Helicity, defined as the inner product of the velocity and the vorticity vector, is elusive to measure even for laminar flows. Theoretical results for turbulence have shown that flow structures of high helicity are associated with low dissipation of kinetic energy, making them candidates for coherent structures that can persist longer than others in the flow field. As scalar transport in turbulence is enhanced by three-dimensional coherent motions, study of helicity could reveal flow structures that contribute more than others to scalar transport. Direct numerical simulations of turbulent channel flow at ${\mathrm{Re}}_{\tau }=300$ in conjunction with Lagrangian tracking of fluid particles and passive scalar markers for Schmidt numbers 0.7 and 200 are used here. Local helicity along the trajectories of these markers and the fluid particles is calculated. Different levels of helicity are found to relate to the probability that these markers will remain inside a coherent region. The Lagrangian helicity distribution is also presented, indicating a change of the alignment between velocity and vorticity vectors for scalar transport markers.

中文翻译：

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使用螺旋线研究壁湍流中的标量传输

螺旋度定义为速度和涡度矢量的内积，甚至对于层流也难以测量。湍流的理论结果表明，高螺旋度的流动结构与动能的低耗散相关，使它们成为相干结构的候选对象，该结构在流场中的持久性可能比其他结构更长。由于三维相干运动增强了湍流中的标量传输，因此对螺旋度的研究可以揭示对标量传输贡献最大的流动结构。湍流通道流动的直接数值模拟${\mathrm{回覆}}_{\tau }=300$结合流体粒子的拉格朗日跟踪和施密特数0.7和200的被动标量标记一起使用。沿着这些标记和流体粒子的轨迹计算局部螺旋度。发现不同程度的螺旋度与这些标记将保留在相干区域内的概率有关。还显示了拉格朗日螺旋度分布，表明标量运输标记的速度和涡度向量之间的对齐方式发生了变化。