Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers. (31st May 2016) - Taddei, S.
Abstract : The wakes and the drag forces of canopy patches with different densities, immersed in turbulent boundary layers, are investigated experimentally. The patches are circular (with outer diameter $D$ ) and are made of several identical circular cylinders (height, $H$, and diameter, $d$ ). The bulk aspect ratio of all of the patches ( $AR=H/D$ ) was fixed at 1 and the patch density ( ${\it\phi}=N_{c}d^{2}/D^{2}$, also referred to as the solidity) is altered by varying the number of cylinders ( $N_{c}$ ) in the patch. Drag measurements show that the patch drag coefficient increases with increasing density. However, the drag coefficient of the highest investigated density ( ${\it\phi}\approx 0.25$ ) is greater than the drag coefficient of a solid patch (i.e. ${\it\phi}=1$, which is a solid cylinder with $AR=1$ ). Particle image velocimetry (PIV) measurements were carried out along the streamwise–wall-normal ( $x$ – $y$ ) plane along the centreline of patch and in the streamwise–spanwise ( $x$ – $z$ ) plane at its mid height (i.e. $y=H/2$ ). Mean velocity fields show that the porosity of the patch promotes bleeding along all directions. It was observed that bleeding along the vertical/horizontal direction increases/decreases with increasing ${\it\phi}$ . Furthermore, it was also observed that bleeding along the lateral direction dictates the point of flow separation along the sides of the patch and makes it independent of ${\it\phi}$ . All of these aspects make wakes for porous patches markedly different from their solid counterpart and provide a rational basis to explain the observed trends in the drag coefficient.