Numerical Simulation of Air Entrainment for Flat-Sloped Stepped Spillway

Abstract. A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydisperse gas–particle mixtures forming volcanic plumes. Starting from the three-dimensional N-phase Eulerian transport equations for a mixture of gases and solid dispersed particles, we adopt an asymptotic expansion strategy to derive a compressible version of the first-order non-equilibrium model, valid for low-concentration regimes (particle volume fraction less than 10−3) and particle Stokes number (St – i.e., the ratio between relaxation time and flow characteristic time) not exceeding about 0.2. The new model, which is called ASHEE (ASH Equilibrium Eulerian), is significantly faster than the N-phase Eulerian model while retaining the capability to describe gas–particle non-equilibrium effects. Direct Numerical Simulation accurately reproduces the dynamics of isotropic, compressible turbulence in subsonic regimes. For gas–particle mixtures, it describes the main features of density fluctuations and the preferential concentration and clustering of particles by turbulence, thus verifying the model reliability and suitability for the numerical simulation of high-Reynolds number and high-temperature regimes in the presence of a dispersed phase. On the other hand, Large-Eddy Numerical Simulations of forced plumes are able to reproduce the averaged and instantaneous flow properties. In particular, the self-similar Gaussian radial profile and the development of large-scale coherent structures are reproduced, including the rate of turbulent mixing and entrainment of atmospheric air. Application to the Large-Eddy Simulation of the injection of the eruptive mixture in a stratified atmosphere describes some of the important features of turbulent volcanic plumes, including air entrainment, buoyancy reversal and maximum plume height. For very fine particles (St → 0, when non-equilibrium effects are negligible) the model reduces to the so-called dusty-gas model. However, coarse particles partially decouple from the gas phase within eddies (thus modifying the turbulent structure) and preferentially concentrate at the eddy periphery, eventually being lost from the plume margins due to the concurrent effect of gravity. By these mechanisms, gas–particle non-equilibrium processes are able to influence the large-scale behavior of volcanic plumes.

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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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Experimental investigations of air entrainment in transition and skimming flows down a stepped chute

Canadian Journal of Civil Engineering ◽

10.1139/l01-084 ◽

2002 ◽

Vol 29 (1) ◽

pp. 145-156 ◽

Cited By ~ 43

Author(s):

H Chanson ◽

L Toombes

Keyword(s):

Free Surface ◽

Water Flow ◽

Air Entrainment ◽

Stepped Spillway ◽

Transition Flow ◽

Flow Properties ◽

Flow Measurements ◽

Surface Aeration ◽

Stepped Chute ◽

Uniform Equilibrium

Stepped spillways have been used for about 3500 years. The last few decades have seen the development of new construction materials, design techniques, and applications, for example, embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes (α ~ 50°). This paper presents an experimental study conducted in a large-size stepped chute (α = 22°, h = 0.1 m, W = 1 m). Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed airwater flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterization is developed for the distributions of void fraction, bubble count rate, and velocity, and flow resistance data are compared with other studies. Transition flows exhibit significantly different airwater flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls.Key words: stepped spillway, air entrainment, two-phase flow properties, skimming flow, transition flow.

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Self-Aeration Modelling Using a Sub-Grid Volume-Of-Fluid Model

International Journal of Nonlinear Sciences and Numerical Simulation ◽

10.1515/ijnsns-2017-0015 ◽

2017 ◽

Vol 18 (7-8) ◽

pp. 559-574 ◽

Cited By ~ 1

Author(s):

Pedro Lopes ◽

Jorge Leandro ◽

Rita F. Carvalho

Keyword(s):

