Skimming flow in a large model of a stepped spillway

The present investigation is the first major contribution to the study of skimming flow in large scale models to develop a better understanding of aerated flow in stepped spillways. Most observations were performed in the developed flow region. Detailed measurements of air concentration and velocity were made. Vortical structures were observed visually with the and of a high speed video camera.

Air entrainment created a highly irregular and wavy surface in which returning water drops ejected from the surface continuously entrained air. Stable recirculating eddies were formed in the grooves of the steps which were maintained through the transmission of the shear stress of tbe main flow. Observations revealed that the inception of air entrainment was strongly influenced by the steps. The enhancement of the growth of the boundary layer and the deflection of the internal flow by steps caused early inception of air entrainment.

Photographic observations and air concentration profiles showed that two zones within the depth of self-aerated flow existed in the developed region. In the lower zone, air bubbles were kept in suspension by the turbulent transport. In the upper zone, water drops ejected from the transition zone, were traveling in a stream of air. Due to the similarity of flow over stepped spillways to the flow in steep channels, a basis for the analysis of air concentration distribution was established. It was found that stepped spillways entrain more air than chute spillways. This indicates that steps enhance turbulence in the flow.

The velocity distribution in the lower zone of the flow depth in the developed region was analyzed using Prandtl-Karman universal velocity equation. Skin friction coefficient was calculated based on the uniform flow expression for the developed region. An empirical equation was derived to estimate the skin friction coefficient for the skimming flow. It appeared that the recirculating eddies in the step grooves and vortical structures at step tips were the major causes for energy dissipation. Results of the energy loss showed that the skimming flow dissipates less energy than the jet flow where significantly more energy dissipation occurs.