Performance and Design of a Stepped Spillway Aerator

Stepped spillways are frequently limited to specific discharges under around 30 m2/s due to concerns about potential cavitation damages. A small air concentration can prevent such damages and the design of bottom aerators is well established for smooth chutes. The purpose of this study is to systematically investigate the performance of a deflector aerator at the beginning of stepped chutes. Six parameters (chute angle, step height, approach flow depth, approach flow Froude number, deflector angle and deflector height) are varied in a physical model. The spatial air concentration distribution downstream of the aerator, the cavity sub-pressure, water discharge and air discharges are measured. The results describe the commonly used air entrainment coefficient, the jet length, as well as the average and bottom air concentration development to design an aerator. The lowest bottom air concentration measured in all tests is higher than the air concentration recommended in literature to protect against cavitation damages. And, unlike smooth chutes, there appears to be no significant air detrainment downstream of the jet impact. One deflector aerator seems therefore sufficient to provide protection of a stepped spillway.

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

RBRH ◽

10.1590/2318-0331.262120210098 ◽

2021 ◽

Vol 26 ◽

Author(s):

Carolina K. Novakoski ◽

Rute Ferla ◽

Priscila dos Santos Priebe ◽

Aline Saupe Abreu ◽

Marcelo G. Marques ◽

...

Keyword(s):

Flow Behavior ◽

Air Entrainment ◽

Stepped Spillway ◽

Air Concentration ◽

Stepped Spillways ◽

Jet Impact ◽

Entrainment Coefficient ◽

The Mean ◽

Inception Point ◽

Channel Bottom

ABSTRACT Stepped spillways can dissipate a great amount of energy during the flow passage over the chute, however these structures have limited operation due to the risk of cavitation damage. The induced aeration may protect the concrete chute through the air concentration near the channel bottom. Furthermore, some research studies have indicated that the presence of air in flows may reduce the mean pressures. The present research aims to analyze mean pressures, air entrainment coefficient and flow behavior over a stepped spillway with aeration induced by two different deflectors, comparing the results to natural aeration flow. Despite the jet impact influence, the induced aeration does not change significantly the mean pressures compared to natural aeration flow. The air entrainment coefficient, as well as the jet impact position, is higher for the deflector with the longer extension and, although air bubbles can be seen throughout the extension of the chute due to the air entrainment through the inferior flow surface, the induced aeration did not anticipate the boundary layer inception point position.

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Uniform flow and energy dissipation of hydraulic-jump-stepped spillways

Water Science & Technology Water Supply ◽

10.2166/ws.2020.056 ◽

2020 ◽

Vol 20 (4) ◽

pp. 1546-1553

Author(s):

Yu Zhou ◽

Jianhua Wu ◽

Fei Ma ◽

Jianyong Hu

Keyword(s):

Energy Dissipation ◽

Water Flow ◽

Hydraulic Jump ◽

Flow Region ◽

Uniform Flow ◽

Stepped Spillway ◽

Flow Depth ◽

Clear Water ◽

Air Concentration ◽

Stepped Spillways

Abstract In skimming flow, a uniform flow can be achieved and the flow depth, velocity and air concentration remain constant if a stepped spillway is sufficiently long. In this study, physical model experiments were performed to investigate the uniform characteristics and energy dissipation of a hydraulic-jump-stepped spillway, which is a new type of stepped spillway for increasing the unit discharge capacity and energy dissipation. Based on the redefinition of uniform flow, experimental results show that at a given stepped spillway slope, a smaller height for the beginning of the uniform flow region, a greater uniform aerated flow depth and a greater uniform equivalent clear water flow depth can be obtained as compared with the traditional stepped spillway due to strong aeration in the aeration basin. Under the condition of uniform flow, the energy dissipation rate of stepped spillways can be estimated by the equivalent clear water flow depth with given inflow conditions. Compared with the traditional stepped spillway, the uniform flow over the hydraulic-jump-stepped spillway has a smaller specific energy, revealing that the hydraulic-jump-stepped spillway is more advantageous for dissipating energy, especially at large unit discharges.

