Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers
Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers
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DOI：
10.1017/jfm.2016.312
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发表时间：
2016-05
影响因子：
3.7
通讯作者：
S. Taddei;Costantino Manes;B. Ganapathisubramani
中科院分区：
文献类型：
--
作者：
S. Taddei;Costantino Manes;B. Ganapathisubramani
实验研究了不同密度冠层在湍流边界层中的尾迹和阻力。补丁是圆形的（外径） $D$ )，由几个相同的圆柱体(高度、 $H$ ，直径， $d$ ）. 所有贴片的整体宽高比( $AR=H/D$ )固定为1，斑块密度( ${\it\phi}=N_{c}d^{2}/D^{2}$ ，也称为固体度)是通过改变圆柱体的数量( $N_{c}$ )在补丁。阻力测量结果表明，阻力系数随密度的增大而增大。然而，最高研究密度的阻力系数( ${\it\phi}\approx 0.25$ )大于固体斑块的阻力系数(即。 ${\it\phi}=1$ ，这是一个带 $AR=1$ ）. 粒子图像测速（PIV）沿流壁法向( $x$ —— $y$ )沿斑块中心线和沿河流方向的平面( $x$ —— $z$ )平面处于中等高度(即 $y=H/2$ ）. 平均速度场表明，贴片的孔隙度促进了各个方向的出血。观察到沿垂直/水平方向的出血随增加而增加/减少 ${\it\phi}$ ． 此外，还观察到沿外侧方向的出血决定了沿斑块两侧的血流分离点，并使其独立于 ${\it\phi}$ ． 所有这些方面都使多孔斑块的尾迹与固体斑块的尾迹明显不同，并为解释观测到的阻力系数变化趋势提供了合理的依据。
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.