Flow Visualization of Transition From Linear to Nonlinear Flow Regimes in Rock Fractures

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.