Flow Visualization of Transition From Linear to Nonlinear Flow Regimes in Rock Fractures. Issue 11 (31st October 2022)

• Record Type: Journal Article
• Authors: Kim, Dahye; Yeo, In Wook
• Is Part Of: Water resources research. Volume 58:Issue 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-10-31
• Subjects: rock fractures -- nonlinear flow -- flow visualization -- micro‐PIV -- velocity measurement; Hydrology -- Periodicals; 333.91
• DOI: 10.1029/2022WR032088
• Languages: English
• ISSNs: 0043-1397

Abstract

Abstract: This study made the first experimental attempt to visualize the transition processes from linear to nonlinear flow regimes in rough‐walled rock fractures for a better understanding of the evolution of flow regimes in rock fractures. The experiments visualized the narrowing of the mainstream channel associated with eddies enlarged near the fracture wall with increasing applied flux. The measured fluid velocity field showed that these flow structures resulted in a significant increase in fluid velocity in the narrowed main flow channel and a relatively small velocity in the enlarged eddy zone. The order‐of‐magnitude analysis of local inertial and viscous forces, using measured fluid velocity vectors, showed that inertial forces overwhelmed viscous forces in the narrowed mainstream channel of rough‐walled segments, which led to the transition to nonlinear flow in rough‐walled fractures. The quantitative comparison between measured fluid velocity and the numerical simulations showed good agreement. This study demonstrates that the micro‐PIV technique can serve for phenomenon‐based experimental research on fluid flow and solute transport in rock fractures. Plain Language Summary: The Darcy‐type equations have been used to quantify fluid flow through rock fractures. These equations are limited to the flow regime that exhibits the linearity between applied pressure gradient and measured flux. However, the breakdown of the linearity takes place in many hydrogeological applications associated with high flowrates, such as geothermal energy extraction, hydrocarbon production, groundwater remediation, and so on. Therefore, this study made the first attempt to visualize the evolution of nonlinear flow through flow visualization and fluid velocity measurement. Experiments observed the narrowing of the mainstream channel associated with eddies enlarged near the fracture wall. The magnitude analysis of measured fluid velocity showed that inertial forces overwhelming viscous forces in narrowed main channel account for nonlinear flow. Flow visualization and analysis demonstrated that fracture roughness determines nonlinear flow characteristics rather than aperture size. The flow visualization technique presented in this study can serve for phenomenon‐based experimental research on fluid flow and solute transport in rock fractures. Key Points: Transition from linear to nonlinear flow regimes are visualized using the micro‐particle image velocimetry for the first time Experiments observe the narrowing of the mainstream channel associated with eddies near fracture wall with increasing applied flux Flow visualization and analysis show that inertial forces overwhelming viscous forces in narrowed main channel account for nonlinear flow