Spillway Aerator Research Articles

The Impact of Decreased Atmospheric Pressure on Forced Aeration of Discharged Flow

To account for changes in the performance of spillway aerator structures of high-altitude dams, depressurization generalized model experiments and theoretical analyses were conducted in this study. Measurements were taken for ventilation hole air velocity, cavity subpressure, cavity length, and air concentration in crucial regions. The study proposed correction formulas for the aeration coefficient and water air concentration in aerator devices operating under low atmospheric pressure. The pressure range of the experiments was between 26.3 kPa and 101.3 kPa. The results indicated that with decreasing atmospheric pressure, ventilation hole air velocity, ventilation volume, cavity subpressure, and water air concentration all showed a decreasing trend. For every 15 kPa decrease in pressure, ventilation hole air velocity decreased by approximately 24%. When the atmospheric pressure dropped from 101.3 kPa to 26.3 kPa, the cavity subpressure decreased and eventually approached zero. The maximum reduction in air concentration was 14.9% in the cavity backwater area, 38.5% at the cavity end, and 38.3% in the downstream bubble escape segment. The proposed correction formulas could be used for a rapid estimation of ventilation volume and air concentration in low-pressure environments. This research provides a scientific basis for the design of aeration devices in water projects located in high-altitude regions.

Advances in Spillway Hydraulics: From Theory to Practice

Over the past decades, significant advances have been achieved in hydraulic structures for dams, namely in water release structures such as spillway weirs, chutes, and energy dissipators. This editorial presents a brief overview of the eleven papers in this Special Issue, Advances in Spillway Hydraulics: From Theory to Practice, and frames them in current research trends. This Special Issue explores the following topics: spillway inlet structures, spillway transport structures, and spillway outlet structures. For the first topic of spillway inlet structures, this collection includes one paper on the hydrodynamics and free-flow characteristics of piano key weirs with different plan shapes and another that presents a theoretical model for the flow at an ogee crest axis for a wide range of head ratios. Most of the contributions address the second topic of spillway transport structures as follows: a physical modeling of a beveled-face stepped chute; the description and recent developments of the generalized, energy-based, water surface profile calculation tool SpillwayPro; an application of the SPH method on non-aerated flow over smooth and stepped converging spillways; a physical model study of the effect of stepped chute slope reduction on the bottom-pressure development; an assessment of a spillway offset aerator with a comparison of the two-phase volume of fluid and complete two-phase Euler models included in the OpenFOAM® toolbox; an evaluation of the performance and design of a stepped spillway aerator based on a physical model study. For the third topic of spillway outlet structures, physical model studies are presented on air–water flow in rectangular free-falling jets, the performance of a plain stilling basin downstream of 30° and 50° inclined smooth and stepped chutes, and scour protection for piano key weirs with apron and cutoff wall. Finally, we include a brief discussion about some research challenges and practice-oriented questions.

Air entrainment scale effects in chute aerator flows

Physical study of air–water flow in a spillway aerator is usually conducted using a downscaled Froude-similar model. The Reynolds number much smaller than that in a prototype structure may adversely affect the understanding and correct interpretation of many physical processes, including air entrainment and turbulence development. In this paper, the similarity and scale effects in the aeration process of flows in chute aerators were systematically investigated by using physical models of three different geometric scales. Air–water flow properties related to the air entrainment, turbulence and bubble characteristics were measured for inflow Froude numbers from 4.9 to 12 and Reynolds numbers from 8.26 × 104 to 9.23 × 105. The experimental results showed that the parameters of air concentration and bubble count rate were scale-sensitive, while the velocity and turbulence intensity were almost free of scale effects. The Reynolds number seemed to have a significant effect on bubble size which was observed to increase with increasing Reynolds number.

