  We examine the characteristics of mean flow and scalar concentration
characteristics of a turbulent boundary layer flow impinging on a cluster of
tall obstacles (which can also be interpreted as a porous obstruction). The
cluster is created with a group of cylinders of diameter $d$ and height $h$
arranged in a circular patch of diameter $D$.The solidity of the
patch/obstruction is defined by $\phi$ (the total planar area covered by
cylinders), which is systematically varied ($0.098 \leq \phi \leq 1$) by
increasing the number of cylinders in a patch ($N_c$). A point source is placed
at ground level upstream of the patch and its transport around the patch is
examined. Time-averaged velocity and scalar fields, obtained from simultaneous
planar PIV-PLIF (particle image velocimetry-planar laser-induced fluorescence)
measurements, reveal that the characteristics of wake and flow above porous
patches are heavily influenced by $\phi$. In particular, we observe that the
horizontal and vertical extent of the wake and scalar concentration downstream
of the patches decreases and increases with $\phi$, respectively. Here, the
recirculation bubble is shifted closer to the trailing edge of the patches as
$\phi$ increases, limiting the flow from convecting downstream, decreasing the
scalar concentration and virtually `extending' the patch in streamwise
direction. As the bubble forms in the trailing edge, vertical bleeding
increases and hence the concentration increases above patch where the cylinders
appear to be `extend' vertically towards the freestream.
