Skewness-induced asymmetric modulation of small-scale turbulence by large-scale structures
Skewness-induced asymmetric modulation of small-scale turbulence by large-scale structures
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DOI：
10.1063/1.4939718
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发表时间：
2016-01-01
影响因子：
4.6
通讯作者：
Gaitonde, Datta
中科院分区：
文献类型：
--
作者：
Agostini, Lionel;Leschziner, Michael;Gaitonde, Datta
最近的几项研究讨论了近壁剪切层中湍流速度波动的偏度的作用，在量化大尺度运动和小尺度波动振幅变化之间的相关性的背景下，被称为“调制”。“本研究的前提是，在确定其包络线的过程中，应明确考虑到小规模波动的偏斜度，包络线是其幅度变化的特征。这就引出了两个包络线的概念，一个是正的，另一个是负的小尺度波动，因此也引出了两个相应的相关系数。这个概念的理由是首先提供了一个检查的高频合成信号进行现实的偏斜诱导调制。一个新的形式主义推导出的两个信封，其保真度证明了合成测试用例。然后将该方法应用于摩擦雷诺数为4200的槽道流动，其中直接数值模拟（DNS）数据可用。用经验模态分解方法分离了大尺度场和小尺度场，研究了大尺度场对小尺度场的调制作用。对于与正和负小尺度波动有关的两个包络，分别导出了相关系数图和两点相关图，后者将大尺度运动和小尺度运动的包络联系起来，这些图表明了对波动定义过程的显著敏感性。特别是靠近壁，在那里小尺度波动的偏斜度是总值的主要贡献者。(C)2016 AIP Publishing LLC.
Several recent studies discuss of role of skewness of the turbulent velocity fluctuations in near-wall shear layers, in the context of quantifying the correlation between large-scale motions and amplitude variations of small-scale fluctuations-referred to as "modulation." The present study is based on the premise that the skewness of the small-scale fluctuations should be accounted for explicitly in the process of defining their envelope, which characterizes their amplitude variations. This leads to the notion of two envelopes, one for positive and the other for negative small-scale fluctuations, and hence also to two corresponding correlation coefficients. Justification for this concept is provided first by an examination of a high-frequency synthetic signal subjected to realistic skewness-inducing modulation. A new formalism is provided for deriving the two envelopes, and its fidelity is demonstrated for the synthetic test case. The method is then applied to a channel flow at a friction Reynolds number of 4200, for which direct numerical simulation (DNS) data are available. The large-scale and small-scale fields are separated by the empirical mode decomposition method, and the modulation of the small-scale fluctuations by the large scales is examined. Separate maps of the correlation coefficient and of two-point correlations, the latter linking the large-scale motions and the envelopes of the small-scale motions, are derived for the two envelopes pertaining to positive and negative small-scale fluctuations, and these demonstrate a significant sensitivity to the envelope-definition process, especially close to the wall where the skewness of the small-scale fluctuations is the dominant contributor to the total value. (C) 2016 AIP Publishing LLC.