Turbulence not just affects our well-being during flights; it also plays a central role in nature and in technology: it influences how pollutants spread in the atmosphere, how efficiently fuel and air mix up in combustion motors, and it limits the transport of liquids in pipelines, to give just a few examples. As a result, researchers have been trying for over a hundred years to better understand how turbulence first arises. Important progress has now been achieved by physicist Björn Hof, professor at IST Austria, and his colleagues. In the current edition of Nature, the team describes for the first time how a fully turbulent flow arises in pipe and square duct flows.
Although turbulence can already appear at lower speeds in localized patches, a large part of the fluid remains unaffected and continues to flow in a well ordered (laminar) fashion. This changes however at larger flow rates. The fluid now possesses higher kinetic energy, thereby stabilizing the turbulent patches which now continuously grow. As a result, all laminar areas are absorbed and the entire flow is transformed into chaotic eddying motion. Fully turbulent flow is now the natural state of the system.
Björn Hof and colleagues from the Max Planck Institute for Dynamics and Self-Organization, the University Erlangen-Nürnberg as well as the University of Warwick could observe this behavior in experiments and high-resolution computer simulations. For the first time, the team was able to verify with a mathematical model which state (i.e. localized or continuously spreading turbulence) arises at which flow rates. A decisive role is played by the fronts that appear at the boundaries between laminar and turbulent patches and change their stability.
"Our findings regarding the onset of turbulence are an important starting point to eventually obtain a better understanding of highly turbulent flows," says Björn Hof. The specific example of oil pipelines shows that this question is not just interesting from a mere fundamental angle: Pumping costs amount to billions of dollars--a fact that is mainly owed to the friction losses resulting from turbulence. Hof adds, "A transformation into a laminar flow could reduce the friction loss by often more than 90% and would thus result in very substantial energy savings."
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