Abstract
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.
Highlights
The design of spillways with stepped channels became common in dams built with the roller-compacted concrete (RCC) technique (Chanson, 1993), which were raised in the 19th and early 20th centuries (Matos & Meireles, 2014)
Specific flows of 0.05 to 0.50 m2/s were analyzed in a channel with an angle of 53.13o
The air entrainment coefficient β increases with the positioning of the aerator system further downstream of the chute, as expected, as this is influenced by the increase in the Froude number of the flow
Summary
The design of spillways with stepped channels became common in dams built with the roller-compacted concrete (RCC) technique (Chanson, 1993), which were raised in the 19th and early 20th centuries (Matos & Meireles, 2014). Hydraulic structures subjected to flows with speeds in the order of 30 m/s or more – as is the case of dam spillways, for example – are susceptible to damage due to the phenomenon of cavitation (Falvey, 1990; Kells & Smith, 1991). It is known, that small amounts of air in the water flow – that is, about 7% of air concerning the volume of water – can reduce or even eliminate the erosion damage resulting from cavitation. Upstream from the inception point, depending on the discharge, there is a high risk of damages due to cavitation
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