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Numerical Simulation of Air–Water Two-Phase Flow on Stepped Spillways Behind X-Shaped Flaring Gate Piers under Very High Unit Discharge

Water ◽

10.3390/w11101956 ◽

2019 ◽

Vol 11 (10) ◽

pp. 1956 ◽

Cited By ~ 4

Author(s):

Dong ◽

Wang ◽

Vetsch ◽

Boes ◽

Tan

Keyword(s):

Two Phase Flow ◽

Stokes Equations ◽

Air Entrainment ◽

Stepped Spillway ◽

Phase Flow ◽

Air Concentration ◽

Two Phase ◽

Unit Discharge ◽

Stepped Spillways ◽

Very High

Stepped spillways are commonly used under relatively low unit discharge, where cavitation pitting can be avoided by self-aerated flow. However, there are several dams in China with stepped spillways in combination with X-shaped flaring gate piers with unit design discharge considerably larger than specified in the available guidelines. Consequently, air–water two-phase flow on stepped spillway behind X-shaped flaring gate piers under very high unit discharge was investigated using Computational Fluid Dynamics (CFD) simulations. The 3-D Reynolds-averaged Navier–Stokes equations were solved, including sub-grid models for air entrainment, density evaluation, and drift-flux, to capture self-aerated free-surface flow over the spillway. The pressure on the vertical step faces was compared with laboratory data. In addition, the air–water two-phase flow characteristics and prototype step failure of the simulated prototype spillway were analyzed based on the numerical results of velocity, pressure, and air concentration. Moreover, an optimized bottom-aeration was further studied. The results reveal that the involved models can predict the air concentration near the steps. The cavitation index at the stepped surface is below the threshold value, and the air concentration is insufficient under high unit discharges. Moreover, with the proposed optimization of the aerator air entrainment can be improved and thereby cavitation erosion risk can be reduced.

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Hydraulic characteristics of the aeration basin in a ski-jump-step spillway

Water Science & Technology Water Supply ◽

10.2166/ws.2020.016 ◽

2020 ◽

Vol 20 (3) ◽

pp. 922-929

Author(s):

Shangtuo Qian ◽

Jianhua Wu

Keyword(s):

Vertical Direction ◽

Air Entrainment ◽

Stepped Spillway ◽

Hydraulic Characteristics ◽

Air Concentration ◽

Unit Discharge ◽

Plunge Pool ◽

Jet Impact ◽

Logarithmic Distribution ◽

Air Concentrations

Abstract The ski-jump-step spillway was designed using a ski-jump and an aeration basin to effectively pre-aerate flow in a stepped spillway. A new experimental study of the hydraulic characteristics of aeration basins was conducted to better understanding their pre-aeration properties and mechanisms. The plunge-pool patterns of aeration basins were classified into partially aerated, fully aerated, and vortex expelled, with increasing unit discharge. Relations between the distributions of the time-averaged pressure and the air concentration of the plunge-pools suggested that the ski-jump jet impact and the recirculating vortices are the main causes of plunge-pool air entrainment. Based on the export cross-section of the aeration basins, the bottom air concentrations remained greater than 3.0%. The sidewall air concentrations were greater than 7.5% and followed a logarithmic distribution in the vertical direction, demonstrating that the export flow attains a completely aerated state without any blackwater zones. In addition, increasing the aeration basin length was found to prevent the occurrence of a vortex expelled plunge-pool, thus promoting the appropriate pre-aeration effect under large unit discharges.

