Three-dimensional green water velocity on a model structure

Flow kinematics of green water due to plunging breaking waves impinging on a simplified, 3D model structure was investigated in the laboratory. Two breaking wave conditions were tested: one with waves impinging on the vertical wall of the model at still water level, and the other with waves impinging on the horizontal deck surface. The bubble image velocimetry (BIV) technique was used to measure flow velocities. Measurements were taken on both vertical and horizontal planes. Evolution of green water flow kinematics in time and space was revealed and was found to be quite different between the two wave conditions, even though the incoming waves are essentially identical. The time history of maximum velocity is demonstrated and compared. In both cases, the maximum velocity occurs near the green water front and beneath the free surface. The maximum horizontal velocity for the deck impinging case is 1.44C with C being the wave phase speed, which is greater than 1.24C for the wall impingement case. The overall turbulence level is about 0.3 of the corresponding maximum velocity in each wave condition. The results were also compared with 2D experimental results to examine the 3D effect. It was found that the magnitude of the maximum vertical velocity during the runup process is 1.7C in the 3D model study and 2.9C in the 2D model study, whereas the maximum horizontal velocity on the deck is similar, 1.2C in both 3D and 2D model studies.     

Green water impact pressure on a three-dimensional model structure

Green water impact pressure due to plunging breaking waves impinging on a simplified, three-dimensional model structure was investigated in the laboratory. Two breaking wave conditions were tested: one with waves impinging on the vertical wall of the model at the still water level and the other with waves impinging on the horizontal deck surface. Pressure measurements were taken at locations in two vertical planes on the deck surface with one at centerline of deck and the other between the centerline and an edge. Impact pressure was found to be quite different between the two wave conditions even though the incoming waves are essentially identical. Two types of pressure variations were observed??impulsive type and non-impulsive type. Much higher pressure was observed for the deck impingement wave condition, even though the flow velocities were quite close. Void fraction was also measured at selected points. Impact pressure was correlated with the mean kinetic energy calculated based on the measured mean velocities and void fraction. Impact coefficient, defined as the ratio between the maximum pressure at a given point and the corresponding mean kinetic energy, was obtained. For the wall impingement wave condition, the relationship between impact pressure and mean kinetic energy is linear with the impact coefficient close to 1.3. For the deck impingement wave condition, the above relationship does not show good correlation; the impact coefficient was between 0.6 and 7. The impact coefficient was found to be a function of the rate of pressure rise.