Wall effect on the hydrodynamics of anguilliform swimming in confined space

Three-dimensional numerical simulations are conducted to investigate the wall effect in a tube, focusing on the hydrodynamic performance of anguilliform swimming. By systematically varying the gap G (the distance between the anguilliform swimmer's midline and the tube wall), wall effect on hydrodynamic forces is quantified by analyzing surface pressure distribution and vortex structures. Results indicate that when G ≥ 4.5 the hydrodynamic performance is no longer influenced by wall effect. However, for G < 4.5, the time-averaged thrust and power coefficients increase as G decreases, while the time-averaged lateral force coefficient is almost negligible. And when the gap decreases to G = 1.5, the time-averaged thrust coefficient and power coefficient increase by 25.5 % and 21.4 %, respectively. The study reveals that wall effect primarily enhances thrust by amplifying the surface pressure and intensifying vortex interactions near the wall. At smaller gaps, the flow evolves from stable double-row vortex structures as if in the unbounded domain to unsteady small-scale vortices. These findings provide insights into hydrodynamics of the anguilliform swimmer in the confined tube and offer practical implications for the design of bio-inspired underwater vehicles and the conservation of aquatic habitats.