Skin-friction generation by attached eddies in turbulent channel flow
Skin-friction generation by attached eddies in turbulent channel flow
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DOI：
10.1017/jfm.2016.665
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发表时间：
2016-12-01
影响因子：
3.7
通讯作者：
Choi, Haecheon
中科院分区：
文献类型：
--
作者：
de Giovanetti, Matteo;Hwang, Yongyun;Choi, Haecheon
尽管最近有越来越多的证据表明，在有壁界的湍流中，以汤森附加涡流形式存在的自相似含能运动的等级组织，但它们在湍流表面摩擦产生中的作用目前还没有得到很好的理解。本文探讨了每一种自相似含能运动对湍流表面摩擦的贡献，其Re-tau相似或等于4000。采用三种不同的方法来量化运动产生的表面摩擦，其展向长度尺度小于给定的截止波长：(i) FIK （Fukagata, Iwamoto, Kasagi）识别结合雷诺剪切应力的展向波数谱；（ii）跨计算域的约束；（iii）待测运动的人为阻尼。研究发现，随着雷诺数的增加，近壁运动在摩擦产生中的作用不断降低，这与之前在低雷诺数下的发现一致。以非常大尺度和大尺度运动形式给出的最大结构也被发现具有有限的重要性：由于非微不足道的尺度相互作用过程，在所有考虑的雷诺数下，它们的完全去除只产生5-8%的皮肤摩擦减少，尽管它们被发现在Re-tau近似或等于2000的总皮肤摩擦中占20-30%。所有三种方法的应用一致地表明，最大的皮肤摩擦是由填充对数区域的自相似运动产生的。进一步表明，这些运动对湍流表面摩擦的贡献随着雷诺数的增加而逐渐增加，并且在足够高的雷诺数下，这些相干结构最终导致了大部分湍流表面摩擦的产生。
Despite a growing body of recent evidence on the hierarchical organization of the self-similar energy-containing motions in the form of Townsend's attached eddies in wall-bounded turbulent flows, their role in turbulent skin-friction generation is currently not well understood. In this paper, the contribution of each of these self-similar energy-containing motions to turbulent skin friction is explored up to Re-tau similar or equal to 4000. Three different approaches are employed to quantify the skin-friction generation by the motions, the spanwise length scale of which is smaller than a given cutoff wavelength: (i) FIK (Fukagata, Iwamoto, Kasagi) identity in combination with the spanwise wavenumber spectra of the Reynolds shear stress; (ii) confinement of the spanwise computational domain; (iii) artificial damping of the motions to be examined. The near-wall motions are found to continuously reduce their role in skin-friction generation on increasing the Reynolds number, consistent with the previous finding at low Reynolds numbers. The largest structures given in the form of very-large-scale and large-scale motions are also found to be of limited importance: due to a non-trivial scale interaction process, their complete removal yields only a 5-8% skin-friction reduction at all of the Reynolds numbers considered, although they are found to be responsible for 20-30% of total skin friction at Re-tau similar or equal to 2000. Application of all the three approaches consistently reveals that the largest amount of skin friction is generated by the self-similar motions populating the logarithmic region. It is further shown that the contribution of these motions to turbulent skin friction gradually increases with the Reynolds number, and that these coherent structures are eventually responsible for most of turbulent skin-friction generation at sufficiently high Reynolds numbers.