Experimental study of air entrainment in the flow past a cylinder close to a free surface

The objective of this paper is to experimentally investigate the air entrainment induced by free surface flow past a cylinder. A high-speed digital camera and a 2D-High Speed Ply system are applied to investigate the wake patterns and velocity field. The wake patterns are defined by occurrence or non-occurrence of air entrainment. A regime map of wake patterns is constructed to depict the wake patterns and states, i.e. Froude number (Fr) and gap ratio (hid): As shown in the wake patterns map, there are two regions with a clear boundary: the upper region represents the air entrainment condition, and the lower region non-air entrainment. With the increasing of Fr, the wake pattern migrates from the non-occurrence of air entrainment state to the occurrence state. While for gap ratio, there only exists a parameter space of 0.64 <= Fr <= 0.8, in which the wake patterns can change from the non-occurrence state to the occurrence state with the gap ratio increasing. Moreover, two type of air entrainment are observed, namely plunging breaker and counter flow entraining, respectively.