Abstract The role of mean shear on the orientational behavior of aspherical particles is studied by Lagrangian tracking of inertial spheroids in a directly simulated turbulent Couette–Poiseuille flow. The effects of particle inertia and shape in the conventional wall-turbulence near the stationary wall closely resemble observations made earlier in turbulent channel flows. The particles’ orientational behavior in the anisotropic turbulence field near the shear-free wall is qualitatively different. Direct comparisons between the orientation statistics near the sheared and the shear-free wall provide an insight into the role played by the mean shear. We observed that disks (rods) with low inertia preferentially oriented in the directions of Lagrangian compression (stretching) of the fluid elements, similarly as already reported for tracer spheroids. Spheroids with intermediate inertia exhibited similar orientations as the low-inertia particles near the shear-free wall, although the behavior of such spheroids was greatly affected by inertia near the sheared wall. This is explained by a six-fold increase of the Kolmogorov time scale across the flow, which leads to a corresponding reduction of the local Stokes number near the shear-free wall. The most inertial spheroids, however, oriented almost randomly near the shear-free wall, a finding we ascribe to the absence of mean shear and inertial filtering of the anisotropic vorticity field. It can therefore be inferred that the presence of mean shear is essential for forming the preferential orientation of inertial spheroids in near-wall turbulence and the particle inertia alone merely randomizes particle alignment due to the inertia-filtering.

中文翻译：

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具有无剪切壁的湍流 Couette-Poiseuille 流中惯性球体的方向

摘要 通过在直接模拟的湍流 Couette-Poiseuille 流中对惯性球体进行拉格朗日跟踪研究平均剪切对非球面粒子取向行为的作用。在固定壁附近的常规壁湍流中，粒子惯性和形状的影响与早先在湍流通道流中所做的观察非常相似。在无剪切壁附近的各向异性湍流场中，粒子的取向行为在性质上是不同的。剪切壁和无剪切壁附近的取向统计之间的直接比较可以深入了解平均剪切所起的作用。我们观察到具有低惯性的圆盘（棒）优先朝向流体元素的拉格朗日压缩（拉伸）方向，类似于已经报道的示踪球体。具有中等惯性的球体表现出与无剪切壁附近的低惯性粒子相似的取向，尽管此类球体的行为受剪切壁附近惯性的影响很大。这是通过整个流动的 Kolmogorov 时间尺度增加六倍来解释的，这导致无剪切壁附近的局部斯托克斯数相应减少。然而，大多数惯性球体在无剪切壁附近几乎随机定向，我们将这一发现归因于各向异性涡量场没有平均剪切和惯性过滤。因此可以推断平均剪切的存在对于在近壁湍流中形成惯性球体的优先取向是必不可少的，并且由于惯性过滤，仅粒子惯性仅使粒子排列随机化。