  Recent numerical simulations indicate that streamwise-preferential
anisotropic porous materials have the potential to reduce skin friction in
turbulent flows through a similar mechanism to riblets. This paper reports
particle image velocimetry (PIV) measurements made in turbulent boundary layers
at $Re_\tau \approx 360$ over 3D-printed porous substrates exhibiting such
streamwise-preferential permeability. The porous material has normalized
streamwise permeability $\sqrt{K_{xx}^+}\approx 3.0$ and wall-normal and
spanwise permeabilities $\sqrt{K_{yy}^+} = \sqrt{K_{zz}^+} \approx 1.1$. This
material is flush-mounted into a cutout in the downstream half of a flat-plate
boundary layer setup in a water channel facility. Measurements made at several
locations along the porous substrate provide insight into boundary layer
development. For fully-developed conditions, the mean profiles show the
presence of a logarithmic region over the porous material with similar
constants to those found over a smooth wall. A technique that estimates the
mean profile at single-pixel resolution from the particle images suggests the
presence of an interfacial slip velocity of $U_s^+ \approx \sqrt{K_{xx}^+}$
over the porous substrate. Friction velocity estimates obtained from outer
layer fits to the mean profile suggest a marginal increase in drag over the
porous substrate. PIV measurements show a decrease in the intensity of
streamwise velocity fluctuations in the near-wall region and an increase in the
intensity of wall-normal velocity fluctuations. These observations are
consistent with simulation results, which suggest that materials with
$\sqrt{K_{yy}^+} > 0.4$ are susceptible to the emergence of spanwise rollers
similar to Kelvin-Helmholtz vortices that degrade drag reduction performance.
Velocity spectra indicate that such structures emerge in the experiments as
well.
