An essential ingredient of turbulent flows is the vortex stretching mechanism, which emanates from the non-linear interaction of vorticity and strain-rate tensor and leads to formation of extreme events. We analyze the statistical correlations between vorticity and strain rate by using a massive database generated from very well resolved direct numerical simulations of forced isotropic turbulence in periodic domains. The grid resolution is up to ${12288}^3$, and the Taylor-scale Reynolds number is in the range $140-1300$. In order to understand the formation and structure of extreme vorticity fluctuations, we obtain statistics conditioned on enstrophy (vorticity-squared). The magnitude of strain, as well as its eigenvalues, is approximately constant when conditioned on weak enstrophy; whereas they grow approximately as power laws for strong enstrophy, which become steeper with increasing $\re$. We find that the well-known preferential alignment between vorticity and the intermediate eigenvector of strain tensor is even stronger for large enstrophy, whereas vorticity shows a tendency to be weakly orthogonal to the most extensive eigenvector (for large enstrophy). Yet the dominant contribution to the production of large enstrophy events arises from the most extensive eigendirection, the more so as $\re$ increases. Nevertheless, the stretching in intense vorticity regions is significantly depleted, consistent with the kinematic properties of weakly-curved tubes in which they are organized. Further analysis reveals that intense enstrophy is primarily depleted via viscous diffusion, though viscous dissipation is also significant. Implications for modeling are nominally addressed as appropriate.

中文翻译：

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雷诺数高湍流中的涡旋拉伸和涡旋产生

湍流的基本成分是涡旋拉伸机制，它是由涡度和应变率张量的非线性相互作用产生的，并导致形成极端事件。我们通过使用一个很好的数据库来分析涡度和应变率之间的统计相关性，该数据库是由周期域内各向同性湍流的很好解析的直接数值模拟生成的。网格分辨率最高${12288}^3$，并且泰勒级雷诺数在范围内 $140-1300$。为了了解极端涡度波动的形成和结构，我们获得了以涡旋（涡度平方）为条件的统计数据。当以弱的涡旋为条件时，应变的大小及其特征值大约是恒定的。然而，随着强定律的幂定律，它们近似增长，随着$ \ re $的增加，斜率变得越来越陡峭。我们发现，对于大涡旋，涡度与应变张量的中间特征向量之间的众所周知的优先排列甚至更强，而对于大涡旋，涡度显示出与最广泛的特征向量弱正交的趋势。然而，对大内旋事件产生的主要贡献来自于最广泛的本征方向，例如$ \ re $增加。然而，强烈的涡度区域的拉伸显着减少，这与组织它们的弱弯曲管的运动学特性一致。进一步的分析表明，尽管粘性耗散也很重要，但强烈的内吞作用主要是通过粘性扩散来消除的。名义上适当地考虑了建模的含义。