Examination of outer-layer similarity in wall turbulence over obstructing surfaces
Examination of outer-layer similarity in wall turbulence over obstructing surfaces
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DOI：
10.1017/jfm.2023.792
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发表时间：
2023-05
影响因子：
3.7
通讯作者：
Zi-shen Chen;Ricardo Garc'ia-Mayoral
中科院分区：
文献类型：
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作者：
Zi-shen Chen;Ricardo Garc'ia-Mayoral
摘要采用直接数值模拟方法研究了中等雷诺数Re_\tau \approximately 550\unicode {x2013}1000$条件下不同密度刚性细丝冠层的湍流流动。树冠的高度为$h^+\approximat110\unicode {x2013}220$，并用作评估外层相似性的阻挡基底实例。我们表明，用于确定零平面位移$\Delta y$的传统方法可以与适当的外层相似性不一致，并且可能不适用于中等Re\tau$的流动。相反，我们确定$\Delta y$和恢复外层相似性的长度和速度尺度，通过最小化光滑壁和冠层诊断功能之间的差异，到处粗糙子层，而不仅仅是在对数层。此外，我们探讨了零平面位移和摩擦速度被独立设置的可能性，但发现外层相似性恢复更一致时，他们耦合。我们观察到，虽然卡门常数，$\kappa$可能没有光滑的壁状值，流量统计是光滑的壁状的对数层和以上，如果表面效应是有限的近壁区域内。这表明了一个修改的外层相似性，其中$\kappa$不是0.39，但湍流是光滑的壁状。当冠层稠密时，叶尖上方的流动基本上是光滑壁状的，光滑壁状$\kappa \约为0.39$，并且基本上起源于叶尖平面。对于具有中等密度的冠层，上覆流感知到更深的零平面位移进入冠层，这与以前的研究报告的观测结果一致，但表现出较低的卡门常数，$\kappa \约0.34\unicode{x2013}0.36$。对于稀疏的冠层，$\kappa$趋于回到其光滑壁值，零平面位移高度在冠层床处。对于所有的冠层研究，在$\kappa$的下降从来没有超过15\，\%$，这是显着小于在以前的一些作品中使用传统的方法来评估外层相似性。
Abstract Turbulent flows over canopies of rigid filaments with different densities are studied using direct numerical simulations at moderate Reynolds numbers $Re_\tau \approx 550\unicode{x2013}1000$. The canopies have heights $h^+\approx 110\unicode{x2013}220$, and are used as an instance of obstructing substrate for the assessment of outer-layer similarity. We show that conventional methods used to determine the zero-plane displacement $\Delta y$ can be at odds with proper outer-layer similarity and may not be applicable for flows at moderate $Re_\tau$. Instead, we determine $\Delta y$ and the length and velocity scales that recover outer-layer similarity by minimising the difference between the smooth-wall and canopy diagnostic function everywhere above the roughness sublayer, not just in the logarithmic layer. In addition, we explore the possibility of the zero-plane displacement and the friction velocity being set independently, but find that outer-layer similarity is recovered more consistently when they are coupled. We observe that although the Kármán constant, $\kappa$ may not have smooth-wall-like values, the flow statistics are smooth-wall-like in the logarithmic layer and above if the surface effect is limited within the near-wall region. This suggests a modified outer-layer similarity, where $\kappa$ is not 0.39, but turbulence is otherwise smooth-wall-like. When the canopy is dense, the flow above the tips is essentially smooth-wall-like, with smooth-wall-like $\kappa \approx 0.39$ and origin essentially at the tip plane. For canopies with intermediate density, the overlying flow perceives a deeper zero-plane displacement into the canopy, which is consistent with observations reported by previous studies, but exhibits a lower Kármán constant, $\kappa \approx 0.34\unicode{x2013}0.36$. For sparse canopies, $\kappa$ tends back to its smooth-wall value, and the zero-plane displacement height is at the canopy bed. For all canopies studied, the decrease in $\kappa$ never exceeds $15\,\%$, which is significantly less than that obtained in some previous works using conventional methods to assess outer-layer similarity.