湍流通道流中的长刚性纤维：实验与模拟的比较
Long rigid fibres in a turbulent channel flow: comparison between experiments and simulations
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中文总结 AI 辅助
研究湍流通道流中长刚性纤维动力学，通过实验和模拟对比，用基于离壁距离的统计表征纤维长度等因素影响，发现实验与模拟在取向、翻滚吻合好，平移速度差异大，突出模型未完全体现的沉降等影响。
中文摘要 AI 辅助
对湍流通道流中长刚性纤维的动力学进行了实验和数值研究。实验使用了三种长度（\(\ell/h = 0.25\)、\(0.5\)和\(1\)）且具有中等惯性（\(St^+\approx20\)）的聚苯乙烯纤维。其沉降速度与摩擦速度相当，导致在底壁附近积聚。将测量结果与基于刚性细长体模型的模拟进行系统比较。通过基于离壁距离的统计来表征纤维长度和限制对平移、取向和翻滚的影响。远离壁时，实验纤维滞后于流体，速度亏缺与长度无明显依赖关系。靠近壁时，最短纤维比局部平均流移动快，较长纤维则较慢，表明对近壁湍流的采样与长度有关。限制也强烈约束取向和旋转。靠近壁的纤维主要经历“皮划艇”运动，在大致平行于壁的平面内翻滚。当壁距离用纤维长度归一化时，取向统计结果一致，确定\(y^+/\ell^+\)为相关几何变量。在纤维可获得显著壁法向取向处，“撑杆跳”事件在\(y^+\approx\ell^+/2\)处产生局部翻滚率最大值，其大小约以\((\ell^+)^{-2}\)减小，与基于沿纤维采样的近壁速度变化的量纲估计一致。实验和模拟在取向和翻滚方面吻合良好，但在平移速度方面差异较大。这些差异突出了沉降、有限纤维厚度和有限滑移雷诺数的影响，而模型未完全体现这些影响。
英文摘要
The dynamics of long rigid fibres transported by turbulent channel flow are investigated experimentally and numerically. Experiments use polystyrene fibres of three lengths, $\ell/h=0.25$, $0.5$ and $1$, with moderate inertia, $St^+\approx20$. Their settling velocity is comparable to the friction velocity, causing accumulation near the bottom wall. Measurements are compared systematically with simulations based on a rigid slender-body model. Statistics conditioned on the distance from the wall are used to characterise the effects of fibre length and confinement on translation, orientation and tumbling. Away from the wall, the experimental fibres lag the fluid, with no clear dependence of the velocity deficit on length. Near the wall, the shortest fibres move faster than the local mean flow, whereas longer fibres remain slower, indicating length-dependent sampling of near-wall turbulence. Confinement also strongly constrains orientation and rotation. Fibres close to the wall predominantly undergo "kayaking" motion, tumbling in planes approximately parallel to it. Orientation statistics collapse when wall distance is normalised by fibre length, identifying $y^+/\ell^+$ as the relevant geometrical variable. Where fibres can acquire a significant wall-normal orientation, "pole-vaulting" events produce a local tumbling-rate maximum at $y^+\approx\ell^+/2$. Its magnitude decreases approximately as $(\ell^+)^{-2}$, consistently with a dimensional estimate based on the near-wall velocity variation sampled along the fibre. Experiments and simulations agree well for orientation and tumbling but differ more for translational velocity. The discrepancies highlight the effect of settling, finite fibre thickness and finite slip Reynolds number that are not fully represented by the model.
