An improved air entrainment model for stepped spillways

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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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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Predicting the Flow Velocity at the Toe of a Labyrinth Stepped Spillway

The Journal of Engineering Research [TJER] ◽

10.53540/tjer.vol18iss1pp20-25 ◽

2021 ◽

Vol 18 (1) ◽

pp. 20-25

Author(s):

Jaafar S. Maatooq

Keyword(s):

Kinetic Energy ◽

Flow Velocity ◽

Smooth Surface ◽

Air Entrainment ◽

Physical Models ◽

Stepped Spillway ◽

Stepped Spillways ◽

Stilling Basin ◽

Major Variable ◽

Jet Velocities

The velocity at the toe of a spillway is a major variable when designing a stilling basin. Reducing this velocity leads to reduce the size of the basin as well as the required appurtenances which needs for dissipating the surplus kinetic energy of the flow. If the spillway chute is able to dissipate more kinetic energy, then the resulting flow velocity at the toe of spillway will be reduced. Typically, stepped spillway is able to dissipate more kinetic energy than that of a smooth surface. In the present study, the typical uniform shape of the steps has been modified to a labyrinth shape. It is postulated that a labyrinth shape can increase the dissipation of kinetic energy through increasing the overlap between the forests of nappe will circulating the flow that in turns leading to further turbulence. This action can reduce the jet velocities near the surfaces, thus minimizing cavitation. At the same time the increasing of circulation regions will maximize the opportunity for air entrainment which also helps to dissipate more kinetic energy. The undertaken physical models were consisted of three labyrinth stepped spillways with magnification ratios (width of labyrinth to width of conventional step) WL/W are 1.1, 1.2, and 1.3 as well as testing a conventional stepped spillway (WL/W=1). It is concluded that the spillway chute coefficient is directly proportional to the labyrinth ratio and its value decreases as this ratio increases.

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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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PREDICTION OF DISSOLVED OXYGEN ON STEPPED SPILLWAY WITH DIFFERENT CONFIGURATION

Jurnal Teknologi ◽

10.11113/jt.v79.9907 ◽

2017 ◽

Vol 79 (4) ◽

Author(s):

Very Dermawan ◽

Djoko Legono ◽

Denik Sri Krisnayanti

Keyword(s):

Water Quality ◽

Dissolved Oxygen ◽

Froude Number ◽

Oxygen Transfer ◽

Air Entrainment ◽

Stepped Spillway ◽

Oxygen Level ◽

Stepped Spillways ◽

Turbulence Flow ◽

Bed Slope

The increase of water quality is related to the presence of dissolved oxygen. Even, the oxygen concentration in surface waters is a main indicator of the water quality for human use as well as for the aquatic biota. Air entrainment on stepped spillway is also recognised for its contribution to the oxygen transfer. The oxygen transfer on stepped spillways in skimming flow regime is increased due to earlier self-aeration and slower flow velocities in comparison to smooth spillways. This paper presents the results gained on a physical model by using a variety of different configurations of stepped spillway. The slopes of stepped spillway (θ) used are 30˚ and 45˚, the number of step (N) are 40 and 20, and two types of steps are flat steps and pooled steps. The experiments were conducted for ten Froude number (Fr) run ranging from 1.117 to 9.909. This research aimed to investigate the influence of different configuration in stepped spillway for predicting of dissolved oxygen. The results showed that the dissolved oxygen of the stepped spillway increases with an increase in chute of slope, number of step, and surface roughness on steps. The increases of Froude number as a function of discharge will cause turbulence flow becomes decreases, and the concentration of air bubble in the water will be decreased. The decreased value of turbulence flow will make dissolved oxygen level decrease. In skimming flow condition, the dissolved oxygen level decreases with increasing discharge per unit width especially for steep bed slope.

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