Turbulent flows moving over a porous, permeable, surface are ubiquitous in many environmental and industrial flows, such as in rivers, over vegetated canopies, in heat exchangers, membrane tube reactors and hydrocarbon wells. However, despite the widespread importance of these flows in many branches of science, the physics of turbulence across permeable boundaries remains poorly understood, and currently hampers our ability to predict the movement of gases, particles, microbes and pollutants into and out of such surfaces. This project will develop and utilize the latest techniques in optical flow measurement to provide unique new data on the structure of 3D flow above and within permeable surfaces, and critically the interactions between these flows. The project is expected to transform the understanding of surface-subsurface turbulent flow interactions, and enable testing of several key questions concerning the nature of turbulence over such surfaces, and how this flow structure changes as the type of permeable bed is altered and the overlying flow velocity is varied. Laboratory experiments will match the refractive index of the flow and sediment, thus permitting optical measurements to be achieved above and within permeable sediment beds. Additionally, new plenoptic camera techniques will be used for the quantification of three-dimensional flow within a 3D volume, both within and above the permeable surface. The combination of these techniques opens up the possibility of quantitative information that has never been gained before within such complex flows. The project will enable the training of two PhD students in state-of-the-art experimental techniques within a collaborative and interdisciplinary research environment. These unique data will allow development of new conceptual models for flow over permeable surfaces and provide essential data to guide subsequent numerical modelling of these flows. These data will be used to explain how turbulent flows moving over a range of permeable surfaces may be modified by the permeability, and thus examine the fluid dynamic controls on the fate of fine particles moving across the fluid-sediment interface. For instance, these results will be of direct relevance in interpreting the behavior of natural river beds, where gravels can become infilled with fine-grained sediment, and which can control the exchange of nanoparticulates and contaminants between surface and subsurface flows.
