No abstract

Stepped spillways have been constructed worldwide and the overflow is characterised by intense turbulent dissipation and strong self-aeration. The current study investigated experimentally the two-phase flow properties on stepped chutes equipped with inclined downward steps. The observations showed a drastic impact of the cavity shape on the recirculation patterns, with less stable cavity recirculation with inclined steps. The two-phase flow measurements showed slightly more aeration and slower velocities over the inclined steps, in comparison to the flat horizontal stepped chute, for the same discharge and step location.

No abstract

At hydraulic structures, some strong interactions may develop between fast flowing waters and the air adjacent to the water in motion that enhance the air-water transfer of atmospheric and volatile gases in the flow. In turn, in-stream structures may contribute to the aeration and re-oxygenation during overflow. The present study aims to characterize the aeration performance of a steep stepped weir, based upon a detailed physical investigation of air-water interfacial properties across a relatively wide range of discharges. The data showed a strong fragmentation of the air-water flows, a very broad range of entrained bubbles and drops, and a large amount of particle clustering. The results implied a monotonic increase in re-aeration with increasing rate of energy dissipation, while the largest aeration efficiency was observed on the horizontal step weir chute, with the smallest on the 1V:2.33H inclined downward steps. Altogether, the study showed that a steep stepped chute can make a sizeable contribution to the re-oxygenation of the waters, although the downward inclined steps reduce the re-aeration performances.

On a stepped spillway, the staircase invert profile generates some intense turbulent dissipation during the spill, associated with a significant reduction of kinetic energy, as well as strong self-aeration. The present study focused on the effects of inclined downward steps on the air-water flow properties, flow resistance, and head losses because these mostly relate to spillway design. Some physical modeling was conducted in a relatively large facility with a 45°stepped chute (1V∶1H) operating with Reynolds numbers 2.8 × 10 3 < Re < 1 × 10 6 . The presence of downward steps induced some elongated asymmetrical cavity shapes, creating a less stable cavity recirculation pattern along the entire chute, leading to different interactions with the main stream. In terms of basic air-water flow properties, the distributions of void fraction and bubble count rate presented very close results for all three stepped geometries, both qualitatively and quantitatively. The interfacial velocities did not reach any uniform equilibrium (i.e., normal flow) condition, and the fastest velocities were recorded with the 1V∶2.33H inclined downward stepped chute geometry (δ ¼ 23.3°and λ=k ¼ 3), and the slowest velocities on the horizontal stepped chute (δ ¼ 0 and λ=k ¼ 2). The Darcy-Weisbach friction factor f and relative head loss ΔH=H max were estimated in the self-aerated flow. The comparative analyses suggested that the largest total drag and head losses were observed on the stepped chute with flat horizontal steps. An inclined downward stepped design yielded lesser head losses for all investigated flow conditions, providing an important information for practical engineers designing these hydraulic structures.