Experiments and simulations for the study of submerged aquatic canopies consisting of long flexible blades

水生生态系统因其丰富程度和在不同尺度上的各种作用（从当地河流的饮用水质量到对气候变化的大范围影响）而成为一个高度相关的主题。在水生冠层中，水流与柔性植物之间的流体力学相互作用决定了水力学以及沉积物、营养物质和污染物的输送。虽然在许多实验室研究中已经研究了具有刚性元素的冠层，但对于具有非常柔性叶片的冠层，即高柯西数的冠层知之甚少。在该项目中，通过明智地结合模拟和实验来研究它们在重构存在下的流体力学及其对标量输运的影响，解决了这种知识的缺乏。一个关键的特点是通过涉及水生植物和生态水力学专家，与生态相关条件紧密联系。针对三种类型的配置进行了实验和模拟：(1)测试单叶片和少量叶片的配置，以开发和验证方法；(2)具有高灵活性的均匀叶片的均匀冠层；(3)间隙模拟斑块尺度问题的冠层。生态水力学专家收集了真实的、叶片状的水生植物和斑块的特征数据，确保了流体力学实验和模拟的最佳参数选择。这些部分解决了完全相同的配置，例如，模拟提供了无法测量的数据。此外，通过进行参数的互补变化，利用了实验和模拟的各自优势。这产生了一个非常健全和庞大的数据库。实验和仿真都采用了创新技术。在实验中，PIV、PLIF和ADVP分别用于同时测量标量浓度、流体速度和叶片瞬时位置。特别是，声波多普勒速度剖面传感器以前还没有被用于这项任务。它允许测量瞬时速度剖面，上面和内部的冠同时与叶片运动。目前尚不存在对柔性构件构成的冠层进行令人信服的模拟。本文采用了一种创新的方法，将高效的浸入边界法与自己的半隐式耦合算法和一种极高效的coserat杆方案相结合。通过这种方式，具有数千个叶片的树冠的高分辨率模拟可能提供大量数据。这些数据的协作评估，也包括生态水力学专家，提供了跨学科知识的理想组合。其目标是详细了解高柯西数冠层内及其上的复杂过程，并将其转化为与水生生态系统有关的信息。

Aquatic ecosystems constitute a topic of high relevance due to their abundance and their various roles on different scales, ranging from the quality of drinking water taken from the local river to the large-scale impact on climate change. The fluid mechanical interaction between the flow and the flexible plants in an aquatic canopy determines hydraulics as well as transport of sediment, nutrients and pollutants. While canopies with rigid elements have been investigated in many laboratory studies, far less is known about canopies with very flexible blades, i.e. high Cauchy number. This lack of knowledge is addressed in the project by a judicious combination of simulations and experiments to investigate their hydromechanics in the presence of reconfiguration and their impact on the transport of scalar quantities. A key feature is the tight connection to ecologically-relevant conditions by involving a specialist for aquatic plants and ecohydraulics. Experiments and simulations are performed for three types of configurations: (1) test configurations of a single blade and a small number of blades to develop and validate methods, (2) homogeneous canopies with uniform blades of high flexibility, (3) canopies with clearances mimicking patch-scale issues. Data for characterization of real, blade-like, aquatic plants and patches are gathered by the ecohydraulics specialist ensuring an optimal choice of parameters for the fluid mechanics experiments and simulations conducted. These partly address exactly the same configuration with, e.g., simulations providing data which cannot be measured. In addition, the respective advantages of experiments and simulations are exploited by performing complementary variations of parameters. This yields a very sound and large database. In both, experiment and simulation, innovative technologies are employed. For the experiments, PIV, PLIF and ADVP are adapted for simultaneous measurements of scalar concentration, fluid velocity and instantaneous position of blades. In particular, the Acoustic Doppler Velocity Profile sensor has not yet been used for this task before. It allows measuring instantaneous velocity profiles both above and inside the canopy simultaneously with blade motion. Convincing simulations of canopies made of flexible elements do not exist up to now. Here, an innovative method is employed combining a highly efficient immersed boundary method with an own semi-implicit coupling algorithm and an extremely efficient scheme for a Cosserat rod. In this way, highly resolved simulations for canopies with thousands of blades are possible furnishing a huge wealth of data. The collaborative assessment of these data, also involving the ecohydraulics specialist, provides an ideal combination of interdisciplinary knowledge. The vision is to generate detailed understanding of the complex processes in and over high-Cauchy number canopies and to turn that into information relevant for aquatic ecosystems.