De-asymmetry of small-scale motions in wall-bounded turbulence
De-asymmetry of small-scale motions in wall-bounded turbulence
复制标题
壁面湍流中小尺度运动的去不对称性
DOI：
10.1063/5.0092548
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发表时间：
2022-05
影响因子：
4.6
通讯作者：
Wang Jinjun
中科院分区：
文献类型：
--
作者：
Zhang Qingfu;Pan Chong;Wang Jinjun
本文主要研究了有壁湍流大到小振幅调制（AM）中的对称性破缺现象。利用最近提出的多分量变分模态分解方法，将直接数值模拟（DNS）得到的空间发展湍流边界层的体积速度场分解为四个三维本征模态函数（IMFs），它们的跨向长度尺度彼此完全分离。发现第一个IMF（1IMF）主要表征流向大尺度（LS）和甚大尺度湍流运动。在第二次IMF（2IMF）中观察到溅射和溅射事件，导致在极端大尺度运动条件下近壁$u_{2text{IMF}}$和$v_{2text{IMF}}$的条件概率密度函数（pdf）偏置。这与前两种imf的雷诺兹剪应力（RSS）的壁法向剖面的分层自相似性形成鲜明对比。当将2IMF作为小尺度湍流运动的组成部分时，这种溅射和溅射事件会导致不对称AM效应，即正负LS运动对应的AM系数彼此不对称。潜在的原因是这些强象限事件在空间上是不对称的，并且与局部LS运动紧密耦合。在此基础上，提出了一种去空间不对称（DSA）方法来获得无不对称的“通用”SS湍流运动。该方法包括去除SS运动中的过渡2IMF、去幅度调制和长度尺度的重新缩放。单点速度统计、RSS和速度谱分析表明，DSA方法得到的SS运动的“通用性”得到了显著提高。
The present work focuses on the symmetry-breaking phenomenon in large-to-small amplitude modulation(AM) of wall-bounded turbulence. Using the recently proposed multi-component variational mode decomposition method, the volumetric velocity fields of a spatially developing turbulent boundary layer being obtained by Direct Numerical Simulation(DNS) is decomposed into four three-dimensional intrinsic mode functions(IMFs), whose spanwise length scales are fully separated from each other. It is found that the first IMF(1IMF) mainly characterizes the streamwise large-scale(LS) and very-large-scale turbulent motions. Splatting and sputtering events are observed in the second IMF(2IMF), leading to the biased conditional probability density functions(PDFs) of near-wall $u_{2text{IMF}}$ and $v_{2text{IMF}}$ under the condition of extreme large-scale motions. This is in distinct contrast to the hierarchical self-similarity of the wall-normal profiles of Reynolds shear stress(RSS) of the last two IMFs. When treating 2IMF as components of small-scale(SS) turbulent motions, such splatting and sputtering events lead to asymmetric AM effect, i.e., the AM coefficients corresponding to positive and negative LS motions are asymmetric to each other. The underlying reason is that these strong quadrant events are spatially asymmetric and are tightly coupled with local LS motions. Based on this observation, a de-spatial-asymmetry(DSA) method is proposed to obtain asymmetry-free `universal' SS turbulent motions. This method includes the removal of the transitional 2IMF from SS motions, the de-amplitude modulation and the length-scale rescaling. Analysis of single-point velocity statistics, RSS, as well as velocity spectrum, shows that the `universality' of SS motions derived from the DSA method is remarkably improved.