Experimental investigations of air entrainment in transition and skimming flows down a stepped chute

Abstract

Stepped spillways have been used for about 3500 years. The last few decades have seen the development of new construction materials, design techniques, and applications, for example, embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes (α ~ 50°). This paper presents an experimental study conducted in a large-size stepped chute (α = 22°, h = 0.1 m, W = 1 m). Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air–water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterization is developed for the distributions of void fraction, bubble count rate, and velocity, and flow resistance data are compared with other studies. Transition flows exhibit significantly different air–water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls.Key words: stepped spillway, air entrainment, two-phase flow properties, skimming flow, transition flow.

Strong interactions between free-surface aeration and turbulence in an open channel flow

“…The air-water velocity was calculated based upon a cross-correlation technique, and the turbulence intensity was deduced from the width of the cross-correlation function. Details of the processing technique were given in [6]. Figure 9 includes transition and skimming flow data for the same flow conditions as in Figure 4.…”

Application of Displacement Height and Surface Roughness Length to Determination Boundary Layer Development Length over Stepped Spillway

“…(4) The turbulent boundary layer is seen at the water surface when the Bauer-defined boundary layer thickness is between 0.72 and 0.79 of the flow depth. (5) The length down the spillway to the inception of air entrainment is best predicted by Equation (13), developed by Hunt and Kadavy [68], especially when the surface roughness F* is equivalent above 64. However, the definition of inception point location in this paper is that the visual observation of the cross section where there is a continuous presence of air within the flow at the sidewalls or within the step cavities, and this conclusion was achieved based on the numerical simulation results under 18 different hydraulic conditions.…”

“…The length on the spillway of non-aerated flow increases as the flow rate increases, with larger velocities that could cause unacceptable pressure fluctuations. Usually, determined by the size of steps and the flow discharge per width, the experimental studies, such as Chamani and Rajaratnam [2,3], Chanson [4,5], Cheng et al [6,7], revealed three types of flows over a stepped spillway, namely, nappe flow, transition flow and skimming flow. For a given chute profile, the flow pattern may be either nappe flow at low flow rates, transition flow for intermediate discharge or skimming flow at larger flow rates.…”

Stepped spillways for embankment dams. Review, progress and development in overflow hydraulics

“…Any single failure of the spillway system can lead to a total dam failure. A number of specific key issues must be assessed accurately and these include stepped chute operation and erosion, embankment seepage, drainage beneath steps, sidewall design (overtopping, scour), chute geometry (effects of chute convergence), and downstream energy dissipation and scour (CHANSON and TOOMBES 2001). Today some hydrodynamic processes are still inadequately understood.…”

“…WOOD 1964, NAUDASCHER andROCKWELL 1994). CHANSON and TOOMBES (2001) hypothesised that a similar mechanism may exist in skimming flows above embankment stepped spillway. At the downstream end of the spillway, submerged steps may be damaged during high tailwater level conditions.…”