Hydroelastic performance of fish-like swimmers in confined flows: Experimental and numerical investigation

Flexible fish that propel themselves through body undulation represent natural examples of hydroelastic coupling, where body deformation and hydrodynamic loading interact to generate thrust. Despite extensive research on unconfined swimming, the effects of environmental confinement and body geometry on hydroelastic performance remain poorly quantified. This study contributes to addressing this gap through a combined experimental-numerical approach linking fish observations with bio-inspired hydrodynamic modelling. Experiments on Atlantic salmon in a recirculating tunnel showed lower critical swimming speeds than wild fish, and suggested that both body bluntness and wall confinement might influence propulsion performance. To help interpret these observations, complementary two-dimensional (2D) self-propelled simulations of fish-like foils extended the analysis to open water and quantified how body thickness and tunnel width influence thrust, drag, and propulsive efficiency. Blunter (thicker) foils generated larger pressure drag and lower net thrust, while relative clearances below O(ten body thicknesses) altered wake features and reduced efficiency. Because the simulations are 2D and laminar, this threshold represents an approximate scale for the onset of wall effects rather than a precise limit, while the experiments also show pronounced confinement effects at comparable relative clearances. Including translational recoil enabled two-way fluid-structure interaction, showing that confinement can increase the Strouhal number required for self-propelled equilibrium, consistent with experimental observations. Overall, the results indicate that the effective propulsion regime is governed not only by Strouhal scaling but also by the level of hydrodynamic confinement. These findings help interpret swim-tunnel experiments and inform the design of bio-inspired flexible swimmers operating in confined environments.