Velocity measurements of a shear flow penetrating a porous medium

This paper reports an experimental investigation of simple shear flow penetrating a model of a fibrous porous medium. The flow field is established between a stationary inner cylinder and a concentric outer cylinder rotating at a constant speed. The model medium is a regular array of rods which are oriented across the flow and which fill a fraction of the annular space between the cylinders. Rods with circular, square and triangular cross-sections are investigated, and the solid volume fraction of the arrays ranges from 0.01 to 0.16. With a viscous oil as the working fluid, the Reynolds number is much less than unity. Velocity measurements made using particle image velocimetry focus on the region around the edge of each array tested. The measurements reveal that eddies form between the two outermost circles of rods, for solid volume fractions above a minimum value which depends on rod shape. The velocity data are used to find the interfacial slip velocity and the average velocity at the interface between the porous medium and the outer shear flow. The data demonstrate that the slip velocity decays with increasing solid volume fraction, as expected, but the velocity is found to be nearly independent of rod shape and of the number of circles of rods comprising an array. It is also found that the slip velocity is only 24–30% of the value predicted from the Brinkman equation.