Plausible Differences between the Laboratory and Prototype Behaviors of Spillway Aerator Flows

An aerator, installed to prevent cavitation damages in spillways and outlet works, generates a typical two-phase flow affected by its configuration and flow conditions. The major parameters of concern include air demand, jet trajectory and streamwise changes of air concentration near chute bottom. This study reviews the theoretical basis and deals with several aspects of physical scale modeling, supported exclusively by field measurements. Analyses reveal that the sub-atmospheric pressure generated in the air cavity should also be scaled in physical modeling, which is seldom the case in the laboratory environment. Thus, the conventional approach to upscale the air demand is controversial. With the data from both fields and laboratories, it is demonstrated that a direct conversion of air flow from model to prototype is justified only if the approach flow velocity in the model exceeds 7.0–7.5 m/s or the Reynolds number exceeds 1.58 × 106. Failing to meet this premise would bring about errors for prototype predictions; the error extent depends on both model scale and flow magnitude. In terms of cavity pressure drop, the prototype differs by a factor of less than 10 from its scale model with sufficient air supply. In a model, the air concentration along the chute bottom drops considerably within one to two trajectory lengths. The prototype differs from its model in such a way that the air concentration decay is much slower, with a higher level that is maintained over a longer distance downstream of the impact location. This study is intended to provide insight for laboratory studies and engineering design.

Air concentration distribution in the impact zone of spillway aerator

Air concentration distribution in the impact zone of spillway aerator

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.

Do the Volume-of-Fluid and the Two-Phase Euler Compete for Modeling a Spillway Aerator?

Spillway design is key to the effective and safe operation of dams. Typically, the flow is characterized by high velocity, high levels of turbulence, and aeration. In the last two decades, advances in computational fluid dynamics (CFD) made available several numerical tools to aid hydraulic structures engineers. The most frequent approach is to solve the Reynolds-averaged Navier–Stokes equations using an Euler type model combined with the volume-of-fluid (VoF) method. Regardless of a few applications, the complete two-phase Euler is still considered to demand exorbitant computational resources. An assessment is performed in a spillway offset aerator, comparing the two-phase volume-of-fluid (TPVoF) with the complete two-phase Euler (CTPE). Both models are included in the OpenFOAM® toolbox. As expected, the TPVoF results depend highly on the mesh, not showing convergence in the maximum chute bottom pressure and the lower-nappe aeration, tending to null aeration as resolution increases. The CTPE combined with the k–ω SST Sato turbulence model exhibits the most accurate results and mesh convergence in the lower-nappe aeration. Surprisingly, intermediate mesh resolutions are sufficient to surpass the TPVoF performance with reasonable calculation efforts. Moreover, compressibility, flow bulking, and several entrained air effects in the flow are comprehended. Despite not reproducing all aspects of the flow with acceptable accuracy, the complete two-phase Euler demonstrated an efficient cost-benefit performance and high value in spillway aerated flows. Nonetheless, further developments are expected to enhance the efficiency and stability of this model.

Estimation of aerator air demand by an embedded multi-gene genetic programming

Abstract A spillway discharging a high-speed flow is susceptible to cavitation damages. As a countermeasure, an aerator is often used to artificially entrain air into the flow. Its air demand is of relevance to cavitation reduction and requires accurate estimations. The main contribution of this study is to establish an embedded multi-gene genetic programming (EMGGP) model for improved prediction of air demand. It is an MGGP-based framework coupled with the gene expression programming acting as a pre-processing technique for input determination and the Pareto front serving as a post-processing measure for solution optimization. Experimental data from a spillway aerator are used to develop and validate the proposed technique. Its performance is statistically evaluated by the coefficient of determination (CD), Nash–Sutcliffe coefficient (NSC), root-mean-square error (RMSE) and mean absolute error (MAE). Satisfactory predictions are yielded with CD = 0.95, NSC = 0.94, RMSE = 0.17 m3/s and MAE = 0.12 m3/s. Compared with the best empirical formula, the EMGGP approach enhances the fitness (CD and NSC) by 23% and reduces the errors (RMSE and MAE) by 48%. It also exhibits higher prediction accuracy and a simpler expressional form than the genetic programming solution. This study provides a procedure for the establishment of parameter relationships for similar hydraulic issues.