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Numerical investigation of flow characteristics over stepped spillways

Water Science & Technology Water Supply ◽

10.2166/ws.2020.283 ◽

2020 ◽

Author(s):

Aytaç Güven ◽

Ahmed Hussein Mahmood

Keyword(s):

Numerical Model ◽

Turbulent Flow ◽

Flow Characteristics ◽

Air Entrainment ◽

Stepped Spillway ◽

Stepped Spillways ◽

Entrainment Velocity ◽

Fluid Flow Characteristics ◽

3D Package ◽

Good Agreement

Abstract Spillways are constructed to evacuate the flood discharge safely not to let the flood wave overtop the dam body. There are different types of spillways, ogee type being the conventional one. Stepped spillway is an example of nonconventional spillways. The turbulent flow over stepped spillway was studied numerically by using the Flow-3D package. Different fluid flow characteristics such as longitudinal flow velocity, temperature distribution, density and chemical concentration can be well simulated by Flow-3D. In this study, the influence of slope changes on flow characteristics such as air entrainment, velocity distribution and dynamic pressures distribution over the stepped spillway was modelled by Flow-3D. The results from the numerical model were compared with the experimental study done by others in the literature. Two models of the stepped spillway with different discharge for each model was simulated. The turbulent flow in the experimental model was simulated by the Renormalized Group (RNG) turbulence scheme in the numerical model. A good agreement was achieved between the numerical results and the observed ones, which were exhibited in terms of graphics and statistical tables.

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NUMERICAL STUDY OF TURBULENT FLOW FOR MODERATE-SLOPE STEPPED SPILLWAYS

Malaysian Journal of Civil Engineering ◽

10.11113/mjce.v30.167 ◽

2018 ◽

Vol 30 (1) ◽

Author(s):

Bentalha Chakib

Keyword(s):

Hydraulic Structure ◽

Numerical Study ◽

Air Entrainment ◽

Work Flow ◽

Stepped Spillway ◽

Cavitation Damage ◽

Stepped Spillways ◽

Vof Model ◽

Computational Fluid Dynamics Cfd ◽

Stepped Chute

Stepped spillway is a power full hydraulic structure for energy dissipation because ofthe large value of the surface roughness. The performance of the stepped spillway is enhancedwith the presence of air that can prevent or reduce the cavitation damage. This work aims tosimulate air entrainment and determine the characteristics of flow at stepped spillways. Withinthis work flow over stepped chute is simulated by using fluent computational fluid dynamics(CFD). The volume of fluid (VOF) model is used as a tool to simulate air-water interaction onthe free surface thereby the turbulence closure is derived in the k −ε turbulence standard model.The found numerical results agree well with experimental results.

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Influence of aeration induced by piers on the starting position of the flow aeration and extreme pressures in stepped spillways

RBRH ◽

10.1590/2318-0331.262120200113 ◽

2021 ◽

Vol 26 ◽

Author(s):

Priscila dos Santos Priebe ◽

Rute Ferla ◽

Carolina Kuhn Novakoski ◽

Aline Saupe Abreu ◽

Eder Daniel Teixeira ◽

...

Keyword(s):

Physical Model ◽

Experimental Analysis ◽

Flow Behavior ◽

Flow Characteristics ◽

Exceedance Probability ◽

Stepped Spillway ◽

Air Concentration ◽

Stepped Spillways ◽

Starting Position ◽

Cavitation Phenomenon

ABSTRACT The operation of stepped spillways is limited by a range of discharges due to the risk of occurrence of the cavitation phenomenon and erosion on its steps. Since there is a demand for spillways with the possibility of overflow of greater discharges, the designs seek to increase the air concentration of the flow, which can occur through the installation of piers in the spillway in order to protect the structure from the above mentioned damage. The aim of this work is to analyze flow characteristics and extreme minimum and maximum pressures with non-exceedance probability of 0.1% and 99.9% acting next to the step edges of the spillway with aeration induced by piers through an experimental analysis in a physical model. Based on the results obtained, flow behavior was defined and equations for predicting the extreme pressures that occur along the stepped spillway with aeration induced by piers were proposed.

