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

Lead Research Organisation: University of Bradford
Department Name: Sch of Engineering Design and Technology

Abstract

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.

Publications

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Description The project aimed to develop a novel measurement system, based on imaging of the chiral properties of liquid crystals, that would be able to measure remotely and directly the surface shear stress experienced by sediment particles in a water-worked sediment deposit at realistic Reynolds Numbers. The research made less progress than expected. Initially, liquid crystal coatings were applied to natural particles, however their variation in background colour meant that a simpler testing configuration had to be adopted. More extensive testing of a wider range of liquid crystal coatings were carried out using flat black targets. The geometric configuration of the targets meant that a simpler hydraulic experimental configuration was used. This comprised of a tilting Perspex duct of a uniform roughness and no free surface. The enabled different time-average boundary shear stresses to be generated on the surface area of the liquid crystal target that was placed on one of the walls of the duct. Given the more extensive range of liquid crystal mixtures tested and the results obtained only steady state time averaged hydraulic measurements were conducted, no turbulence based measurement was carried out. However a liquid crystal coating was developed that was sufficiently resillience to water to remain and was sufficiently sensitive for time averaged boundary shear stresses to be measured.
Exploitation Route This was a feasibility study and demonstrated that liquid crystal coatings could be developed that could be used to remotely measure boundary shear stress under moving water.
Sectors Environment
 
Description This award was to test the feasibility of using liquid crystal coatings to measure boundary shear stress in hydrauic environments. The findings have not been used outside the research team.