MICRO-PIV TECHNIQUE FOR DIRECT MEASUREMENT OF FLUID FLOW VELOCITY IN ROUGH-WALLED FRACTURES AT HIGH REYNOLDS NUMBER

Direct measurement of flow structure and fluid flow velocity in rough-walled rock fractures are of importance in geological and environmental sciences, such as risk assessment of geologic storage of radioactive waste, the formation of hydrothermal ore deposits by magmatic fluids, and groundwater management and remediation. Nevertheless, difficulty in directly observing fluid flow and solute transport phenomena actually occurring within microscale rough-walled fractures has led only to speculative studies and concepts, e.g., the diffusive mass transfer processes between mobile and immobile fluid flow zones. Numerical studies indicated that tailing, due to a large eddy, was more persistent with an increase of the Reynolds number (Re) up to about 10, but another numerical study presented the opposite result that the velocity transient fluctuation in an idealized sinusoidal-walled tube channel at Re of 449 decreased the amount of tailing in eddies. The opposing results demand more direct observation (or measurement)-based-studies for a fundamental understanding of the role of mobile and immobile zones in fluid flow and solute transport in rough-walled fractures. Fluid flow measurement with our previous micro Particle Image Velocimetry (micro-PIV) system was limited to Re < 0.1, with which the evolution of fluid flow velocity was impossible to measure/observe at mobile flow channels and immobile eddy areas. We presented a new setup for a microscale visualization technique of micro-PIV to enable flow velocity measurement up to Re = 20. A rough-walled fracture, 200-mm long by 1-mm wide, was prepared by scanning both surfaces of a rough-walled fracture and engraving them on acrylic using a numerically controlled computer modeling machine. Preliminary result indicated that large eddies were fully developed between Re = 10 and 20 at large aperture area. With increasing Re from 0.1 to 20, the magnitude of fluid flow velocity measured in the mobile main flow channel increased accordingly with applied flux, whereas in the immobile large eddy area, it did not change so much, only to increase much less compared to applied flux. However, fluid flow velocity in fully-developed eddies was more accelerated than that of main flow channels, when Re increased from 10 to 20.