  This paper reports turbulent boundary layer measurements made over open-cell
reticulated foams with varying pore size and thickness, but constant porosity
($\epsilon \approx 0.97$). The foams were flush-mounted into a cutout on a flat
plate. A Laser Doppler Velocimeter (LDV) was used to measure mean streamwise
velocity and turbulence intensity immediately upstream of the porous section,
and at multiple measurement stations along the porous substrate. The friction
Reynolds number upstream of the porous section was $Re_\tau \approx 1690$. For
all but the thickest foam tested, the internal boundary layer was fully
developed by $<10 \delta$ downstream from the porous transition, where $\delta$
is the boundary layer thickness. Fully developed mean velocity profiles showed
the presence of a substantial slip velocity at the porous interface ($>30\%$ of
the free stream velocity) and a mean velocity deficit relative to the canonical
smooth-wall profile further from the wall. While the magnitude of the mean
velocity deficit increased with average pore size, the slip velocity remained
approximately constant. Fits to the mean velocity profile suggest that the
logarithmic region is shifted relative to a smooth wall, and that this shift
increases with pore size until it becomes comparable to substrate thickness
$h$. For all foams, the turbulence intensity was found to be elevated further
into the boundary layer to $y/ \delta \approx 0.2$. An outer peak in intensity
was also evident for the largest pore sizes. Velocity spectra indicate that
this outer peak is associated with large-scale structures resembling
Kelvin-Helmholtz vortices that have streamwise length scale $2\delta-4\delta$.
Skewness profiles suggest that these large-scale structures may have an
amplitude-modulating effect on the interfacial turbulence.
