speeds. The special feature is that the new method now presented in "Science" requires so little energy that energy can be saved for the first time overall.

Gleeful and bubbling like a mountain stream, turbulent currents make their way. Part of the energy that drives the flow loses itself in whirls and eddies. Whether oil is pumped through a pipeline or water through a municipal service pipe - the turbulences often consume more than ten times as much energy as a calm current of the same speed. The situation is quite different with a calm, laminar flow. Here alone the friction is responsible for energy losses. "Calming turbulent flows is therefore of great interest to many industrial applications, " explains Björn Hof of the Max Planck Institute for Dynamics and Self-Organization.

The scientists from the Max Planck Institute in Göttingen, together with colleagues from Harvard University in the USA, have now succeeded in deliberately ending such turbulence at low speeds. In their experiments, the researchers first turned to a special case: in a twelve-meter-long glass tube with a diameter of only three centimeters, they created a turbulent vortex that moved downstream with the otherwise laminar flow of water. Tiny particles in the water, which a laser lit up brightly, allowed to follow the movement of the water exactly.

Rear eddy area crucial

It was found that the velocity distributions in the turbulent and laminar sections of the flow are very different: While the laminar flow flows slowly at the tube edge and very fast in the middle, the turbulent flow is relatively fast on the edge as well. "In order to destroy the turbulent vortex, especially its rear boundary to the laminar flow is crucial, " explains Hof. "Because at this point, the turbulence that drives the turbulence arises."

In computer simulations, the researchers changed the velocity distribution over a short period of time at this point: in the middle of the glass tube, they slowed the flow and accelerated it at the edge, while the flow rate remained constant. In this way, there is not enough energy left in the middle of the pipe to effectively "push" the turbulence. The result: The turbulent disorder breaks down - and remains gone. display

Checkpoint in the glass tube

But how can the flow be manipulated in the same way in an experiment? The solution to the problem sounds paradoxical at first. Because the scientists extended the glass tube around a kind of "control point", at which they produced purposefully additional vortexes in regular succession. The trick: If these vortices follow each other closely enough, the subsequent vortex with its turbulent velocity distribution influences the rear boundary area of ​​its predecessor in a manner similar to computer simulation.

The flow velocity in the middle of the tube decreases and the vortex swirls. At the checkpoint, a chain of vertebrae is created, of which the subsequent one always robs its predecessor of the necessary energy. Vortexes that are further upstream due to other influences are also intercepted at the control point in this way. Beyond the control point, the flow is thus completely laminar. The situation in the glass tube is thus comparable to a regatta, in which at a certain point - the control point - constantly new sailboats go into the race. Finally, the boats are so close behind each other, that the respective Hintermann takes the wind out of the sails of the front man and the race comes to a complete standstill. In this way, the route beyond the checkpoint remains free of boats.

A fifth of the energy saved

"Because our process only starts in one place, it requires little energy, " says Hof. The researchers use only a fifth of the energy that they could save on the whole. In earlier experiments, the G ttingen researchers had already shown that at moderate flow velocities over time any turbulence would be destroyed even without external intervention. However, this can often take many years. Because in principle only the laminar state is stable, a small "push" suffices to purposefully deworm the flow, "says Hof. In future experiments, the researchers now want to expand their method to extensive turbulence.

(Max Planck Society, 19.03.2010 - NPO)