The measurement of velocity fields of a plunging wave impacting on a structure in a two-dimensional wave flume was investigated experimentally. As the wave impinged and overtopped the structure, a large highly aerated region was created in front of the structure and water splashed on top of the structure. The broken wave in front of the structure and associated greenwater on top of the structure are highly aerated containing not only a large number of bubbles but also very large sizes of bubbles. A modified PIV method incorporating the traditional PIV method with the shadowgraphy technique was introduced to obtain the velocity in the highly aerated region and the splashing water on the deck by correlating the "texture" of the bubble images. It was found that the maximum fluid particle velocity in front of the structure during the impinging process is about 1.5 times the phase speed of the wave, while the maximum horizontal velocity above the deck is less than the phase speed. It was also found that the dam breaking solution does not work well in predicting the greenwater velocity.
It is well known that extreme waves have caused significant damages to offshore structures due to the tremendous forces of wave impingement (e.g., Buchner, 1995; Hamoudi and Varyani, 1998; Schoenberg and Rainey, 2002). Frequently, the impinging waves rush upward to the deck and create so-called greenwater on the deck that washes out and damages equipment and, in some cases, causes the injury or death to persons working on the deck.
The interaction of extreme waves and a structure has been studied for decades. Although some studies were carried out using numerical models, most of the models are based on the potential theory therefore the multiphase highly turbulent problem was not realistically simulated.