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Mathematical simulation of air-water flow along Ski Jump Jet

Water Science & Technology Water Supply ◽

10.2166/ws.2021.293 ◽

2021 ◽

Author(s):

Roghayeh Ahmadpour ◽

Hamed Sarkardeh ◽

Hazi Azamathulla

Keyword(s):

Numerical Simulation ◽

Water Flow ◽

Concentration Distribution ◽

Analytical Data ◽

Geometrical Parameters ◽

Flow Depth ◽

Air Concentration ◽

Black Water ◽

Core Length ◽

Analytical Simulation

Abstract In the present study, using a quasi 3D analytical simulation, air concentration distribution in ski jump generated jet is calculated. A numerical simulation is also performed to verify the results of the analytical model in parallel with the available experimental and another analytical data. By solving continuity and momentum equations in case of air-water flow for three different cases, it was confirmed that the air concentrations along the ski jet are uniquely linked to the relative black water core length. Results showed that the black water core length is also influenced by the approach flow depth, Froude number, geometrical parameters of ski jump and the chute bottom angle. Finally, an analytical equation is proposed to predict the air concentration distribution along the ski jump jet regarding different hydraulic and geometric parameters. By calculating the velocity profiles along the jet, it showed that increasing the air concentration reduces the jet velocity profile.

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Experimental Investigation of Artificial Aeration on a Smooth Spillway with a Crest Pier

Water ◽

10.3390/w10101383 ◽

2018 ◽

Vol 10 (10) ◽

pp. 1383 ◽

Cited By ~ 2

Author(s):

Juan Luna-Bahena ◽

Oscar Pozos-Estrada ◽

Víctor Ortiz-Martínez ◽

Jesús Gracia-Sánchez

Keyword(s):

Shock Waves ◽

Hydraulic Structure ◽

Standing Waves ◽

Experimental Tests ◽

Air Entrainment ◽

Air Concentration ◽

Cavitation Damage ◽

Stepped Spillways ◽

Artificial Aeration ◽

And Shock Waves

Crest piers placed on overflow spillways induce standing waves at the downstream end of them and the supercritical flow expands after flowing past the rear of the pier. The expanding flow from each side of a pier will intersect and form disturbances or shock waves that travel laterally as they move downstream and eventually reach the chute sidewalls. Recently, investigations regarding crest piers are related with artificial aeration on stepped spillways to eliminate the risk of cavitation damage. However, there is a lack of studies on standing and shock waves in smooth spillways concerning the air entrainment into the flow in presence of crest piers. This paper presents the study of the combined effect on air entrainment of a crest pier and an aerator on the bottom of a smooth spillway (configuration 1). For comparison, experimental tests were developed in the spillway without pier, that is in presence of aerator only (configuration 2). The configuration 1 results show that the air concentration distribution on the spillway bottom across the width and length of the chute increases in comparison with configuration 2, reducing even more the risk of cavitation damage and enhancing the safety of the hydraulic structure.

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Stepped Spillway Slope Effect on Air Entrainment and Inception Point Location

Water ◽

10.3390/w13101428 ◽

2021 ◽

Vol 13 (10) ◽

pp. 1428

Author(s):

Awais Raza ◽

Wuyi Wan ◽

Kashif Mehmood

Keyword(s):

Air Entrainment ◽

Stepped Spillway ◽

Point Location ◽

Step Size ◽

Zone Length ◽

Flow Rates ◽

Stepped Spillways ◽

Flow Surface ◽

Channel Slope ◽

Inception Point

Spillway is a crucial hydraulic structure used to discharge excess water from the dam reservoir. Air entrainment is essential to prevent cavitation damage on the spillway, however, without air entrainment the risk of cavitation over the spillway increases. The most important parameter for the determination of air entrainment in stepped spillways is the inception point. The inception point is the location where the air starts to inter into the water flow surface over the spillway. It occurs when the turbulent boundary layer meets the free surface. The location of the inception point depends upon different parameters like flow rate, geometry, step size, and slope of the spillway. The main aim of this study was applying numerical simulation by using the realizable k-ϵ model and the volume of fluid (VOF) method to locate the location of the inception point. For this purpose, by using different stepped spillways with four different slopes (12.5°, 19°, 29°, and 35°) different flow rates were simulated, which gives the location of the inception point of different channel slopes of stepped spillways at different flow rates. The results demonstrated that the inception point location of mild slopes is farther from the crest of the spillway than the steep slope stepped spillway. Non-aerated flow zone length increases when the channel slope decreases from steep to mild slope.

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