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Turbulent boundary layer measurements over permeable surfaces

Christoph Efstathiou
Dept. of Aerospace and Mechanical Engineering University of Southern California Los Angeles, California 90089

Mitul Luhar
Graduate Aerospace Laboratories California Institute of Technology Pasadena, CA 91125, USA; Aerospace and Mechanical Engineering University of Southern California Los Angeles, CA 90089, USA

Sinopsis

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 140 µm (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.