ABSTRACT The topological evolution near the turbulent/non-turbulent interface (TNTI) in turbulent mixing layer is studied by means of statistical analysis of the invariants of velocity gradient tensor (VGT) based on direct numerical simulation data. The dynamics of topological evolution is investigated in terms of the source terms of the evolution equations for the invariants, including the pressure effect term, viscous effect term and interaction term among the invariants. It is found that the local topology of fluid particles at the TNTI evolves from non-focal region to focal region in the plane of the second (Q) and the third (R) invariants of the VGT. The topological evolution is mainly associated with the pressure effect term in the TNTI region. According to the analysis of the evolution of enstrophy and dissipation, the enstrophy increase and the dissipation decrease are revealed in the TNTI region, which are caused by viscous vorticity diffusion near the TNTI. A weak correlation between the strain rate and the rotation rate is found in the TNTI region which is related to the reduction of enstrophy production. The alignments between vorticity and strain near the TNTI are investigated and a strong alignment of the vorticity with the extensive eigenvector direction is identified in the TNTI region.

中文翻译：

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湍流混合层中湍流/非湍流界面附近的拓扑演化

摘要 基于直接数值模拟数据，通过速度梯度张量（VGT）不变量的统计分析，研究了湍流/非湍流界面（TNTI）附近的拓扑演化。从不变量演化方程的源项研究拓扑演化的动力学，包括不变量之间的压力效应项、粘性效应项和相互作用项。发现在 TNTI 处流体粒子的局部拓扑在 VGT 的第二个 (Q) 和第三个 (R) 不变量的平面中从非焦点区域演变为焦点区域。拓扑演化主要与 TNTI 区域的压力效应项有关。根据对熵增耗散演化的分析，在 TNTI 区域揭示了熵增加和耗散减少，这是由 TNTI 附近的粘性涡度扩散引起的。在 TNTI 区域发现应变速率和旋转速率之间的相关性较弱，这与熵产的减少有关。研究了 TNTI 附近的涡度和应变之间的对齐，并在 TNTI 区域确定了涡度与广泛的特征向量方向的强对齐。