On the influence of outer large-scale structures on near-wall turbulence in channel flow
On the influence of outer large-scale structures on near-wall turbulence in channel flow
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DOI：
10.1063/1.4890745
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发表时间：
2014-07
影响因子：
4.6
通讯作者：
L. Agostini;M. Leschziner
中科院分区：
文献类型：
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作者：
L. Agostini;M. Leschziner
直接数值模拟（DNS）在1025的通道流动数据被用来分析对数律区域的大尺度外尺度（称为超条纹）和受粘度影响的近壁层的小尺度、条纹、顺流速度波动之间的相互作用。这项研究受到了Mathis， Marusic和Hutchins的广泛实验调查的启发，最终得出了一个预测模型，该模型以一种被认为是普遍的方式描述了外部结构对小规模近壁运动的“足迹”和“调制”效应。本文使用的方法是基于对以大尺度足迹区域为条件的小尺度波动的联合pdf的检查。采用Huang-Hilbert经验模态分解方法对大尺度和小尺度进行了分离，并通过两种尺度的预乘能谱、相关图和能量剖面证明了其有效性。然后，从pdf中得出的观测结果构成了评估模型所依据的假设有效性的基础。虽然目前的观测结果支持模型的某些要素，但结果表明，负大尺度波动和正大尺度波动的调制差别很大，这是一种与模型不相容的不对称响应。因此，这项研究扩展到检验另一种建议的有效性，该建议基于这样一个假设，即对大尺度运动的小尺度响应的普遍描述必须依赖于用大尺度修正的局部摩擦速度而不是用平均值对速度波动进行标度。本分析部分支持这一建议。最后，本文提出并检验了另一种新的现象学模型。
Direct Numerical Simulation (DNS) data for channel flow at 1025 are used to analyse the interaction between large outer scales in the log-law region – referred to as super-streaks – and the small-scale, streaky, streamwise-velocity fluctuations in the viscosity-affected near-wall layer. The study is inspired by extensive experimental investigations by Mathis, Marusic, and Hutchins, culminating in a predictive model that describes, in a supposedly universal manner, the “footprinting” and “modulating” effects of the outer structures on the small-scale near-wall motions. The approach used herein is based on the examination of joint PDFs for the small-scale fluctuations, conditioned on regions of large-scale footprints. The large and small scales are separated by means of the Huang-Hilbert empirical-mode decomposition, the validity of which is demonstrated by way of pre-multiplied energy spectra, correlation maps, and energy profiles for both scales. Observations derived from the PDFs then form the basis of assessing the validity of the assumptions underlying the model. Although the present observations support some elements of the model, the results imply that modulation by negative and positive large-scale fluctuations differ greatly – an asymmetric response that is not compatible with the model. The study is thus extended to examining the validity of an alternative proposal, which is based on the assumption that a universal description of the small-scale response to the large-scale motions has to rely on the velocity fluctuations being scaled with the large-scales-modified local friction velocity, rather than with the mean value. This proposal is partially supported by the present analysis. Finally, an alternative, new phenomenological model is proposed and examined.