Flow aeration by an offset aerator: Air entrainment, bubbly flow turbulence, and adaptive-window cross-correlation processing

Abstract

Chute aerators are used to induce flow aeration to prevent cavitation damages to flood discharge structures. The aerated chute flows are traditionally studied in terms of the air concentration distributions and air entrainment rate, whereas further detailed characterization of the bubbly flow associated with the turbulence modification has been missing. Herein the air–water flow turbulence was studied experimentally for a simple offset aerator model in a sloping chute. Advanced interfacial turbulence analyses were applied to the air–water flow measurement results using a dual-tip phase-detection probe. The new adaptive-window cross-correlation approach was first applied to the turbulence intensity estimate in both regions upstream and downstream the impact point. The overall spatial distributions of basic air–water flow properties agreed favorably with literature, and highlighted different patterns of air–water flow development in different sections of the flow. The turbulence level of the bubbly flow, and the bubble-turbulence interactions, were quantitatively described with the interfacial turbulence intensity, correlation turbulent length scale, and some measures of the bubble clustering behaviors. The interfacial turbulence intensity was obtained in the same order of magnitude as the common water-phase turbulence intensity observations, except for the impact zone. The results revealed the shear flow development and turbulence dissipation in the bottom layer of the aerated chute flow, which affected the local average bubble counts, the relative motions of neighboring bubbles, the diffusive transport of bubbly eddy structures, and consequently the near-wall bubble distributions critical for the protection of chute surface from cavitation erosion.