Stepped spillways with aerators: hydrodynamic pressures and air entrainment

Abstract

The possibility of damage due to the phenomenon of cavitation leads the design of stepped spillways considering maximum specific discharges of 15 to 30 m2/s, a limit considerably lower than that practiced on smooth chutes. Aerators promote the insertion of air in the flow, allowing for the increase of specific flow rates. This work analyzes the pressures on the vertical faces of the steps and the air entrainment coefficient in the flow, through an experimental study in a physical model with a stepped chute angle of 53.13o, considering the installation of aerators in different places of the channel. Comparing the tested conditions with the natural aeration, it is concluded that the installation of the aerator does not change the magnitude of the minimum extreme hydrodynamic pressures, but anticipates the beginning of the flow aeration. A new equation is proposed to estimate the air entrainment coefficient, as well as a methodology for forecasting extreme pressures on the vertical faces of the steps, both valid in the range 3.0 ≤ Fr ≤ 6.0.

Hydrodynamic pressures on a stepped spillway with an aerator system subjected to different air flow rates

“…In this region, the higher β, the higher is also P 0.1% (that is, the less negative is P 0.1% ), so the results associated with the NAT condition are more extreme (more negative) than those of the induced aeration conditions, in general. Such behavior is consistent with what was measured by Priebe (2020) and Ferla et al (2021).…”

“…Similar conclusions were also observed by Ferla et al (2021), when they evaluated experimental results of extreme negative pressures (P 0.1% , P 1% and P 5% ) on the vertical face of the steps of a spillway (α ≈ 53º) subjected to different conditions of induced aeration, compared to natural aeration. However, the results of Ferla et al (2021) identified a tendency to increase pressures in conditions where the incorporation of air in the flow was greater when compared to natural aeration.…”

“…This is because some of the studies already carried out are divergent, even under similar conditions, and it is not possible to identify, at the moment, whether or not air insertion is associated with increased pressures. While Marques et al (2019), Novakoski (2021) and Priebe et al (2021), for example, did not identify significant variations between the different conditions, Arantes (2007), Priebe (2020) and Ferla et al (2021) verified them in some cases -which also concurs with Peterka (1953) and Dong & Su (2006), for example.…”

Induced aeration flow over stepped spillways: mean pressures, air entrainment and flow behavior

“…Air can be inserted into the flow by aerator elements such as piers and deflectors combined with an air supply system. Authors such as Dong et al (2019), Koen et al (2019), Ma & Wu (2021) and Priebe et al (2021) studied the aeration induced in stepped spillways by piers and Pfister et al (2006a), Pfister et al (2006b), Mojtaba et al (2015), Terrier (2016), Novakoski et al (2020) and Ferla et al (2021) with deflectors.…”

“…It is important to emphasize that β considers only the air entrance through the aerator elements and not the air-water changes through the free surface. Some authors who proposed equations which allows estimating this coefficient for different kinds of aerators elements are Kökpınar & Gogus (2002), Zamora et al (2008), Pfister and Hager (2010), Terrier (2016) and Ferla et al (2021). Generally, the equations proposed are a function of Froude number, aerator geometry, flow depth, mean flow velocity, or the reach of the jet.…”