Time-averaged velocity and scalar fields of the flow surrounding a group of cylinders

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 $\varphi$ (the total planar area covered by cylinders), which is systematically varied ($0.098\le \varphi \le 1$) 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 $\varphi$. In particular, we observe that the horizontal and vertical extent of the wake and scalar concentration downstream of the patches decreases and increases with $\varphi$, respectively. Here, the recirculation bubble is shifted closer to the trailing edge of the patches as $\varphi$ 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.