Experimental investigation of the effects of upstream perturbations in the near-wall and core of a high porosity random-wire porous medium.

Abstract

Experiments were conducted on a high-porosity porous medium to investigate the influences of upstream perturbations. The porous medium was fabricated by stacking stainless steel wire-mesh screens in a container of circular cross-section. A step change in porosity is considered a perturbation to the flow as these perturbations would offer an alternate path for flow. In the present set of experiments, perturbations were offered by gaps or blockages to the flow, which would allow jets or wakes to evolve inside the porous medium, with the gap offereing a low-resistance path and the blockage offereing a high-resistance path. These perturbations were positioned at two radial locations: a near-wall case, with a gap or blockage adjacent to walls of the test-section container, and a core-flow case, with a hole or a blockage at the center of the metal screens. In each case, the perturbation was for half the streamwise distance of the test-section. For the remainder of the streamwise distance, flow was allowed to travel unperturbed through the porous medium. Hotwire measurements of velocity were taken at the exit of the test-section, which provided insight on the characteristics of transport in different regions of the porous medium. It was discovered that pore-scale eddies, shed from the mental wires that constitute the porous medium were suppressed near the walls of the container by the wall boundary layer, leading to very poor transport in the near-wall region. As a result, effects of the perturbations were felt even after twenty hydraulic diameters of streamwise travel. No such effects were observed when the core-flow was perturbed. Thes results indicated that transport charactersitics are vastly different in different regions of the flow and near-wall transport in porous media flows is far less effective than core transport.