Free Surface ◽

Source Term ◽

Air Entrainment ◽

Volume Of Fluid ◽

Free Surface Flows ◽

Stepped Spillway ◽

Main Concern ◽

Advection Diffusion Equation ◽

Computational Performance ◽

Good Agreement

AbstractThe accurate prediction of self-aerated flow is not always easy to obtain, particularly if the computational performance is the main concern. Two-fluid formulation is suitable to simulate the dispersed air in a continuous water phase (e.g. bubbly flows) in a fine mesh, whereas the interface tracking methods are used for sharp interfaces with two continuous and contiguous phases (e.g. free-surface flows). Several approaches have emerged to combine both methods; however all found a gap in the transition between resolved and unresolved scales of air at the interface. Including a source term that predicts the self-aeration process is viewed as a promising step to overcome such difficulty. In this work, we added to the volume-of-fluid formulation an extra advection-diffusion equation connected to a source of air at the free surface to simulate the dispersed bubble phase. One-way coupling and two-way coupling versions of this model are tested along with sensitivity tests to show the accuracy of the new source term that does not require calibration. The location of the aeration is analysed and investigated. Results are obtained in terms of free-surface flow depths, air–concentration profiles and velocity fields and compared to experimental data acquired in a scaled stepped spillway model with good agreement. The free-surface given by the air-entrainment model is in good agreement in both non-aerated and aerated zone of the spillway.

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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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Free surface profile and inception point as characteristics of aerated flow over stepped spillway: Numerical study

Journal of Water and Land Development ◽

10.2478/jwld-2019-0043 ◽

2019 ◽

Vol 42 (1) ◽

pp. 42-48

Author(s):

Chakib Bentalha ◽

Mohammed Habi

Keyword(s):

Free Surface ◽

Kinetic Energy ◽

Water Depth ◽

Numerical Study ◽

Surface Profile ◽

Air Entrainment ◽

Stepped Spillway ◽

Ansys Fluent ◽

Vof Model ◽

Inception Point

Abstract Stepped spillway is hydraulic structure designed to dissipate the excess in kinetic energy at the downstream of dams and can reduce the size of stilling basin at the toe of the spillway or chute. The flow on a stepped spillway is characterised by the large aeration that can prevent or reduce the cavitation damage. The air entrainment starts where the boundary layer attains the free surface of flow; this point is called “point of inception”. Within this work the inception point is determined by using software Ansys Fluent where the volume of fluid (VOF) model is used as a tool to track the free surface thereby the turbulence closure is derived in the k − ε turbulence standard model. This research aims to find new formulas for describe the variation of water depth at step edge and the positions of the inception point, at the same time the contour map of velocity, turbulent kinetic energy and strain rate are presented. The found numerical results agree well with experimental results like the values of computed and measured water depth at the inception point and the numerical and experimental inception point locations. Also, the dimensionless water depth profile obtained by numerical method agrees well with that of measurement. This study confirmed that the Ansys Fluent is a robust software for simulating air entrainment and exploring more characteristics of flow over stepped spillways.

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A NUMERICAL SIMULATION OF THE WOS AND THE WAVE PROPAGATION ALONG A COASTAL DIKE

Coastal Engineering Proceedings ◽

10.9753/icce.v33.waves.49 ◽

2012 ◽

Vol 1 (33) ◽

pp. 49 ◽

Cited By ~ 1

Author(s):

Panayotis Prinos ◽

Maria Tsakiri ◽

Dimitris Souliotis

Keyword(s):

Numerical Simulation ◽

Wave Propagation ◽

Free Surface ◽

Numerical Models ◽

Field Measurements ◽

Air Entrainment ◽

Empirical Relationships ◽

Wave Overtopping ◽

Unsteady Rans ◽

The Waves

Wave overtopping and the propagation of the waves on the crest and the landward slope of a coastal dike is investigated numerically. Wave overtopping conditions are simulated using the concept of the Wave Overtopping Simulator (WOS). Two numerical models of the WOS are constructed using the FLUENT 6.0.12 (FLUENT Inc. 2001) and the FLOW 3D 9.4 (FLOW 3D 2010) CFD codes. The former simulates the WOS without accounting for air entrainment while the latter accounts for air entrainment. The unsteady RANS equations, the RNG k-ε turbulence model and the VOF method are solved numerically, for "tracking" the free surface and the head of the "current" from the dike crest to the landward dike slope. The computed results from the two models are compared with each other and also against field measurements and proposed empirical relationships (Van der Meer et al. 2010).

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