Turbulent Flow Over Large Roughness Elements: Effect of Frontal and Plan Solidity on Turbulence Statistics and Structure.
Turbulent Flow Over Large Roughness Elements: Effect of Frontal and Plan Solidity on Turbulence Statistics and Structure.
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大粗糙度元素上的湍流：锋面和平面密实度对湍流统计和结构的影响。
DOI：
10.1007/s10546-017-0317-3
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发表时间：
2018
影响因子：
4.3
通讯作者：
Placidi M
中科院分区：
文献类型：
--
作者：
Placidi M
风洞实验是在具有大粗糙度的全粗糙边界层上进行的(其中粗糙元的高度为边界层厚度)。使用统一高度的乐高™砖创造了12种不同的表面条件。6例进行了固定平面密度()的测试，额面密度()变化，另外6例进行了固定的变化。进行了粒子图像测速仪和浮动元件阻力天平测量。目前的结果补充了Placidi和Ganapathisubramani(J Fluid Mech 782：541-566,2015)中的结果，将先前的分析扩展到湍流统计和空间结构。结果表明，缺陷形状的平均速度分布符合Townsend的相似假设，但当缺陷形状发生变化时，这一假设的一致性较差。流向和壁法向湍流应力，以及雷诺剪切应力，在大多数被研究的情况下显示出缺乏相似性。这表明，外层相似的粗糙度的临界高度不仅取决于粗糙度的高度，还取决于局部壁面的形态。一个新的基于遮蔽物坚固性的判据被定义为单位平行墙面的遮蔽物面积，它类似于Raupach和Shaw(边界层气象22：79-90,1982)中的“有效遮蔽物面积”，它反映了湍流统计与外层相似的偏离。尽管在湍流统计中缺乏这种相似性，但固有的正交分解分析以及两点空间关联表明，存在某种形式的通用流动结构，因为所有的情况都表现出几乎相同的固有的正交分解模式形状和关联场。最后，基于本征正交分解的简化模型表明，湍流的小尺度在评估外层相似性方面起着重要的作用。
Wind-tunnel experiments were carried out on fully-rough boundary layers with large roughness (, wherehis the height of the roughness elements andis the boundary-layer thickness). Twelve different surface conditions were created by using LEGO™ bricks of uniform height. Six cases are tested for a fixed plan solidity () with variations in frontal density (), while the other six cases have varyingfor fixed. Particle image velocimetry and floating-element drag-balance measurements were performed. The current results complement those contained in Placidi and Ganapathisubramani (J Fluid Mech 782:541–566, 2015), extending the previous analysis to the turbulence statistics and spatial structure. Results indicate that mean velocity profiles in defect form agree with Townsend’s similarity hypothesis with varying, however, the agreement is worse for cases with varying. The streamwise and wall-normal turbulent stresses, as well as the Reynolds shear stresses, show a lack of similarity across most examined cases. This suggests that the critical height of the roughness for which outer-layer similarity holds depends not only on the height of the roughness, but also on the local wall morphology. A new criterion based on shelter solidity, defined as the sheltered plan area per unit wall-parallel area, which is similar to the ‘effective shelter area’ in Raupach and Shaw (Boundary-Layer Meteorol 22:79–90, 1982), is found to capture the departure of the turbulence statistics from outer-layer similarity. Despite this lack of similarity reported in the turbulence statistics, proper orthogonal decomposition analysis, as well as two-point spatial correlations, show that some form of universal flow structure is present, as all cases exhibit virtually identical proper orthogonal decomposition mode shapes and correlation fields. Finally, reduced models based on proper orthogonal decomposition reveal that the small scales of the turbulence play a significant role in assessing outer-layer similarity.