Numerical modeling of spillway aerators in high-head dams

Due to high flow velocity, the spillway surfaces of high-head dams can expose to cavitational damage. The most effective and economical method of protection from this damage is aerated to flow using aerators. In this study, a spillway aerator of the roller-compacted concrete dam of 100 m height was analyzed using two-phase computational fluid dynamic model to overcome the cavitation damage on the spillway surface. The numerical analysis with prototype dimensions was performed for various flow conditions (5223, 3500, 1750 and 1000 m3/s of flow rate), and obtained results were compared with some experimental observation in the literature. Numerical and experimental results indicated that the cavitation occurs on the surface after a certain downstream point based on cavitation indices. The air entrainment rate and air concentrations supplied by means of the aerator were determined to avoid the cavitational damage. While the experimental results can contain considerable scale effect in terms of air entrainment rate owing to, e.g., viscous effects especially for small scales, the numerical models with prototype dimensions gave much more accurate results. In other words, it can be also mentioned that the actual aeration amount is much greater than that obtained from the model experiments. The results based on numerical analysis showed that the aerator device meet air demand to prevent the cavitation damage.

Experimental and numerical studies on orifice spillway aerator of Bunji Dam

ABSTRACTIn this study, hydraulic design of the aerators of steep slope orifice spillways (26°–32°) was evaluated using physical and numerical modeling techniques. The Bunji Dam spillway, which is proposed to be constructed on the Indus River in Pakistan, was selected for physical and numerical modeling of the aerators. Extensive experimentation was carried out on a physical model of the Bunji Dam spillway by changing the air vent size and ramp angle of the aerator. Three air vent sizes and five ramp angles were constructed to assess the hydraulic performance of the aerator. The numerical model was formulated using Computational Fluid Dynamics (CFD)—FLOW 3D. The air vent size showed a significant impact on the dimensionless performance indicators of the aerators. Empirical equations were developed based on the outputs of physical and numerical models of the aerator. It is believed that the proposed equations will be useful for the estimation of nondimensional cavity length and air entrainment coefficient of the orifice spillway aerators having a short horizontal stretch leading to a convex curvature with slope varied from 26° to 32°.

A new integrated model of the group method of data handling and the firefly algorithm (GMDH-FA): application to aeration modelling on spillways

Due to the high flow velocity over dam spillways and outlets, severe cavitation damage might occur to the structures. Aeration (introducing air into the passing flow) is a useful remedy for preventing or decreasing cavitation, however, proper estimation of aerators air demand is a complex problem. On that account, the standard GMDH model, integrated GMDH-HS (with the harmony search algorithm) model and a novel integrated GMDH-FA model (with the firefly algorithm), were developed and applied to estimate air demand on spillway aerators in dams. Input parameters including flow rate (Qw), flow depth (d0), relative pressure under the jet (hs), ramp angle (α), step height (s), and spillway slope (θ) were applied as the effective factors for estimating the amount of air flow of the aerators (Qa). General results based on several statistical measures (NRMSE, PCC, NMAE, NSE) and the test of ANOVA for models’ residuals, showed that the standard GMDH improved the accuracy of estimating air flow in comparison to empirical equations (an average enhanced efficiency of 59.86% in terms of NRMSE) and multiple linear regression method (an enhanced efficiency of 37.15% in terms of NRMSE). Moreover, findings of the research revealed that the FA and HS algorithms improved the performance of the standard GMDH equal to 17% and 13%, respectively.

Air-vent layouts and water-air flow behaviors of a wide spillway aerator

A spillway aerator should guarantee favorable flow conditions in the coupled water-air system even if the aerator is unconventionally wide. Eight air-vent configurations are devised and incorporated into a 35-m wide chute aerator for a generalized study. Computational fluid dynamics (CFD) simulations are performed to explore their effects on water-jet and air-cavity features. The Re-normalisation group (RNG) k – ɛ turbulence model and the two-fluid model are combined to predict the two-phase flow field. The results demonstrate appreciable influences of the vent layouts on the water-air flow. The air vents stir the air motion and re-distribute the cavity air pressure. Once the vent layout is modified, reciprocal adjustments exist between the jet behavior and air-pressure field in the cavity, thus leading to considerable differences in air-flow rate, jet-trajectory length, vent air-flow distribution across the chute, etc. The large width plays a discernable role in affecting the aerated flow. Telling differences exist between the near-wall region and the central part of the chute. To improve the duct pressure propagation, a gradual augment of the vent area should be assigned towards the chute center. Relative to single-slot vents across the flow, the layouts with segregated vents gain by comparison. A designer should see to it that a vented aerator operates satisfactorily for a given range of flow discharges.

