Inertial Flow‐Driven Enhancement of Solute Mixing and Partitioning at Rough‐Walled Fracture Intersections: Experimental and Numerical Investigations

Abstract This study investigates the impact of the transition from viscous linear to inertial nonlinear flows on solute mixing and partitioning at rough‐walled fracture intersections, using direct observations of flow dynamics and solute partitioning processes through microscopic particle image velocimetry. It is generally known that mixing at fracture intersections decreases when transport shifts from diffusion‐dominated to advection‐dominated processes, but this trend holds only in viscous linear flows. The experimental results conducted in this study reveal that in inertial flows, significant changes in flow structures occur at rough‐walled fracture intersections, including the straightening and stretching of main streamlines and the formation of fully developed eddies. Fluid stretching and the formation of eddies contribute to advection‐driven diffusive mixing. The straightened streamlines deliver solutes to the outflow leg along a direct path. More importantly, fully developed eddies generate spiral advective paths that reconnect to the main flow channels, enhancing solute redistribution at the intersection. Microscopic measurements and quantitative analyses show that flow nonlinearity—including the formation of eddies, along with enhanced flow straightening and stretching—contributes to increased flow heterogeneity and solute redistribution at fracture intersections. This phenomenon appears as an increase in “apparent” mixing at rough‐walled fracture intersections.