Wall-attached structures of velocity fluctuations in a turbulent boundary layer
Wall-attached structures of velocity fluctuations in a turbulent boundary layer
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DOI：
10.1017/jfm.2018.727
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发表时间：
2018-04
影响因子：
3.7
通讯作者：
Jinyul Hwang;H. Sung
中科院分区：
文献类型：
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作者：
Jinyul Hwang;H. Sung
壁湍流是自然界和工程应用中普遍存在的现象，但由于其复杂性，预测壁湍流是困难的。高雷诺数湍流出现在大多数实际流动中，并且由于其尺度范围广而特别复杂。虽然汤森提出的附着涡假设可以用来预测湍流强度，并作为湍流渐近行为的统一理论，但在瞬时流场中还没有观察到对对数行为有贡献的相干结构的存在。在这里，我们证明了湍流强度的对数区域，通过识别附壁结构的速度波动（$u_{i}$）通过直接数值模拟的中等雷诺数边界层（$Re_{\unicode[STIX]{x1 D 70 F}}\approx 1000$）。附壁结构的高度（$l_{y}$）是自相似的，特别是流向分量的种群密度（$u$）与$l_{y}$成反比，使人想起附壁涡的层次结构。包含在壁面平行分量（$u$和$w$）的湍流强度表现出对数行为。u$的高附着结构（$l_{y}^{+}>100$）由多个流向范围较长的均匀动量区（UMZ）组成，而横流分量（$v$和$w$）的附着结构相对较短，宽度相当，表明存在与多个UMZ相关的高涡结构。在流向湍流强度中观察到的近壁峰的大小随着l_{y}$的增加而增加，反映了附着的u$结构的嵌套层次。这些研究结果表明，所确定的结构是汤森的附着涡假设的主要候选人，他们可以作为基石，了解高雷诺数边界层的多尺度现象。
Wall turbulence is a ubiquitous phenomenon in nature and engineering applications, yet predicting such turbulence is difficult due to its complexity. High-Reynolds-number turbulence arises in most practical flows, and is particularly complicated because of its wide range of scales. Although the attached-eddy hypothesis postulated by Townsend can be used to predict turbulence intensities and serves as a unified theory for the asymptotic behaviours of turbulence, the presence of coherent structures that contribute to the logarithmic behaviours has not been observed in instantaneous flow fields. Here, we demonstrate the logarithmic region of the turbulence intensity by identifying wall-attached structures of the velocity fluctuations ( $u_{i}$ ) through the direct numerical simulation of a moderate-Reynolds-number boundary layer ( $Re_{\unicode[STIX]{x1D70F}}\approx 1000$ ). The wall-attached structures are self-similar with respect to their heights ( $l_{y}$ ), and in particular the population density of the streamwise component ( $u$ ) scales inversely with $l_{y}$ , reminiscent of the hierarchy of attached eddies. The turbulence intensities contained within the wall-parallel components ( $u$ and $w$ ) exhibit the logarithmic behaviour. The tall attached structures ( $l_{y}^{+}>100$ ) of $u$ are composed of multiple uniform momentum zones (UMZs) with long streamwise extents, whereas those of the cross-stream components ( $v$ and $w$ ) are relatively short with a comparable width, suggesting the presence of tall vortical structures associated with multiple UMZs. The magnitude of the near-wall peak observed in the streamwise turbulent intensity increases with increasing $l_{y}$ , reflecting the nested hierarchies of the attached $u$ structures. These findings suggest that the identified structures are prime candidates for Townsend’s attached-eddy hypothesis and that they can serve as cornerstones for understanding the multiscale phenomena of high-Reynolds-number boundary layers.