Friction and Free‐Surface Flow over Porous Media

Abstract

This paper examines frictional resistance for steady free‐surface flow over a porous bed of uniformly‐sized glass beads. This process is important in mass transport problems, such as non‐point source pollution, in which flow extracts chemicals from porous boundaries. Laboratory data show that superposed shear flow interacts intimately with the permeable boundary. Observed reduced frictional resistance for laminar flow supports the Beavers and Joseph's 1967 postulation of interfacial velocity slip. Turbulent flow, however, produced significant momentum transfer across the bed interface, which acted to increase frictional resistance and, thereby, counteract the reduction of frictional resistance caused by velocity slip, A predominance of either mechanism during turbulent flow depends upon media permeability and, possibly, the scale of turbulent eddies. As permeability increases, lateral momentum exchange begins to dominate overall flow resistance, because small scale eddies are able to penetrate the larger pore spaces.