Greenwater due to plunging breaking wave impingement on a deck structure. Part 2: Impact pressure and air fraction

Abstract

Greenwater generated by plunging wave breakers can impose impulsive and highly localized loads on deck structures, yet the influence of entrained air remains insufficiently quantified because pressure and air fraction are rarely measured at nearly co-located positions. Extending the work of Chuang et al. (2023, Ocean Engineering, 287, 115859), this study simultaneously measures impact pressure and local air fraction on a rigid horizontal deck using a 40-point grid of flush-mounted pressure transducers and fiber-optic reflectometers. Two plunging-wave conditions representing near-edge and more inboard impacts were each repeated 30 times. Incident-wave repeatability and bootstrap convergence analyses were used to assess the adequacy of local ensemble means. Results show a narrow primary pressure peak when the water-rich plunging tongue reaches the deck, followed in some cases by delayed reloading associated with deformation and collapse of entrapped air cavities. The maximum ensemble-averaged pressure reached 15.58 kPa at X50-Z60 under the inboard-impact condition. Air-fraction measurements indicate rapid wetting near the lower probes and intermittent aeration near the upper probes. Higher local air fraction was associated with lower normalized peak pressure under the tested conditions, although the weak correlations and the influence of impact velocity and morphology preclude an isolated causal interpretation.