AI 中文总结
该研究针对海洋表面浮粒输运模型的偏差问题，通过实验测量浮粒扩散率与上升速度，验证了轨迹交叉理论，修正了湍流施密特数的取值，为浮粒输运模型提供了实验依据。
AI 中文摘要
预测自由表面边界层中浮粒的输运对研究诸多环境系统至关重要，包括上层海洋中的微塑料。当前输运模型改编自沉积物输运理论，通常依赖于静止上升速度和梯度扩散的假设，其中湍流施密特数$Sc_t$不确定。本研究通过实验对这类模型进行验证，研究近中性浮力、有限尺寸的球体、杆状颗粒和盘状颗粒在风驱动的波浪自由表面流中的垂直混合，从拉格朗日轨迹直接测量颗粒扩散率，并与基于欧拉浓度的估计值进行比较。总体而言，研究发现颗粒浮力是扩散率的主要控制因素，且当颗粒上升速度相对于湍流波动增大时，扩散率会降低。这些观察结果与轨迹交叉理论一致，即使存在波浪时也是如此。此外，研究发现，采用假设的静止上升速度从浓度剖面推断扩散率会使扩散率高估多达5倍，这与有效上升速度比相应静止值低多达80%的结果一致。还直接测量得到，中性浮力颗粒的$Sc_t \approx 1$，而浮粒的$Sc_t > 1$。这些实验结果共同表明，当标准模型闭合应用于海洋表面的浮粒时，其扩散率和上升速度都可能存在偏差。
英文摘要
Predicting the transport of buoyant particles in a free-surface boundary layer is important to the study of many environmental systems, including microplastics in the upper ocean. Current transport models, adapted from sediment transport theory, typically rely on assumptions of a quiescent rise velocity and gradient diffusion with an uncertain turbulent Schmidt number $Sc_t$. Here, we test this type of model against experiments by studying the vertical mixing of near-neutrally buoyant, finite-size spheres, rods, and disks in a wind-driven, wavy free-surface flow. We measure particle diffusivity directly from Lagrangian trajectories and compare against Eulerian concentration-based estimates. Overall, we find that particle buoyancy is the main control on the diffusivity, and that the diffusivity decreases as particle rise velocity grows relative to the turbulent fluctuations. These observations we find to be consistent with the crossing-trajectories theory, even in the presence of waves. In addition, we find that inferring the diffusivity from concentration profiles with an assumed quiescent rise velocity overestimates the diffusivity by up to a factor of $5$, consistent with effective rise velocities up to $80\%$ lower than the corresponding quiescent values. We also directly measure $Sc_t \approx 1$ for the neutrally-buoyant particles and $Sc_t > 1$ for the buoyant particles. Together, these experimental results demonstrate how standard model closures may be biased in both their diffusivities and rise velocities when applied to buoyant particles at the ocean surface.
