Rigid fiber transport in a periodic non-homogeneous geophysical turbulent flow

From anthropogenic litter carried by ocean currents to plant stems traveling through the atmosphere, geophysical flows are often seeded with elongated, fiber-like particles. In this study, we used a large-scale laboratory model of a tidal current -- representative of a widespread class of geophysical flows -- to investigate the tumbling motion of long, slender, and floating fibers in the complex turbulence generated by flow interactions with a tidal inlet. Despite the non-stationary, non-homogeneous, and anisotropic nature of this turbulence, we find that long fibers statistically rotate at the same frequency as eddies of similar size, a phenomenon called \textit{scale selection}, which is known to occur in ideal turbulence. Furthermore, we report that the signal of the instantaneous transverse velocity difference between the fiber ends differs significantly from the signal produced by the flow in the fiber surroundings, although the two are statistically equivalent. These observations have twofold implications. On the one hand, they confirm the reliability of using the end-to-end velocity signal of rigid fibers to probe the two-point transverse statistics of the flow, even under realistic conditions: oceanographers could exploit this observation to measure transverse velocity differences through elongated floats in the field, where superdiffusion complicates collecting sufficient data to probe two-point turbulence statistics at a fixed separation effectively. On the other hand, by addressing the dynamics of inertial range particles floating in the coastal zone, these observations are crucial to improving our ability to predict the fate of meso- and macro-litter, a size class currently understudied.

从洋流携带的人为垃圾到大气中传输的植物茎秆，地球物理流场中常常混杂有细长纤维状颗粒。本研究采用代表一类典型地球物理流场的大型潮流实验室模型，探究了细长漂浮纤维在潮汐口流场相互作用产生的复杂湍流中的翻滚运动。尽管该湍流具有非定常、非均匀且各向异性的特性，我们仍发现长纤维的统计旋转频率与同尺寸涡旋的旋转频率完全一致，这种现象被称为尺度选择（scale selection），此前已知仅存在于理想湍流场景中。此外，我们发现纤维两端的瞬时横向速度差信号与纤维周围流场产生的信号存在显著差异，尽管二者在统计意义上等价。上述观测结果具有双重意义：其一，即便在真实野外场景下，利用刚性纤维的端到端速度信号探测流场的两点横向统计特性依然可靠；海洋学家可借助这一发现，通过野外细长浮标测量横向速度差——此前由于超扩散效应的存在，通过固定间距有效采集足够数据以探测两点湍流统计特性的工作难度极大。其二，针对漂浮于近岸海域的惯性区颗粒动力学展开研究，上述观测结果对提升中尺度与大尺度海洋垃圾的归宿预测能力至关重要，而该尺寸区间的垃圾目前仍未得到充分研究。