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

• Applicant: Professor Dr.-Ing. Jochen Fröhlich
• Subject Area: Fluid Mechanics ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 316798177

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.

• DFG Programme: Research Grants
• International Connection: France
• Partner Organisation: Agence Nationale de la Recherche / The French National Research Agency
• Cooperation Partners: Dr. Delphine Doppler; Dr. Sara Puijalon