Author(s): Islam Abdelghafar; Jonathan D. Bolland; Dominique Thevenin; Philip A. Rubini; Rosalind M. Wright; Stefan Hoerner

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Keywords: Closed-conduit; pipe fittings; eel motion; CFD-DEM; overset 6-DOF

Abstract: The European eel (Anguilla anguilla) is distributed from the North Cape in Norway, southwards along the coast of Scandinavia and the Baltic states throughout Atlantic Europe and all the coasts of the Mediterranean including North and West African Coast. It can be found in a wide variety of freshwater and estuarine habitats. In the past three decades, a dramatic decline in the number of eels reaching European river systems has been recorded. European eel is listed as "critically endangered" under the IUCN red list of the threatened species. Hydraulic structures across rivers (e.g. low-head weirs) and other in-river anthropogenic barriers (e.g., pumping stations, hydropower facilities) impede downstream migration of European eels. The identification of hazardous regions in technical facilities and their assessment in an early design phase is necessary to achieve safe migration. Computational fluid dynamics (CFD) provide a great variety of approaches to simulate fluid flows and allow for the implementation of fish and eel surrogate models without the need of doing live fish tests. Various numerical approaches with varying modelling complexities for tracking European eel motion including streamlines, particle-based methods, and Dynamic fluid/body Interaction (DFBI) were investigated in the present study. A closed-conduit component, typically, a flanged eccentric reducer is considered as a test case to demonstrate the different numerical approaches. One of the early attempts is to treat a fish trajectory as a streamline and all relevant flow quantities (e.g., velocity, pressure, shear stress) could be derived along this path. A more recent tracking method for fish or eel motion is to include a Lagrangian approach, in which the eel is approximated as a super-ellipsoid particle, composite particles, or a Flexible Fiber composed of bonded segments with the aid of the Discrete Element Method (DEM). Finally, a boundary resolved approach, Dynamic Fluid-Body Interaction (DFBI), can be used efficiently to track the eel motion by solving the equations of motion with 6 degrees of freedom (6-DOF) coupled with the fluid flow. The DFBI motion can be combined either with overset Chimera or overlapping meshes to track rigid bodies or with an additional body morphing function which allows for the tracking of flexible bodies. DFBI combined with overset meshes is a highly accurate approach and provides very precise insights in the fluid-body interactions of eels in a surrounding fluid due to the resolved boundaries. However, it has drawbacks in terms of computational cost and complexity of mesh construction. A qualitative comparison of the proposed numerical approaches for tracking eel motion through a closedconduit is addressed in terms of accuracy, computational cost, and contact modelling.

DOI: https://doi.org/10.3929/ethz-b-000675921

Year: 2024