Influence of intersection angle on nonlinear flow behavior and solute transport through three-dimensional crossed rock fractures

Accurately evaluating the flow characteristics in fracture intersections is important to advance the understanding of groundwater flow and solute transport in crossed rock fractures. However, mainly two-dimensional (2D) intersection models have been adopted in previous studies, and the influence of intersection angles and related three-dimensional (3D) effects (channel flow and transverse flow) on the fracture seepage and the solute transport is still neglected. In this study, the 3D crossed fracture models, coupled with various intersection angles, were established through the intersection of two rough-walled fractures. The characteristic parameters of the fluid flow and the solute transport under different inlet velocity conditions were calculated by the Navier–Stokes equation and the advective–diffusion equation, respectively. The results indicated that the intricate geometry of the intersection in 3D rough-walled models led to channeling flows, which subsequently impacted mixing behavior depending on velocity. Due to the presence of channeling flows, the velocity ratio at the outlet was different from that of a 2D fracture as the inlet hydraulic conditions evolved. The coefficient matrices describing nonlinear flow behavior in different fracture intersection angles were quantified simultaneously. The reallocation of fluid pathways induced by intersecting angles affects mixing behavior by influencing the geometrical structure of fracture intersections. Moreover, the breakthrough curves and solute mixing process were significantly dependent on the intersection angle and the inlet velocity. In the linear region, the mixing ratio is random due to the intersection of heterogeneous, while in the nonlinear region, the mixing ratio decreases with the increase in water flow. Above all, the correlation established in this study between hydraulic parameters and the intersection angle parameter can enhance their efficacy in predicting solute transport in fractured rocks.