Turbulent boundary layer measurements over permeable surfaces

Laser Doppler Velocimetry (LDV) measurements were made in turbulent boundary layers over a flat plate. Commercially available, open cell foams with pore sizes varying from 0.3-2mm were flush mounted into a cutout in the downstream half of the plate. The friction Reynolds number over the smooth wall upstream of the porous section was Reτ ≈ 1690. Measurements made to within 140μm (3-4 viscous units) of the porous interface revealed a substantial slip velocity at the interface (> 30% of the free stream velocity) that was approximately constant across changing substrate geometry. A mean velocity deficit, increasing with pore size, was observed further from the wall in the region corresponding to the log-layer in smooth wall boundary layers. For all foams, an elevated streamwise turbulence intensity was found further into the boundary layer (until y/δ ≈ 0.2). An outer peak in turbulence intensity was found for the largest pore sizes.