Low-frequency water wave absorption by Helmholtz-resonant oscillating water column: Numerical and experimental study

Oscillating water columns incorporating Helmholtz resonator features were analysed using numerical and experimental techniques, with particular focus on low-frequency wave absorption. When the Helmholtz resonance is triggered, the reciprocating flow at the sharp lip-wall base is strongly accelerated, causing flow separation, vortex shedding, and viscous dissipation. To improve wave energy capture, a parametric optimisation was conducted by varying the lip-wall draft, rounded lip radius, and chamber opening ratio. The numerical results show that replacing the sharp lip-wall base with a rounded transition suppresses vortex-induced dissipation during low-frequency resonance, thereby allowing more incident wave energy to be converted into water-column motion available for pneumatic power extraction. Compared with conventional oscillating water columns, the main efficiency peaks of which generally occur at 1.3 < kh < 1.7, the proposed rounded-lip Helmholtz-resonant oscillating water column exhibits efficient absorption in the lower frequency range, with a peak efficiency exceeding 0.90 near kh = 0.77. Froude-scaled experiments further show that the rounded-lip configuration increases the peak hydrodynamic efficiency from 0.59 to 0.90 compared with the sharp-lip configuration. The combined results demonstrate that the proposed modification enhances low-frequency wave absorption by reducing vortex-induced losses near the submerged neck.