Abstract
The characteristics of the circulation flow were investigated for the water flow with the air entrainment slightly from the free water surface in a three-dimensional rectangular outlet pit. At first, the air entrainment pattern was classified into two types by the visual observations.Measurement of the flow velocity distribution in the pit revealed that the dominant types of flow include circulating flow and vertical eddies. And relation of the three-dimensional steady flow field and the primary factor of the air entrainment were clarified. The primary factor of air entrainment made clear that it was the velocity of the radiated surface flow from the end wall, the flow velocity along the side wall and the instantaneous velocity of downward flow at water surface in outlet pit. Turbulent flow velocities in the three-dimensional flow field near the water surface were also investigated. It was concluded from these results that air entrainment is caused by interference between the steady circulating flow and instantaneous velocity variations near the water surface.
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