论文标题
二维Navier-Stokes湍流中的相干结构检测和逆级联机理
Coherent structure detection and the inverse cascade mechanism in two-dimensional Navier-Stokes turbulence
论文作者
论文摘要
在自然界，实验和数值模拟中观察到了二维Navier-Stokes湍流中的相干结构。本研究基于Okubo-Weiss标准，涡度幅度和Lagrangian相干结构（LCSS）进行了几种结构检测方案的比较，重点是二维流体动力湍流中的逆级联。最近引入的涡流缩放现象学[B. H. Burgess，R。K。Scott，J。FluidMech。，811：742--756，2017]允许根据流量的物理特性来量化这些方法所需的相应阈值。所得的提高可比性可以识别使用的结构检测技术之间检测灵敏度的特征相对差异。对于能量反向级联，相干结构可如预期的那样贡献，而跨尺度通量的贡献要比流量的残留不一致部分少得多，尽管能量占主导的相干结构导致了能量光谱的重要大规模变形。可以通过增加一致结构内应变率和亚网格应力量张量的未对准来理解这种级联反应。同时，这些结构表现出强烈和局部的非线性跨尺度相互作用，似乎可以稳定它们。我们根据多尺度梯度方法来量化和解释相干结构的形状保存[G. L. Eyink，J。FluidMech。，549：191--214，2006]，因为应变旋转和涡度梯度拉伸的耗竭，而残留波动的动力学与涡旋变薄一致。
Coherent structures in two-dimensional Navier-Stokes turbulence are ubiquitously observed in nature, experiments and numerical simulations. The present study conducts a comparison between several structure detection schemes based on the Okubo-Weiss criterion, the vorticity magnitude, and Lagrangian coherent structures (LCSs), focusing on the inverse cascade in two-dimensional hydrodynamic turbulence. A recently introduced vortex scaling phenomenology [B. H. Burgess, R. K. Scott, J. Fluid Mech., 811:742--756, 2017] allows the quantification of the respective thresholds required by these methods based on physical properties of the flow. The resulting improved comparability allows to identify characteristic relative differences in the detection sensitivity between the employed structure detection techniques. With respect to the inverse cascade of energy, coherent structures contribute, as expected, substantially less to the cross-scale flux than the residual incoherent parts of the flow although the energetically dominant coherent structures lead to an important large-scale deformation of the energy spectrum. This cascade inactivity can be understood by an increased misalignment of strain-rate and subgrid stress tensors within coherent structures. At the same time, the structures exhibit strong and localised nonlinear cross-scale interactions that appear to stabilize them. We quantify and interpret the resulting shape preservation of coherent structures in terms of a multi-scale gradient approach [G. L. Eyink, J. Fluid Mech., 549:191--214, 2006] as the depletion of strain rotation and vorticity gradient stretching while the dynamics of the residual fluctuations are consistent with the vortex thinning picture.
