Abstract
This study experimentally investigates the hydrodynamic optimisation and scaling fidelity of five oscillating water column (OWC) configurations: three single-chamber geometries (Conventional, U-OWC, L-OWC) and two novel dual-chamber hybrids (DCOWC-1 and DCOWC-2). Conducting 792 tests at 1:30 scale under regular waves, the campaign quantified the influence of air compressibility by contrasting correct thermodynamic scaling (using external expansion chambers) against the traditional incompressible method across pneumatic damping levels of Or = 0.4–1.6%. The compressible configuration creates a softer air spring effect, increasing wave discharge rates while decreasing pneumatic pressure; consequently, incompressible methods typically underestimated airflow and overestimated pressure damping. Under correctly scaled conditions, the U-OWC demonstrated superior capability and broad bandwidth, achieving a peak capture width ratio (CWR) of 0.79 at a dimensionless frequency B/L = 0.08, surpassing the Conventional (CWR = 0.48) and L-OWC (CWR = 0.46) geometries. The U-OWC proved highly sensitive to thermodynamic modelling: neglecting compressibility caused a 43% underestimation of peak efficiency under high damping, indicating incompressible experiments may misrepresent prototype-scale performance. Conversely, the L-OWC remained robust (5.4% deviation), while the Conventional OWC showed moderate sensitivity (12.5% underestimation and a resonance shift). For total energy extraction, the dual-chamber hybrids extended operational bandwidth via a double-peak mechanism: the superior DCOWC-2 reached a peak CWR of 0.85, nearly 31% higher than DCOWC-1, and maintained CWR > 0.50 across B/L = 0.03–0.09. This validates a mixed-damping strategy applying high pneumatic damping to the front U-OWC and lower pneumatic damping to the rear L-OWC to independently optimise each chamber.