1 CHARACTERISTICS OF CLUSTERED PARTICLES IN SKIMMING FLOWS ON A STEPPED SPILLWAY

Abstract: Air-water flows at hydraulic structures are commonly observed and called white waters. The free-surface aeration is characterised by some intense exchanges of air and water leading to complex air-water structures including some clustering. The number and properties of clusters may provide some measure of the level of particle-turbulence and particle-particle interactions in the high-velocity air-water flows. Herein a re-analysis of air-water clusters was applied to a highly aerated free-surface flow data set (Chanson and Carosi 2007). A two-dimensional cluster analysis was introduced combining a longitudinal clustering criterion based on near-wake effect and a side-by-side particle detection method. The results highlighted a significant number of clustered particles in the high-velocity free-surface flows. The number of bubble/droplet clusters per second and the percentage of clustered particles were significantly larger using the two-dimensional cluster analysis than those derived from earlier longitudinal detection techniques only. A number of large cluster structures were further detected. The results illustrated the complex interactions between entrained air and turbulent structures in skimming flow on a stepped spillway, and the cluster detection method may apply to other highly aerated free-surface flows