Quantitative Characterization of Mass Transfer in Rough Conduits Under Eddy Effects

The prevention and control of groundwater pollution in karst regions has long been a focus and challenge in the field of hydrogeology. As a typical aquifer structure commonly found in karst areas, karst conduits are particularly susceptible to contamination. The two-region model (TRM) is widely used in conduit solute transport research due to its clear conceptual framework and concise equation forms. To further reveal the mechanism of eddy effect on solute transport, this study designed a series of artificially roughened conduits with varying shapes and relative roughness. The breakthrough curves under different working conditions were obtained through a series of physical models and numerical simulation experiments by controlling different flow velocities, shape coefficients, and relative roughness, and the non-Fickian coefficient was defined to quantify the trailing degree of the breakthrough curves. This study finds that an increase in the inlet flow velocity not only expands the size of the eddy area but also boosts the speed of eddy rotation as this eddy area approaches the asymptotic value. Simultaneously, the interaction of solutes between the main flow stream and the eddy region intensifies, resulting in a gradual reduction of the non-Fickian coefficient. When the inlet flow velocity remains constant, the non-Fickian coefficient rises as the shape coefficient and relative roughness increase. Besides, the mass transfer coefficient under eddy effect is dynamic rather than a constant, increasing first and then decreasing gradually to a stable value, which is positively correlated with inlet flow velocity and negatively correlated with shape coefficient and relative roughness.