The present study investigates the void fraction as well as velocity of an overtopping flow on a structure through laboratory measurements. The overtopping green water was generated by the impingement of a plunging breaking wave on the structure. The flow is multi-phased and turbulent with significant aeration. In this study, fiber optic reflectometer and bubble image velocimetry were employed to measure the void fraction and velocity in the flow, respectively. Various mean properties were obtained by ensemble-, time-, and depth-averaging the repeated instantaneous measurements. The temporal and spatial variations of void fraction reveal very high aeration near the front of green water and relatively low aeration in the region near the deck surface. Using the measured data, a similarity profile was obtained for the depth-averaged green water void fraction on top of the structure. The void fraction profile suggests that using only velocity data may be insufficient if the flow momentum is to be estimated. The accuracy of the void fraction measurement was validated by comparing the directly measured water volume of the overtopping flow with the calculated water volume from the measured velocity and void fraction. Copyright 2008 by The International Society of Offshore and Polar Engineers (ISOPE).
