Unsteady foil propulsion over sediment beds: wake alteration and efficiency loss in particle-laden flows

The current study examines the interaction between a pitching foil and sediment-laden flows, focusing on how particles affect propulsion performance and the sediment dynamics. We show that the presence of particles weakens vortex formation, reduces flow flux and alters the pressure distribution, leading to a decrease in propulsion efficiency. Additionally, sediment deformation and particle suspension under the flow are observed, driven by fluid–structure coupling rather than direct contact with the foil. The coupled dynamics between the foil’s motion, sediment evolution and particle transport creates a feedback loop that influences both the wake structure and sedimentary patterns. Our results underscore the importance of considering the interactions between the foil, flow and particles for a comprehensive understanding of bio-inspired propulsion in sediment-laden environments. This work provides insights for optimising the design and performance of hydrofoils in real-world granular flows and highlights the need for further investigation into the effects of particle types, concentrations and foil motion modes on propulsion efficiency.