Turbulence in rough-wall boundary layers: universality issues
Turbulence in rough-wall boundary layers: universality issues
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DOI：
10.1007/s00348-011-1049-7
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发表时间：
2011-02
影响因子：
2.4
通讯作者：
M. Amir;I. Castro
中科院分区：
文献类型：
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作者：
M. Amir;I. Castro
在风洞中对三维粗糙表面上的紊流边界层进行了测量，以确定临界粗糙度高度，超过该高度，粗糙度会影响整个边界层的紊流特性。实验进行了三种类型的表面，包括城市型表面与正方形随机高度元素，菱形图案丝网和砂纸型砂砾。在动量厚度雷诺数（Reθ）为1，300 - 28，000的范围内使用双分量激光多普勒风速仪（LDA）和热线风速仪（HWA）进行测量。粗糙元高度与边界层厚度之比δ的变化范围较宽（）。结果证实，当h/δ值达到令人惊讶的大值时（可能高达0.2），所有表面的平均剖面在速度亏损形式下都能很好地塌陷，但如前所述，其外层尾流强度比光滑壁流动的要大一些。在较低的h/δ值时（至少达到0.15），所有表面的雷诺应力在整个边界层内表现出良好的一致性。在近壁区域之外产生具有光滑壁的塌陷。然而，随着h/δ的增大，近壁区上方的湍流度逐渐改变，直至影响到整个流动。象限分析证实，粗糙壁边界层的变化确实存在，但仅限于低h/δ时的近壁区;当h/δ超过约0.2时，象限事件表明结构变化遍及边界层的大部分。总之，数据表明，h/δ ≥ 0.15时，粗糙度的细节对外流中湍流结构多快（随着h/δ的增大）不再符合经典边界层特性的影响很小。目前的结果提供了支持汤森的壁相似性假设在低h/δ，也表明，一个单一的临界粗糙度高度，超过它失败不存在。对于完全粗糙流，数据也证实了平均流和湍流量基本上与Re θ无关;在很高Re θ时，所有雷诺应力都与光滑壁流的雷诺应力相匹配。尽管如此，在近壁区，即使在相当低的h/δ下，强扫掠事件的应力贡献也有明显的增加。
Wind tunnel measurements of turbulent boundary layers over three-dimensional rough surfaces have been carried out to determine the critical roughness height beyond which the roughness affects the turbulence characteristics of the entire boundary layer. Experiments were performed on three types of surfaces, consisting of an urban type surface with square random height elements, a diamond-pattern wire mesh and a sand-paper type grit. The measurements were carried out over a momentum thickness Reynolds number (Reθ) range of 1,300–28,000 using two-component Laser Doppler anemometry (LDA) and hot-wire anemometry (HWA). A wide range of the ratio of roughness element heighthto boundary layer thickness δ was covered (). The results confirm that the mean profiles for all the surfaces collapse well in velocity defect form up to surprisingly large values ofh/δ, perhaps as large as 0.2, but with a somewhat larger outer layer wake strength than for smooth-wall flows, as previously found. At lowerh/δ, at least up to 0.15, the Reynolds stresses for all surfaces show good agreement throughout the boundary layer, collapsing with smooth-wall results outside the near-wall region. With increasingh/δ, however, the turbulence above the near-wall region is gradually modified until the entire flow is affected. Quadrant analysis confirms that changes in the rough-wall boundary layers certainly exist but are confined to the near-wall region at lowh/δ; forh/δ beyond about 0.2 the quadrant events show that the structural changes extend throughout much of the boundary layer. Taken together, the data suggest that aboveh/δ ≈ 0.15, the details of the roughness have a weak effect on how quickly (with risingh/δ) the turbulence structure in the outer flow ceases to conform to the classical boundary layer behaviour. The present results provide support for Townsend’s wall similarity hypothesis at lowh/δ and also suggest that a single critical roughness height beyond which it fails does not exist. For fully rough flows, the data also confirm that mean flow and turbulence quantities are essentially independent ofReθ; all the Reynolds stresses match those of smooth-wall flows at very highReθ. Nonetheless, there is a noticeable increase in stress contributions from strong sweep events in the near-wall region, even at quite lowh/δ.