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

剪切湍流的瞬态特性的前景引起了工程、应用数学、天文学、物理学和生物系统等领域的极大兴趣。湍流研究中的普遍共识是，一旦流动成为湍流，并且保持流速恒定，它将永远动荡不安。然而，在最近的管流实验中，已经发现，在比流动的固有时间尺度大得多的时间尺度上，湍流衰减。在这里，湍流的寿命已被发现与流量呈指数增长。由于在管道和通道中，湍流区域以平均速度向下游移动，因此即使在非常长的设置中，调查也仅限于相对较短的观察时间。我们在这里设计了一种新的实验方法，使我们能够观察湍流结构更长的时间，并测试湍流的稳定性，以前不可能的方式。在石油和天然气管道等应用中，可以利用湍流的有限寿命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.