New insights into turbulence

International research team explains the occurrence of fully turbulent flow for the first time

Turbulence does not just influence our comfort on flights, it also plays a key role in nature and in technology. It determines how exhaust fumes spread through the atmosphere or how efficiently fuel and air mix in combustion engines and it limits the transportation of liquids through pipelines. Researchers have been trying to improve our understanding of turbulence and how it occurs for over a hundred years. An international research group involving researchers from FAU has now made a significant breakthrough. In the latest edition of Nature*, the team describes for the first time how fully turbulent flow occurs in pipe and duct flows.

Although turbulence can occur in localised patches at low speeds, the majority of the fluid remains unaffected and flows in a straight line in what is known as laminar flow. However, this changes fundamentally when flow speed increases. The fluid then has a higher kinetic energy, causing turbulent patches to become stabilised and grow unstoppably. This eliminates all the laminar areas, causing all of the fluid to swirl. From this moment onwards, the flow is fully turbulent.

Researchers at IST Austria, FAU, the Max Planck Institute for Dynamics and Self-Organisation, and the University of Warwick observed this behaviour in experiments and high-resolution computer simulations. For the first time, the team were able to use a mathematical model to determine which of the forms of turbulence – localised or expanding – occurs at which flow speeds. The fronts that form between the laminar and turbulent areas and change their stability are crucial here.

‘Our findings regarding the origin of turbulence are an important starting point that will allow highly turbulent flows to be better understood in the future,’ says Prof. Dr. Björn Hof from IST Austria. In addition to its theoretical importance, the issue is also of practical relevance, as can be illustrated using the example of oil pipelines. Here the pumping costs – which run into millions – are largely due to friction loss caused by turbulence. ‘The insight that we have gained into the inner workings of turbulence are a step towards taming it,’ explains FAU professor Dr. Marc Avila from the Chair of Fluid Mechanics. ‘There are a few more hurdles to overcome before this can be achieved, but if it were possible to maintain laminar flow permanently, a large portion of current transport costs would simply disappear. Expanding the simulations to external flows such as those around a wing would have enormous potential and preparation work is currently being done.’

*doi:10.1038/nature15701

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