Competitive roles of contact ratio and aperture in nonlinear flow transition through stressed three-dimensional rough fractures

Fracture geometric parameters, such as contact ratio and mechanical aperture, often evolve coupled under stress. However, existing studies have not effectively decoupled these parameters, obscuring their individual effects on fluid flow and leading to deviations in flow behavior predictions. This study employed numerical simulations to systematically examine the independent and coupled influences of contact ratio and aperture on nonlinear flow in rough fractures. The results reveal that both increasing contact ratio and enlarging aperture intensify flow heterogeneity. Specifically, a rising contact ratio promotes vortex expansion and multi-vortex systems formation, while aperture enlargement transforms vortex type from enclosed to spiral and extends their spatial influence. These vortex evolutions collectively reduce the Rec and enhance flow nonlinearity. The progression of nonlinear flow under normal stress depends on the relative magnitudes of contact ratio increase and aperture reduction: nonlinearity strengthens when the contact ratio increase dominates but weakens when aperture reduction is predominant. Moreover, compared to apertures, fractal dimension exerts a stronger regulatory effect on how contact ratio influences nonlinear flow, such as a pronounced reduction in Rec. These findings clarify the competing roles of fracture geometric parameters under stress and provide valuable insights into the mechanisms governing flow regime transitions in rough fractures.