Common wisdom has long held that once a flow of a fluid has become turbulent, that turbulence would persist.

But researchers at the Institute of Science and Technology Austria (IST Austria) have shown that this is not the case. In their experiments, published in Nature Physics, they destabilized turbulence in a pipe so that the flow turned to a laminar (non-turbulent) state. They observed that the flow remained laminar thereafter.

The researchers say that eliminating turbulence can save as much as 95 percent of the energy required to pump a fluid through a pipe.

Everything from water to oil and natural gas is transported across the globe through pipes. The amount of energy used to pump these fluids adds up to around 10 percent of the global electricity consumption.

A major part of these energy losses is caused by turbulence, a phenomenon that leads to increased frictional drag that requires more energy to pump the fluid. Previous approaches have aimed to locally reduce turbulence levels. The research group at IST Austria took a different approach -- rather than temporarily weakening turbulence, they destabilized existing turbulence so that the flow automatically becomes laminar.

The researchers say that in a so-called laminar flow, a fluid flows in parallel layers which do not mix. The opposite of this is a turbulent flow, which is characterized by vortices and chaotic changes in pressure and velocity within the fluid.

Most flows are turbulent, such as the smoke of an extinguished candle. In pipes, both laminar and turbulent flows can, in principle, exist and be stable. But a small disturbance can make a laminar flow turbulent. Turbulence in pipes was until now assumed to be stable, and efforts to save energy costs focused on reducing its magnitude but not to completely extinguish it. In their proof of principle, the research group showed that this assumption is wrong, and that a turbulent flow can be transformed to a laminar one.

Velocity Profile Is Key

The secret lies in the velocity profile -- that is, in the variation of the flow velocity when looking at different positions in the pipe's cross section.

The flow is fastest in the middle of the pipe while it is much slower near the walls. By placing rotors in the flow that reduced the difference between the fluid in the center and that close to the wall, the researchers managed to obtain a "flatter" profile. For such flow profiles the processes that sustain and create turbulent eddies fail and the fluid gradually returns to smooth laminar motion and it remained laminar until it reached the end of the pipe.

Another way to obtain the flat velocity profile was to inject liquid from the wall. Yet another implementation of the idea of a flat velocity profile was a moving part of the pipe -- by moving the walls quickly over a stretch of the pipe, they also obtained the same flat profile that restored the laminar flow.

The group has already registered two patents for their discovery. And the concept has been proven for relatively small velocities. However, for use in pipelines, an application working at larger velocity will be necessary.