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

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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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 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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Re-Aeration on Stepped Spillways with Special Consideration of Entrained and Entrapped Air

Geosciences ◽

10.3390/geosciences8090333 ◽

2018 ◽

Vol 8 (9) ◽

pp. 333 ◽

Cited By ~ 1

Author(s):

Daniel Bung ◽

Daniel Valero

Keyword(s):

Mass Transfer ◽

Free Surface Flows ◽

Stepped Spillway ◽

Water Interface ◽

Mean Flow ◽

Air Concentration ◽

Flow Measurements ◽

Stepped Spillways ◽

Air Water Interface ◽

The Mean

As with most high-velocity free-surface flows, stepped spillway flows become self-aerated when the drop height exceeds a critical value. Due to the step-induced macro-roughness, the flow field becomes more turbulent than on a similar smooth-invert chute. For this reason, cascades are oftentimes used as re-aeration structures in wastewater treatment. However, for stepped spillways as flood release structures downstream of deoxygenated reservoirs, gas transfer is also of crucial significance to meet ecological requirements. Prediction of mass transfer velocities becomes challenging, as the flow regime differs from typical previously studied flow conditions. In this paper, detailed air-water flow measurements are conducted on stepped spillway models with different geometry, with the aim to estimate the specific air-water interface. Re-aeration performances are determined by applying the absorption method. In contrast to earlier studies, the aerated water body is considered a continuous mixture up to a level where 75% air concentration is reached. Above this level, a homogenous surface wave field is considered, which is found to significantly affect the total air-water interface available for mass transfer. Geometrical characteristics of these surface waves are obtained from high-speed camera investigations. The results show that both the mean air concentration and the mean flow velocity have influence on the mass transfer. Finally, an empirical relationship for the mass transfer on stepped spillway models is proposed.

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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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Analysis of the longitudinal distribution of pressures near the ends of the vertical and horizontal faces in stepped spillway of slope 1V: 0.75H

RBRH ◽

10.1590/2318-0331.0318170057 ◽

2018 ◽

Vol 23 (0) ◽

Author(s):

Fabrício Machado Osmar ◽

Alba Valéria Brandão Canellas ◽

Priscila dos Santos Priebe ◽

Lorena Silva Saraiva ◽

Eder Daniel Teixeira ◽

...

Keyword(s):

Energy Dissipation ◽

Physical Model ◽

Longitudinal Distribution ◽

Stepped Spillway ◽

Dimensionless Numbers ◽

Stepped Spillways ◽

Pressure Coefficients ◽

General Finding ◽

Pressure Fields ◽

Negative Pressures

ABSTRACT Stepped spillways have been widely used for flood runoff due to of their constructive advantages and in regards to energy dissipation during the drop, allowing downstream dissipation structures to be designed in smaller dimensions. Due to the need for further studies on the effect of runoff in the chute of the stepped spillway and the analysis of dimensionless numbers used to characterize the longitudinal distribution of the pressures acting on the faces of the steps of a spillway, this study aims to characterize the acting forces on the steps of said spillways. Analyses were performed on a physical model of spillway with a slope of 1V: 0.75H and steps with a height of 6 centimeters. The pressure fields on the steps were analyzed and compared with results found in literature. As a general finding concerning the pressure in the horizontal faces, it was ascertained that the traction and the compression loads have relatively similar magnitudes, the negative pressures being more intense. Regarding the maximum pressures at the horizontal faces of the steps, it should be emphasized that the maximum values of compression have occurred at the end of the chute and that the lower discharges yielded higher values of pressure coefficients.

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Numerical Simulation of Flow over Stepped Spillways with varying Step-Angle

Hydro Science & Marine Engineering ◽

10.30564/hsme.v2i2.2341 ◽

2020 ◽

Vol 2 (2) ◽

Author(s):

Mohsen Nasrabadi ◽

Younes Aminpour

Keyword(s):

