Turbulent boundary layers over permeable walls: scaling and near-wall structure
Turbulent boundary layers over permeable walls: scaling and near-wall structure
复制标题
DOI：
10.1017/jfm.2011.329
复制
发表时间：
2011-10
影响因子：
3.7
通讯作者：
C. Manes;D. Poggi;L. Ridolfi
中科院分区：
文献类型：
--
作者：
C. Manes;D. Poggi;L. Ridolfi
摘要本文介绍了一个实验研究，致力于研究渗透率对壁湍流的影响。通过激光多普勒风速仪在明渠水流中进行了速度测量，其特征在于具有广泛的渗透性。以往的研究提出，冯卡门系数与渗透性的墙壁上的平均速度剖面是显着低于标准值报告的流动光滑和粗糙的墙壁。此外，有人指出，湍流在可渗透的墙壁不完全尊重广泛接受的外层相似性的范例。我们的数据表明，这两个异常可以解释为效果差的内外尺度分离，如果渗透壁内的剪切穿透深度被认为是内层的代表性长度尺度。我们观察到，随着渗透率的增加，近壁结构逐渐演变成一个更有组织的状态，直到它达到的扰动混合层的条件下，弯曲的平均速度分布的剪切不稳定性决定的主导漩涡的规模。在我们的实验中，这样的剪切不稳定涡流只检测到了最高渗透率的墙壁。与此相反，附加涡存在于所有其他壁条件。在这些研究结果的基础上，我们认为，近壁结构的湍流渗透性的墙壁是由附着和剪切不稳定涡之间的竞争机制。我们还认为，剪切穿透深度和边界层厚度之间的比率量化的比例之间的这种涡流尺度，因此，可以被用来作为一个诊断参数，以评估涡流结构占主导地位的近壁区域不同的壁渗透率和流动条件。
Abstract This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions.