Coherent structure generation in near-wall turbulence
Coherent structure generation in near-wall turbulence
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DOI：
10.1017/s002211200100667x
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发表时间：
2002-02
影响因子：
3.7
通讯作者：
W. Schoppa;F. Hussain
中科院分区：
文献类型：
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作者：
W. Schoppa;F. Hussain
我们提出了一种新的机制产生的近壁流向涡-在边界层中占主导地位的湍流现象-使用线性扰动分析和直接数值模拟的湍流通道流。由平均速度剖面和低速条纹（没有任何初始涡）组成的基流，只有在相对较强的条纹，即条纹涡线的壁面倾角超过50°时，才表现出对正弦简正波的线性不稳定性。从充分发展的近壁湍流条纹提取的分析表明，在缓冲层中的条纹区域的约20%超过不稳定的强度阈值。更重要的是，这些不稳定的条纹表现出只有适度的（两倍）正常模式放大，增长被逮捕的自湮灭条纹侧翼正常涡由于粘性交叉扩散。我们在这里提出了一种替代，条纹瞬态生长（STG）机制，能够产生更大的（十倍）的线性antiflcation的x依赖的干扰。请注意STG的区别-负责条纹速度分布U（y，z）上的扰动增长-从（无条纹）平均速度U（y）的先前瞬态增长分析。我们揭示了流向涡是通过一个新的基于STG的方案从更多的正常模式稳定条纹产生的：（i）扰动的瞬时增长导致流向涡面ωx的形成（通过涡量产生的“剪切”机制），（ii）随着STG达到非线性振幅，（iii）ωx片通过拉伸（而不是卷起）而坍缩成流向涡。值得注意的是，由STG产生的x-交替符号的（瞬时）流向涡的三维特征与完全湍流产生的（整体平均）相干结构一致。STG诱导的内部剪切层的形成，沿着象限雷诺应力和其他湍流措施，也符合充分发展的湍流。结果表明，突出的-可能占主导地位的-这种新的，瞬态增长为基础的涡流生成的情况下，并提出了有趣的可能性强大的控制阻力和传热。
We present a new mechanism for generation of near-wall streamwise vortices – which dominate turbulence phenomena in boundary layers – using linear perturbation analysis and direct numerical simulations of turbulent channel flow. The base flow, consisting of the mean velocity profile and low-speed streaks (free from any initial vortices), is shown to be linearly unstable to sinuous normal modes only for relatively strong streaks, i.e. for wall inclination angles of streak vortex lines exceeding 50°. Analysis of streaks extracted from fully developed near-wall turbulence indicates that about 20% of streak regions in the buffer layer exceed the strength threshold for instability. More importantly, these unstable streaks exhibit only moderate (twofold) normal-mode amplification, the growth being arrested by self-annihilation of streak-flank normal vorticity due to viscous cross-diffusion. We present here an alternative, streak transient growth (STG) mechanism, capable of producing much larger (tenfold) linear ampliflcation of x-dependent disturbances. Note the distinction of STG – responsible for perturbation growth on a streak velocity distribution U(y, z) – from prior transient growth analyses of the (streakless) mean velocity U(y). We reveal that streamwise vortices are generated from the more numerous normal-mode-stable streaks, via a new STG-based scenario: (i) transient growth of perturbations leading to formation of a sheet of streamwise vorticity ωx (by a ‘shearing’ mechanism of vorticity generation), (ii) growth of sinuous streak waviness and hence ∂u/∂x as STG reaches nonlinear amplitude, and (iii) the ωx sheet’s collapse via stretching by ∂u/∂x (rather than rollup) into streamwise vortices. Significantly, the three-dimensional features of the (instantaneous) streamwise vortices of x-alternating sign generated by STG agree well with the (ensemble-averaged) coherent structures educed from fully turbulent flow. The STG-induced formation of internal shear layers, along with quadrant Reynolds stresses and other turbulence measures, also agree well with fully developed turbulence. Results indicate the prominent – possibly dominant – role of this new, transient-growth-based vortex generation scenario, and suggest interesting possibilities for robust control of drag and heat transfer.