Hydrodynamic evaluation of a spider-inspired underwater robot using distributed flapping fin propulsion

This paper presents the design and hydrodynamic evaluation of a spider-inspired underwater hexapod robot employing distributed flapping-fin propulsion. Unlike conventional bio-inspired underwater robots with centralized fin actuation, the proposed system integrates flexible bilateral side fins between leg triads, enabling decentralized lift generation and inherent body stabilization. A three-tier validation framework is adopted, comprising (i) quasi-steady analytical estimation of lift, drag, Reynolds number, and lift-to-power scaling, (ii) transient Computational Fluid Dynamics (CFD) simulations in ANSYS Fluent resolving pressure distribution, vortex shedding, and wake evolution, and (iii) preliminary experimental validation using a laboratory-scale prototype in controlled water-tank conditions. Theoretical, numerical, and experimental results show close agreement, with lift predictions within ± 10-15% across operating regimes. CFD results indicate stable hydrodynamic performance over fin-tip velocities U = 0.6-0.8 m/s, yielding a nearly constant lift-to-drag ratio of ≈ 1.3. Non-dimensional analysis confirms operation within an efficient Strouhal number range. The study establishes the hydrodynamic feasibility and energetic advantages of distributed flapping-fin propulsion integrated with a multi-legged body architecture. The results provide validated design insights for the development of manoeuvrable and energy-efficient underwater robotic platforms for inspection, monitoring, and exploration tasks.