Transient or permanent? Is turbulence in shear flows a sustained state?

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This project addresses the fundamental question if turbulence in shear flows persists indefinitely or if it decays after a finite time The prospect of shear turbulence being transient is of great interest to a variety of areas ranging from engineering and applied mathematics to astronomy, geophysics and biological systems.The common consensus in turbulence research is that once a flow has become turbulent, and provided the flow rate is kept constant, it will remain turbulent for all times. However in a recent experiment in pipe flow evidence has been found that, on a timescale which is very much greater than the intrinsic timescales of the flow, turbulence decays. Here the lifetime of turbulence has been found to grow exponentially with the flow rate. Since in pipes and channels the turbulent region travels downstream at the mean velocity, investigations even in very long set ups are limited to relatively short observation times. We here devise a novel experimental approach that allows us to observe turbulent structures for far longer times and to test the stability of turbulence in ways previously not possible. A finite lifetime of turbulence could be exploited in applications such as oil and gas pipelines to control turbulence and to run these flows much more efficiently.