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

• Final Report Year: 2022

Final Report Abstract

Vegetation layers in water bodies are essential components of ecological systems. Their fluid mechanical properties are extremely complex, as the individual structural elements, i.e. the plants, are usually deformable and interact with the flow. Furthermore, there are collective effects, resulting in a multi-scale system of different interacting phenomena. So far, almost exclusively rigid elements have been studied in the literature. Before the project started, only a handful of works on moving elements existed, and in these the number of elements was small and the modelling of the solids highly simplified. The present project was the first worldwide to experimentally and numerically model a vegetation layer with very long, very flexible structures characterised by a high Cauchy number. The aim was to obtain fundamental information about such vegetation layers and in particular about the inter- actions between the flexible structures and the flow passing through and over this arrangement. This goal was pursued in a joint German-French process involving three teams. In the project part at TU Dresden, first numerical methods were developed able to cope with the very demanding physical situation. This includes a new fluid-structure coupling algorithm, as well as an improved method of representing collisions between the structures. These methods have been extensively validated and then used to simulate configurations of varying complexity. They ranged from singular structures, to configurations with a single row of structures, layers consisting of elements of medium Cauchy number, to a configuration with very high Cauchy number featuring side walls, which had been built exactly like this in the Lyon flume. First, statistics of the flow and the structural movement were determined. These results are new and interesting. For example, the secondary flow effects in the area of the vegetation layer were were found to be clearly weaker than near the free surface. Furthermore, frequency spectra of the structural movement were de- termined. They prove that in the investigated situation no so-called monami occurs, a wave-like collective movement of the structures, an observation consistent with the experimental measure- ments. The large amounts of data provide extensive material beyond the project for further inves- tigation, which is currently underway. The numerical method has proven to be extremely robust and successful. It is now available for similar questions in the biological as well as in the technical field.

Publications

• A constraint-based collision model for Cosserat rods. Archive of Applied Mechanics, 89(2):167–193, 2019
  Silvio Tschisgale, Louis Thiry, and Jochen Fröhlich
  
• A first simulation of a model aquatic canopy at high Cauchy number. In Proceedings of the 10th Conference on Fluvial Hydraulics (Delft, Netherlands, 7-10 July 2020), London. CRC Press, 2020
  Bastian Löhrer, Delphine Doppler, Sara Puijalon, Nicolas Rivière, J. John Soundar Jerome, and Jochen Fröhlich
  
• An immersed boundary method for the fluid-structure interaction of slender flexible structures in viscous fluid. Journal of Computational Physics, 423:109801, 2020
  Silvio Tschisgale and Jochen Fröhlich
  
• Velocity field in a single row canopy made of flexible blades for a static reconfiguration case and a monami case. In Proceedings of the 10th Conference on Fluvial Hydraulics (Delft, Netherlands, 7-10 July 2020), London. CRC Press, 2020
  Bastian Löhrer, J. John Soundar Jerome, Sylvie Barsu, Nicolas Rivière, Delphine Doppler, and Jochen Fröhlich
  
• An efficient solver for a Cosserat rod of non-constant width applied to a problem of fluidstructure interaction. PAMM, 21(1):e202100152, 2021
  Karl Schoppmann, Bastian Löhrer, Silvio Tschisgale, Jochen Fröhlich, and Emmanuel de Langre
  
• Large eddy simulation of the fluid–structure interaction in an abstracted aquatic canopy consisting of flexible blades. Journal of Fluid Mechanics, 916:A43, 2021
  Silvio Tschisgale, Bastian Löhrer, Richard Meller, and Jochen Fröhlich