When Chaos Meets Chaos - Turbulent Entrainment from a Turbulent Background

湍流在自然界中非常丰富，可以通过树木和建筑物等物体后面的尾流来说明。尾迹通常是随着流体粒子向物体的下游移动而形成的。尾迹体积的增加必须伴随着从背景流体向尾迹的净质量传递。用于定义导致这种传质的所有物理过程的首要术语被定义为夹带。夹带可以分为吞没（大规模的波纹“吞没”大量的背景流体）和小规模的在尾流和自由流之间的界面上的蚕食。在最小的物理长度尺度上，由于粘度的作用而发生的咬痕参考了界面上涡度的扩散。尽管在了解非湍流环境下的夹带过程方面做了大量工作，但背景流体本身为湍流时的夹带在很大程度上尚未被探索和了解。该研究项目旨在通过研究湍流自由流到湍流尾流的夹带，为获得对控制湍流-湍流夹带（TTE）的物理学的普遍理解奠定基础并取得初步进展。TTE的应用是广泛的，因为大多数工业和环境流动存在于湍流背景中。最近发生在深水地平线的自然灾害就是一个很好的例子。海洋是一个高度动荡的环境，更好地了解TTE可能会导致更准确地估计石油泄漏的增长。进一步的研究可以导致更好的策略来限制任何此类灾难造成的损害。更好地了解TTE也可以为大城市的决策者提供重要的咨询资产；世界上几个城市的空气污染正接近令人担忧的水平。公共汽车等车辆的废气在湍流背景下释放，因此了解污染物如何在这种环境中分散会非常有帮助。这个项目的关键性质是非常明显的，潜在的应用确实可以非常广泛。在这个项目中，通过观察圆柱体后面的尾迹在湍流自由流中的行为，实验探索了湍流环境中的夹带。一个完全表征的湍流自由流通过使用几个湍流产生网格受到圆柱的影响，这些网格被专门设计为在网格下游产生不同水平的湍流强度(u)、积分长度尺度(L)和湍流动能耗散率(e)。目前文献中存在的少数研究表明，这三个参数中的每一个在单独意义上都很重要。虽然，对每个参数的影响及其各自的灵敏度没有达成共识。这主要是由于缺乏独立改变这三个参数的技术。然而，随着近年来空间填充分形网格的发展，我们能够做到这一点，并评估每个参数对TTE的单独影响。为此，在位于伦敦帝国理工学院的0.6m x 0.6m水槽中，采用同步粒子图像测速（PIV）和平面激光诱导荧光（PLIF）进行了实验活动。初步结果强调了自由蒸汽湍流（FST）影响夹带的两个主要机制。首先，对FST的近场效应与气缸脱落机理进行了研究。其次，我们的目标是确定FST中参数（u， L和e）对近尾流区域下游尾流夹带的直接影响。这种双管齐下的方法将为大大提高我们对TTE现象的理解提供基础。

Turbulent flows are very abundant in nature and can be illustrated by a wake behind objects such as trees and buildings. A wake typically grows as fluid particles move downstream of the object. This increase in wake volume must be accompanied by a net mass transfer into the wake from the background fluid. The overarching term used to define all physical processes that lead to this mass transfer is defined as entrainment. Entrainment can be classed into engulfment (large scale corrugations 'engulfing' a significant mass of background fluid) and small scale nibbling at the interface between the wake and the free-stream. Nibbling references the diffusion of vorticity at the interface, and occurs due to the action of viscosity at the smallest physical length scale. Despite a large amount of work going into the understanding of entrainment processes from a non-turbulent environment, entrainment when the background fluid is itself turbulent is largely unexplored and poorly understood. This research project aims to set the foundation and make initial strides into gaining a universal understanding into the physics that govern turbulent-turbulent entrainment (TTE), by examining entrainment from turbulent free-streams into turbulent wakes.Applications of TTE are vast as a majority of industrial and environmental flows exist in a turbulent background. The recent natural disaster involving Deepwater Horizon is a good example. The ocean is a highly turbulent environment and better understanding of TTE could have led to more accurate estimations of growth of the oil spill. Further research can lead to better strategies for damage limitation caused by any such catastrophes. Better understanding of TTE can also be a great advisory asset to policy makers in metropolitan cities; air pollution is approaching concerning levels in several cities around the world. Exhaust gasses from vehicles such as buses are released in turbulent backgrounds and therefore understanding how the pollutants are dispersed in this environment can be very helpful. The critical nature of this project is very apparent and potential applications can indeed be very extensive.In this project, entrainment from a turbulent environment is explored experimentally by observing the behaviour of a wake behind a cylinder when subjected to a turbulent free-stream. A fully characterised turbulent free-stream is subjected to a circular cylinder through the use of several turbulence generating grids that are specifically designed to produce varying levels of turbulence intensity (u), integral length scale (L) and turbulent kinetic energy dissipation rate (e) downstream of the grids. The few studies that are present in the current literature have shown the importance of each of these three parameters in a solitary sense. Although, there is no consensus on the influence of each parameter and their respective sensitivities. This is largely due to the lack of technology available to vary all three parameters independently. However, with the recent development of space filling fractal grids, we are able to accomplish just that and assess the separate effects of each parameter on TTE. To this end, experimental campaigns employing simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) have been conducted in a 0.6m x 0.6m water flume located at Imperial College London.Preliminary results have highlighted two main mechanisms by which entrainment is affected by free-steam turbulence (FST). Firstly, an investigation on near field effects of FST with regards to the shedding mechanism of the cylinder will be conducted. Secondly, we will aim to ascertain the direct effects of parameters, (u, L & e) in the FST, on entrainment into the wake downstream of the near-wake region. This dual-pronged approach will provide the basis to greatly enhance our understanding of the TTE phenomenon.