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

这个项目解决了一个基本的问题，如果剪切流中的湍流无限期地存在或在有限时间后衰减，剪切湍流是瞬变的前景是从工程和应用数学到天文学、地球物理和生物系统的各种领域都非常感兴趣的。湍流研究中的普遍共识是，一旦流动变成湍流，并且如果流速保持不变，它将一直保持湍流。然而，在最近的一次管流实验中，有证据表明，在比流动的固有时间尺度大得多的时间尺度上，湍流会衰减。在这里，我们发现湍流的寿命随流速呈指数增长。由于在管道和渠道中，湍流区以平均速度向下游移动，所以即使在非常长的设置中，调查也仅限于相对较短的观测时间。我们在这里设计了一种新的实验方法，使我们能够观察更长时间的湍流结构，并以以前不可能实现的方式测试湍流的稳定性。在石油和天然气管道等应用中，可以利用有限寿命的湍流来控制湍流，并更有效地运行这些流动。

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.