Homogeneous shear turbulence – bypass concept via interplay of linear transient growth and nonlinear transverse cascade
Homogeneous shear turbulence – bypass concept via interplay of linear transient growth and nonlinear transverse cascade
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均匀剪切湍流 â 通过线性瞬态增长和非线性横向级联相互作用的旁路概念
DOI：
10.1088/1742-6596/708/1/012001
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发表时间：
2016
期刊：
影响因子：
--
通讯作者：
H. Foysi
中科院分区：
文献类型：
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作者：
G. Mamatsashvili;S. Dong;G. Khujadze;G. Chagelishvili;J. Jimenez;H. Foysi
本文对均匀剪切湍流进行了直接数值模拟，研究了谱稳定剪切流中亚临界湍流的自维持机制。为此，我们分析了湍流动力学在傅立叶/波数/谱空间的基础上的模拟数据的域纵横比1：1：1。具体来说，我们研究了傅立叶谐波和非线性过程的线性瞬态增长的相互作用。谐波的瞬态增长在谱空间中是强各向异性的。这反过来又导致频谱空间中的非线性过程的各向异性，因此，主要的非线性过程似乎不是直接/反向的，而是傅立叶空间中谐波的横向/角度重新分布，称为非线性横向级联。结果表明，湍流是由线性瞬态或非模态增长和横向叶栅的相互作用维持的。这一过程可靠地证实了在谱稳定剪切流中亚临界湍流的著名旁路方案。这些过程主要在大的长度尺度下操作，与盒子尺寸相当。因此，傅立叶空间的中心小波数区域（其大小在下面确定）在自我维持中至关重要，并被标记为重要区域。在临界区以外，瞬态增长和横向级联是次要的，非线性直接级联将傅里叶谐波转移到耗散尺度。积极参与自我维持过程的谐波数量（即，在进化过程中至少有一次能量增长超过最大光谱能量的10%的谐波）是相当大的-对于所考虑的盒纵横比，它等于36-显然不能用低阶模型来描述。
We performed direct numerical simulations of homogeneous shear turbulence to study the mechanism of the self-sustenance of subcritical turbulence in spectrally stable (constant) shear flows. For this purpose, we analyzed the turbulence dynamics in Fourier/wavenumber/spectral space based on the simulation data for the domain aspect ratio 1: 1: 1. Specifically, we examined the interplay of linear transient growth of Fourier harmonics and nonlinear processes. The transient growth of harmonics is strongly anisotropic in spectral space. This, in turn, leads to anisotropy of nonlinear processes in spectral space and, as a result, the main nonlinear process appears to be not a direct/inverse, but rather a transverse/angular redistribution of harmonics in Fourier space referred to as the nonlinear transverse cascade. It is demonstrated that the turbulence is sustained by the interplay of the linear transient, or nonmodal growth and the transverse cascade. This course of events reliably exemplifies the wellknown bypass scenario of subcritical turbulence in spectrally stable shear flows. These processes mainly operate at large length scales, comparable to the box size. Consequently, the central, small wavenumber area of Fourier space (the size of which is determined below) is crucial in the self-sustenance and is labeled the vital area. Outside the vital area, the transient growth and the transverse cascade are of secondary importance-Fourier harmonics are transferred to dissipative scales by the nonlinear direct cascade. The number of harmonics actively participating in the self-sustaining process (ie, the harmonics whose energies grow more than 10% of the maximum spectral energy at least once during evolution) is quite large-it is equal to 36 for the considered box aspect ratio-and obviously cannot be described by low-order models.