Balancing thrust and stability: design and simulation of a miniature median–paired-fin underwater robot

This work presents the design, fabrication, experimental testing, and numerical analysis of a 15-cm bio-inspired robotic fish that swims by a median–paired-fin (MPF) traveling-wave mechanism. A twin-crankshaft/three-rocker transmission, fabricated entirely by fused-filament printing (PLA+ fin skeleton) and cast silicone membranes, drives the fin with only two low-cost brushed DC motors. A simplified moving-mesh model implemented in COMSOL Multiphysics® couples a piecewise kinematic input to an extremely fine, dynamically remeshed domain and reproduces the fluid–structure interaction in 60 min of CPU time per case. Experiments conducted in a 1 m × 0.6 m × 0.4 m tank show that increasing the supply voltage from 3 V to 5 V raises the mean swimming speed from 1.44 cm s−1 to 1.93 cm s−1; the model predicts these speeds within ±5%. Flow-field visualizations reveal that the performance gain is accompanied by larger vortex structures and higher turbulent kinetic energy, identifying a trade-off between maximum speed and directional stability.