Experimental Data ◽

Froude Number ◽

Flow Regime ◽

Flow Characteristics ◽

Surface Profile ◽

Free Surface Flow ◽

Surface Flow ◽

Stepped Spillway ◽

Air Concentration ◽

Vof Model

In the present study, the flow over the stepped spillway was ‎ numerically investigated by using Flow3D model. The effect of step angle on different properties of Nappe flow regime such as the water surface profile, location of free-surface‎ aeration inception, Froude number at the ‎spillway’s toe, and pressure, flow velocity, air concentration and cavitation index were evaluated. The realizable k–ε was applied as the turbulence ‎model, and Volume of Fluid (VOF) model was used to determine the free surface flow profiles of the spillway. ‎The model was verified using experimental data. In order to investigate the different ‎characteristics of Nappe flow regime, 17 numerical runs was designed, in which, ‎four step angles, four flow discharge were considered to investigate the flow characteristics over the stepped spillway. The results indicated that the numerical model is well suited with the experimental data over the stepped spillway (RMSE = 0.147 and ARE = 6.9%). In addition, with increasing the step angles, the aeration inception point is generally moved ‎downstream. By increasing the step angles from zero to 10 degrees, the Froude number does not change ‎significantly, however, at the angle of 15 degrees, the Froude number decreases by about 42 ‎percent.

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Analysis of Energy Dissipation of Interval-Pooled Stepped Spillways

Entropy ◽

10.3390/e24010085 ◽

2022 ◽

Vol 24 (1) ◽

pp. 85

Author(s):

Xin Ma ◽

Jianmin Zhang ◽

Yaan Hu

Keyword(s):

Renormalization Group ◽

Energy Dissipation ◽

Turbulence Model ◽

Dissipation Rate ◽

Flow Characteristics ◽

Stepped Spillway ◽

Flat Panel ◽

Stepped Spillways ◽

Capture Technique ◽

Better Than

The water flow characteristics over an interval-pooled stepped spillway are investigated by combining the renormalization group (RNG) k-ε turbulence model with the volume of fluid (VOF) interface capture technique in the present study. The results show that the energy dissipation performance of the interval-pooled stepped spillway was generally better than that of the pooled, stepped spillways and the traditional flat-panel stepped spillway. The omega vortex intensity identification method is introduced to evaluate the energy dissipation. Due to the formation of “pseudo-weir”, the energy dissipation did not increase with the growth of the pool’s height. In addition, the average vortex intensity can characterize the dissipation rate to some extent.

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Stepped spillway with pre-aeration by a deflector: flow characteristics

RBRH ◽

10.1590/2318-0331.252020190051 ◽

2020 ◽

Vol 25 ◽

Author(s):

Carolina Kuhn Novakoski ◽

Rute Ferla ◽

Maurício Dai Prá ◽

Alba Valéria Brandão Canellas ◽

Marcelo Giulian Marques ◽

...

Keyword(s):

Flow Characteristics ◽

Air Entrainment ◽

Stepped Spillway ◽

Cavitation Damage ◽

Flow Passage ◽

Stepped Spillways ◽

Air Supply ◽

Stepped Chute ◽

Airtight Chamber ◽

Reduced Scale

ABSTRACT Stepped spillways aim to dissipate part of the upstream energy during the flow passage by the chute. However, the use of these structures is limited to a restrict range of specific discharges due to the risk of cavitation damage. As the air entrainment into the flow assists the concrete protection against the aforementioned damages, a possible solution by aerators installed along the chute, already disseminated to smooth chutes, is being studied to be used also in stepped spillways. The purpose of the present paper is to characterize a flow over a stepped chute with induced aeration by deflector and air supply by an airtight chamber trough tests conducted on a reduced scale physical model. The main regions observed during the tests are presented and were developed four equations that allow to approximately predict the location of the main regions for a given spillway.

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Stepped spillway flows and air entrainment

Canadian Journal of Civil Engineering ◽

10.1139/l93-057 ◽

1993 ◽

Vol 20 (3) ◽

pp. 422-435 ◽

Cited By ~ 60

Author(s):

Hubert Chanson

Keyword(s):

Energy Dissipation ◽

Flow Characteristics ◽

Air Entrainment ◽

Stepped Spillway ◽

Calculation Methods ◽

Stepped Spillways ◽

Popular Method ◽

Roller Compacted Concrete ◽

Construction Techniques ◽

Recent Developments

Stepped spillways have become a popular method for handling flood releases. The steps significantly increase the rate of energy dissipation taking place on the spillway face and reduce the size of the required downstream energy dissipation basin. The compatibility of stepped spillways with roller compacted concrete and gabion construction techniques results in low additional cost for the spillway. This paper presents a review of recent developments for the design of stepped spillways, provides a discussion of the effects of air entrainment, and presents new calculation methods that take into account the effects of flow aeration on the flow characteristics and the rate of energy dissipation. Key words: stepped spillway, air entrainment, dam, spillway, energy dissipation.

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