Plane jets impinging on porous walls

The flow of a two-dimensional, plane turbulent jet impinging on a porous screen has been studied experimentally. It is shown how the overall flow structure depends on the porosity of the surface. For low screen porosity (β < 0.41, say), transverse wall jets can be formed on both sides of the screen and in extreme cases the axial momentum flux some way downstream of the screen falls to zero, so that the screen has the same drag as would a solid wall. For high screen porosity (β > 0.57, say) the axial volume flux is largely preserved through the screen but the dominant eddy structures present in the upstream jet are largely destroyed so that entrainment rates downstream of the screen can be very low. The relatively small, intermediate range of porosities (0.41 < β < 0.57, where β is the screen open area ratio), is associated with dramatic changes in flow pattern and recirculating regions can exist on the upstream side of the screen. These flows, although all geometrically very simple, provide a serious challenge for computational modelling.