Author(s): Hubert Chanson
Linked Author(s): Hubert Chanson
Keywords: Hydraulic structures; Air entrainment; Prototype observations; Spillway operation; Inception of air entrainment; Surface velocity
Abstract: In self-aerated free-surface flows, the upstream flow is typically non-aerated and the flow becomes a strong air-water mix once the turbulent shear stress acting next to the interface exceeds the combined resistance of capillary effect and gravity. The onset location of air entrapment is a rapidly-varied region with a rapid, sometimes explosive transition from monophase liquid to two-phase gas-liquid flows. In this presentation, field observations were conducted in three large hydraulic structures during major flood events, with corresponding Reynolds numbers ranging from 2.6×106 to 1.1×108. Visual observations showed that the inception of selfaeration was a complicated three-dimensional transient phenomenon. An optical technique was applied to gain detailed surface velocity data. While the streamwise surface velocities were reasonably close to theoretical considerations, the streamwise surface velocity fluctuations showed large streamwise turbulence in excess of 100%, consistent with self-aerated free-surface flow measurements based upon dual-tip phase detection probes in laboratory. The current findings yields a seminal question: how large the Reynolds number of a prototype flow needs to be truly representative of large dam spillway self-aerated flows?