Mass Transfer Between Recirculation and Main Flow Zones: Is Physically Based Parameterization Possible?

Recirculation zones (RZs) are common in many geophysical flows. These zones form near irregular solid boundaries and are separate from the main flow. Since mass can be trapped and later released locally from RZs, bulk transport can exhibit long tails—an anomalous behavior that is challenging to predict. The underlying RZ mass transfer and retention in this situation is poorly understood despite common parameterization by effective exchange coefficients in mobile‐immobile (MIM) domain models. We analyzed the mass transfer process using computationally resolved flow and transport fields inside two‐dimensional rough fractures. RZs were delineated by a novel technique followed by quantification of mass transfer across the interface with the main flow zone. The results showed that the first‐order mass transfer coefficient is a function of Reynolds number and velocity difference between the RZ and bulk flow. A distributed mobile‐immobile model with the directly estimated parameters accurately reproduced bulk anomalous transport. While the distributed mobile‐immobile model is not yet predictive, its development showed that mass transfer coefficients for flows involving RZs are physically meaningful, potentially predictable, and useful for elucidating local mass transfer processes within the general framework of mobile‐immobile transport modeling.