Effect of swarm configuration on fluid transport during vertical collective motion

Understanding the hydrodynamics of self-propelled organisms is critical to evaluate the role of migrating zooplankton aggregations in sustaining marine ecosystems via the transport of nutrients and mixing of fluid properties. Analysis of transport and mixing during swimming is thus essential to assess whether biomixing is a relevant source of kinetic energy in the upper ocean. In this study, dilute swarms of the ephyral Aurelia aurita were simulated under different configurations to analyze the effects of inter-organism spacing and structure of a migrating aggregation on fluid transport. By using velocimetry data instead of numerically simulated velocity fields, our study integrates the effects of the near- and far-field flows. Lagrangian analysis of simulated fluid particles, both in homogeneous and stratified fluid, shows that the near-field flow ultimately dictates fluid dispersion. The discrepancy between our results and predictions made using low-order models (both in idealized fluid and within the Stokes limit) highlights the need to correctly represent the near-field flow resulting from swimming kinematics and organism morphology. Derived vertical stirring coefficients for all cases suggest that even in the limit of dilute aggregations, self-propelled organisms can play an important role in transporting fluid against density gradients.

理解自推进生物的流体动力学对于评估浮游动物集群迁移通过营养物质运输和流体性质混合在维持海洋生态系统中的作用至关重要。因此，对游泳过程中的运输和混合进行分析对于评估生物混合是否是上层海洋动能的一个相关来源是必不可少的。在这项研究中，对不同配置下的碟状幼体海月水母稀疏集群进行了模拟，以分析生物间间距和迁移集群结构对流体运输的影响。通过使用测速数据而非数值模拟的速度场，我们的研究综合了近场和远场流动的影响。对均匀和分层流体中模拟流体粒子的拉格朗日分析表明，近场流最终决定了流体的扩散。我们的研究结果与使用低阶模型（在理想流体和斯托克斯极限内）所做的预测之间的差异，凸显了正确呈现由游泳运动学和生物形态产生的近场流的必要性。所有情况下推导出的垂直搅拌系数表明，即使在稀疏集群的极限情况下，自推进生物在逆着密度梯度运输流体方面也能发挥重要作用。