The presence of a fluid layer adjacent to a porous medium is common in many environmental, chemical, mechanical and petroleum engineering problems. The flow behavior in the fluid layer is determined by the properties of the fluid (e.g., viscosity and density) and surface (e.g., pressure and shear) and body (e.g., gravity, and electric field) forces. However, flow in the porous medium depends additionally on the properties of the medium which include, among others, porosity and permeability.
In fluid dynamic analysis of groundwater systems, it is commonly found that water flows through combined free (nonporous) and porous pathways between surface and subsurface systems. Such coupled regimes may also be present due to construction of industrial utilities and structures in the ground. Important engineering processes that may involve combined free and porous flow domains are, for example, fluid losses/leaks from underground pipes and storage tanks in old gas works sites; dig‐and‐treat, pump‐and‐treat, and permeable reactive barrier technologies for groundwater treatment; and drilling/extraction of oil from underground reservoirs. However, in most cases, the associated transport phenomena are determined by natural hydroenvironmental conditions. These include, for example, combined surface and subsurface flow, lake–groundwater interactions, seepage through preferential flow channels and macropores, circulatory flows and rise and fall of groundwater, glaciology, and multiple karstic regions interfaced with granular porous sections. The subdomains in coupled flow systems are generally distinguished by an interfacial surface, which represents the transitional zone for contaminants/fluid mobility from free to porous sections or vice versa. To gain on understanding of the combined flow behavior, it is necessary to develop realistic methodologies for evaluating mass and momentum transfer across the free/porous flow interfaces.