Transition from linear to nonlinear flow in single rough fractures: effect of fracture roughness

A series of laboratory experiments on water flow through rough fractures was performed using self-designed experimental devices to investigate the effect of fracture roughness on the flow behavior. Nine models of single rough fractures—with three joint roughness coefficients (JRCs) of 0–2, 8–10 and 18–20, and three apertures for each JRC—were prepared using three-dimensional printing technology. In the flow experiments, the values of Reynolds numbers ranged widely from less than 10 to around 10,000. According to the experimental data, the fracture roughness has an obvious influence on the hydraulic properties of fractures. A parametric expression for the Forchheimer equation was proposed to quantitatively describe the influence of fracture roughness on the flow behaviour in the fractures. The relations between the parameters for nonlinear flow (such as critical Reynolds number, non-Darcy effect coefficient and friction factor) and the JRCs were obtained. It was found that the critical Reynolds number decreased significantly from 566 to 67 as the JRC increased from 2 to 20. The increase in fracture roughness causes more extra energy losses and enhances the degree of flow nonlinearity in single fractures.