Ocean mixing by active swimmers

活跃游泳者的海洋混合

• 批准号: 328915886
• 负责人: Dr.-Ing. Thomas Köllner
• 金额: --
• 依托单位: Department of Mechanical Engineering
• 依托单位国家: 德国
• 项目类别: Research Fellowships
• 财政年份: 2017
• 资助国家: 德国
• 起止时间: 2016-12-31 至 2018-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/328915886?language=en
• 关键词: Ocean; mixing; active; swimmers

The mixing of seawater caused by the daily vertical migration of zooplankton received growing attention in the field of oceanography and biophysical research in the last years. Besides the desire to understand processes in the biosphere, this interest originates from a suggested impact of swimming microorganism on the mixing of seawater on scales even critical for the global oceanic circulation. Current theories for biomixing are mostly based on simple scaling laws or closure models that poorly consider effects from a collective migration of large swarms.In this project, we will study the vertical migration of swimmers that interact with their surrounding water column. We will develop a model for the vertical migration of swimmers based on experimental observations from literature and by collaborating with an experimental group. On this basis, highly resolved direct-numerical simulations will be conducted simultaneously accounting for the individual motion of a host of active swimmers. A first goal is to reveal the conditions for the formation of large-scale flow structures that have been observed for the collective vertical migration of brine shrimp. Furthermore, we explore the mixing efficiency of theses swarms of swimmers on water that is stable stratified due to vertical salinity and temperature gradients. These numerical results will provide a basis for simplified models of vertical temperature and salinity transport that do not resolve length scales of individual swimmers.A second goal is to clarify the impact of active swimmers on the development of salt fingers, which can be a sensitive factor in the mixing of the upper water column. Salt fingers are a certain type of double-diffusive convection, i.e., their emergence crucially depends on the relative diffusivity of temperature and salinity (measured by the Lewis number), which is effectively rendered by the small-scale motion of active swimmers. Actually, salt fingers arise in near-surface water that is saltier and warmer than in deeper layers. Such a situation will be studied by solving the transport equation for temperature and salinity together with the momentum balance for seawater including swimmers motion. This will reveal the preconditions for salt fingers influenced by biomixing.A better understanding of the interplay between zooplankton motion and mixing processes will improve our understanding of oceanic dynamics on large scales and can thus improve coupled large-scale atmospheric-ocean models.

近年来，由浮游动物的垂直迁移引起的海水混合问题在海洋学和生物物理学研究领域受到越来越多的关注。除了希望了解生物圈中的过程之外，这种兴趣还源于游泳微生物对海水混合的影响，这种影响甚至对全球海洋环流至关重要。目前的生物混合理论大多是基于简单的比例定律或封闭模型，很少考虑大群体集体迁移的影响。在这个项目中，我们将研究游泳者与周围水柱相互作用的垂直迁移。我们将开发一个模型的垂直迁移的游泳运动员的实验观察文献的基础上，并与实验组合作。在此基础上，高分辨率的直接数值模拟将同时进行占一个活跃的游泳者主机的个人运动。 第一个目标是揭示大规模的流动结构，已观察到的集体垂直迁移的卤虫形成的条件。此外，我们探讨的混合效率，这些成群的游泳者的水是稳定的分层，由于垂直盐度和温度梯度。这些数值结果将提供一个简化的模型的温度和盐度的垂直传输，不解决的长度尺度的单个游泳者的基础。第二个目标是澄清的影响，积极的游泳者对盐指的发展，这可能是一个敏感的因素，在混合的上层水柱。盐指是某种类型的双扩散对流，即， 它们的出现主要取决于温度和盐度的相对扩散率（用刘易斯数测量），这是由活跃的游泳者的小尺度运动有效地呈现的。实际上，盐指出现在近地表水，比深层水更咸，更温暖。这种情况将通过求解温度和盐度的输运方程以及包括游泳者运动的海水的动量平衡来研究。研究浮游动物运动与混合过程之间的相互作用，有助于加深对大尺度海洋动力学的认识，进而完善大尺度海气耦合模式。