High Resolution, Non-intrusive Shear Stress Measurement in Fluvial Environments

在许多情况下，包括砾石床河流、陆上水流和部分填满的管道中的水流都是湍动的。一个关键问题是这种湍流对沉积物颗粒施加的动量和作用力，这些颗粒构成了许多自然河道的边界。目前，一个被称为剪切的空间平均参数被用来描述从湍流传递到泥沙沉积的动量/力。流体和床层之间的动量交换是一个关键的物理过程--能够理解这个过程将有助于理解沉积物和污染物如何移动，以及流动如何损失能量，从而确定流动深度。大多数河床是由多孔的、空间复杂的三维颗粒状沉积物组成的，因此动量的时空分布将控制水流和河床之间的污染物交换，以及个别泥沙颗粒是否会移动。目前，环境科学家只能以非常粗糙的方式测量边界剪应力，这些方法只能提供时间和空间平均测量结果，其中许多依赖于经验参数，而这些参数在当地尺度上是不可能确定的。该项目建议开发一种能够在颗粒尺度和频率上测量水流中边界剪应力的系统，该系统能够确定由观察到的湍流结构引起的边界剪应力的波动。该系统使用了最初由航空工程师使用的概念。该项目团队将使用新的化学方法来创造能够附着在天然沉积物上的薄涂层，可以直接测量剪应力。新涂层将包含手性向列相液晶(CLCs)，这种液晶会随着剪应力的变化而改变颜色。使用薄膜涂层，结合适当的照明和图像捕获技术，将意味着有可能首次测量受湍流深度限制的水处理砾石床中颗粒所受的时间和空间脉动力。

Flows found in many situations, including gravel bed rivers, overland flows, and in partially filled pipes, are turbulent. A key issue is the momentum and forces that this turbulent flow imparts on the sediment grains that make up the boundaries of many natural channels. Currently a spatially averaged parameter, termed shear, is used to describe the momentum/force that is transmitted from the turbulent flow into the sediment deposit. The exchange of momentum between the fluid and bed is a key physical process - being able to understand the processes will help understanding of how sediments and pollutants move, and how flows lose energy and so determine flow depths. Most river beds are composed of porous, spatially complex, three-dimensional granular deposits so the spatial and temporal distribution of momentum will control the exchange of pollutants between the flow and the bed and whether individual sediment grains will move. Currently environmental scientists can only measure boundary shear stress in very crude ways, which only provide time and space averaged measurements, many of which rely on empirical parameters that are impossible to determine at a local scale. This project proposes to develop a system that would be able to measure boundary shear stress in a water flow at a grain scale and at a frequency capable of determining the fluctuations in boundary shear stress caused by observed turbulent flow structures. The system uses a concept originally used by aeronautical engineers. The project team will use novel chemistry to create thin coatings capable of being attached to natural sediments that can measure shear stress directly. The new coatings will contain chiral nematic liquid crystals (CLCs), which change colour in response to changes in shear stress. The use of thin film coatings, combined with suitable illumination and image capture techniques, will mean that it is possible to measure, for the first time, the temporal and spatial fluctuating forces on grains in water-worked gravel beds subjected to turbulent depth-limited flows.