Evaluating the air spring effect in U-OWC devices: A comparative study of incompressible and compressible CFD solvers

Air compressibility plays a central but underappreciated role in Oscillating Water Column (OWC) wave energy devices. Standard incompressible computational fluid dynamics (CFD) solvers treat the trapped air as a rigid fluid, an assumption holding only in narrow laboratory conditions and breaking down at prototype scale or when the air volume is thermodynamically corrected. This study quantifies that breakdown by directly comparing the incompressible interFoam and fully compressible compressibleInterFoam solvers within an OpenFOAM numerical wave tank, validated against flume experiments on a 1:15 scale U-OWC model at the COAST Laboratory, University of Plymouth. Two configurations were tested: a Froude-scaled blanking setup (76 L internal air volume) and a thermodynamically corrected expansion-chamber configuration (976 L total active air volume, including approximately 900 L of additional pneumatic volume), covering orifice ratios Or = 0.9%–1.0% over the dimensionless range B/L = 0.039–0.103. In the blanking configuration, compressibleInterFoam consistently reduced the resonant-peak capture width ratio ( CWR) error, roughly halving it at the higher-damping orifice. In the expansion chamber, interFoam overestimated CWR by up to 30% because it cannot treat the large air volume as a compliant pneumatic spring, whereas compressibleInterFoam reproduced the measured CWR with a resonant-peak error of only 3.6%, demonstrating that explicitly resolving air compressibility provides substantially improved prediction of the pneumatic response in the thermodynamically scaled expansion-volume configuration.