Objective Flexible hydrofoils are widely used in ship propulsion and fluid machinery. Yet, how their deformation affects unsteady flow fields and hydrodynamic performance remains insufficiently understood.

Method This study combines numerical simulation with Dynamic Mode Decomposition (DMD) to systematically examine the flow around a NACA66 hydrofoil at 6° angle of attack under medium-to-high Reynolds number conditions. The rigid hydrofoil is compared with three deformed configurations: pure bending, pure torsion, and combined bending–torsion. DMD serves as a quantitative diagnostic tool to decompose the time-resolved velocity fields into coherent spatiotemporal modes, allowing analysis of their energy distribution and spatial structure, and thereby connecting flow dynamics directly to deformation.

Results Results show that hydrofoil deformation substantially alters lift–drag characteristics, with torsional deformation exerting the strongest influence on both mean values and fluctuation amplitudes. A key observation is the shift of the dominant spectral peak from 35 Hz (natural vortex shedding in the rigid case) to the imposed deformation frequency of 43 Hz across all deformed configurations, indicating global synchronization of the flow. Notably, through the first application of DMD to this problem, we reveal how torsional deformation regulates modal energy: it selectively amplifies the energy of the 43 Hz deformation-driven mode. This amplified mode exhibits stronger coherence and periodicity in its spatial vortex pattern, which is identified as the primary mechanism modifying hydrodynamic performance.

Conclusion DMD analysis confirms that torsional deformation actively reorganizes wake dynamics by intensifying and regularizing vortex structures through energy amplification at the deformation frequency. These findings offer important theoretical insight and a refined analytical framework for the optimized design and unsteady behavior prediction of flexible hydrofoils.