Wall effects in fishes-like undulating hydrofoils

Researchers have been inspired to study the mechanics of aquatic animals to develop advanced underwater vehicles and robots, as they demonstrate swimming abilities that are far superior to those of human-made propulsion devices. Autonomous underwater vehicles (AUVs) designed for military, scientific, and environmental applications often require efficient propulsion near boundaries, motivating the study of boundary effect in swimming organisms. This research investigates the hydrodynamic characteristics of wall effects in undulating hydrofoils, exploring variations in undulation wavelength (λ), Strouhal number (St), wall clearance (h/c), and Reynolds number (Re). Our current work aims to elucidate how these parameters influence lift coefficient, drag coefficient, power consumption, and swimming efficiency compared to the free-stream conditions. We have explained how the kinematics of the undulating hydrofoils affect the direction of forces over their different locations and result in variations of lift and drag with the kinematic parameters. We have also explored the interaction of vortices shed from the trailing edge and their influences on the flow characteristics in swimmers near boundaries. Comparative analyses with rigid pitching hydrofoils highlight distinct hydrodynamic behaviors in undulatory motion, emphasizing the importance of flexibility and kinematic adjustments near boundaries. The wall effect increases viscous drag across the parameters explored in this study. Pressure thrust is appreciably enhanced at higher values of (St) and (λ). However, while the amplitude of lift increases monotonically with the wall clearance ratio for all λ, the time-averaged lift departs from a monotonic trend with clearance at lower λ ranges. Swimming power consumption increases near boundaries, but swimming efficiency decreases with lower wall clearance in moderate Reynolds number scenarios.