Surface velocity and impact pressure of green water flow on a fixed model structure in a large wave basin

• First attempt to measure full‐field green water velocities in a large wave basin.
• Correlate velocity and impact pressure.
• Examine scale effects by comparing measurements taken from different physical scales.

The present study investigated green water velocities and impact pressures caused by the impact of overtopping waves on a fixed deck structure in a large-scale, three-dimensional deep-water wave basin. Using the bubble image velocimetry technique, detailed two-dimensional surface flow structures on a horizontal plane, including the temporal and spatial distributions of the maximum horizontal velocities, were successfully obtained. Pressure measurements were also obtained along four different vertical positions at three different locations on the horizontal plane. Based on the mean velocity distributions on the deck surface, the most significant spatial variability of the propagating green water flow is the protruding wave front near the center of the deck during the early stages of the wave overtopping. The maximum front speed of 1.5C was first observed near the midpoint of the deck along the deck centerline with C being the wave phase speed. The flow velocities decreased to below 1C   once the wave front passed the rear edge of the deck. Most of the measured pressures showed impulsive impacts characterized by a sudden rise of the pressure peak. The highest pressure was observed as 1.65ρC21.65ρC2 at a midpoint and a rear edge of the deck with ρρ being the water density. Correlations between wave kinetic energy and dynamic pressure were examined to determine the impact coefficients. The phase speed based impact coefficient was found to vary within a narrow range between 0.29 and 1.69 and a practical value of 1.5 may be used in applications. It appears that the impact pressures on the structures were strongly affected by the changing front shape of the broken wave and the impulsiveness of the impinging wave that contains a considerable amount of air entrainment.