Challenging soft computing optimization approaches in modeling complex hydraulic phenomenon of aeration process

ABSTRACT This study investigates and challenges the capability of standard and hybrid soft computing models of fuzzy c-means clustering adaptive neuro-fuzzy inference system (ANFIS), wavenet and artificial neural networks (MLPNN and RBFNN) to estimate the spillway aerator air demand in dams. For the learning process, four different meta-heuristic optimization methods (particle swarm optimization (PSO), genetic algorithm (GA), firefly algorithm (FA) and biogeography-based optimization (BBO)) are considered as alternatives to the classical optimization algorithms of the data-driven models. In addition to the data-driven models, the multiple linear regressions and some empirical relations are used to evaluate the performance of the models. Evaluation of the models is assessed with five different statistical parameters as well as the diagnostic tool of the Taylor’s diagram. Analysis of the models’ outcome reveals that the ANFIS-GA has the best performance associated with a standard root mean square error of 0.309 and a coefficient of determination (R2 ) of 0.93.

Evaluation of air vents and ramp angles on the performance of orifice spillway aerators

The performance of steep slope (θ > 30 o ) orifice spillway aerators by varying air vent size and ramp angles were experimentally studied. Three air vents of different sizes and five ramps with different angles were tested on a physical model of Bunji dam spillway, which was constructed at Irrigation Research Station Nandipur, Gujranwala. In each case, the cavity length, cavity pressure, flow velocity and water depth at the aerator were measured by changing operating conditions. Non-dimensional jet length (λ), air entrainment co-efficient (β) and non-dimensional cavity pressure (P n ) were computed to evaluate the performance of the aerator against ramp angle and air vent size. Results noted an improvement in the performance of aerator with the increase of air vent size. However, the ramp initially improved the performance of the aerator but at higher reservoir level with large gate opening, no significant improvement in the performance of the aerator was noted, rather negative impact was observed due to reduction in cavity pressure.

Flow characteristics of orifice spillway aerator: numerical model studies

Orifice spillways are provided in dams situated in hilly regions for carrying out dual function of passing the flood and flushing of sediments. Aerators are required on these spillways for mitigating cavitation damage for head over spillway exceeding 50 m. The available current literature describes mainly the investigations for aerators on overflow spillways and no systematic work has been reported for aerators on orifice spillways. Traditionally, physical model studies have been used for designing the aerators on overflow spillways. However, extensive experimentation with sophisticated instrumentation is required to understand the flow field comprising pressure and velocity distribution and air entrainment characteristics. Numerical modelling using computational fluid dynamic techniques provides a complete picture on flow field in the vicinity of aerator. This study describes numerical simulation of flow over an aerator on deep-seated orifice spillway. The two-phase three-dimensional (3D) numerical model has been developed with 934,136 hexahedral cells and solved using FLUENT. The numerical model has been validated with data obtained from a 1:25 scale physical model. The pressure, velocity and air concentration profiles along the flow and across the depth at various locations highlights the air entrainment process on an orifice spillway aerator. The global air entrainment coefficient described as the ratio of entrained air and water discharge, average air concentration across depth and the bed air concentration profiles obtained from the numerical model completely describe the characteristics of aerator on orifice spillway and is found to be different than the one for the overflow spillway.

Discussion of “Orifice Spillway Aerator: Hydraulic Design” by V. V. Bhosekar, V. Jothiprakash, and P. B. Deolalikar

Discussion of “Orifice Spillway Aerator: Hydraulic Design” by V. V. Bhosekar, V. Jothiprakash, and P. B. Deolalikar

Closure to “Orifice Spillway Aerator: Hydraulic Design” by V. V. Bhosekar, V. Jothiprakash, and P. B. Deolalikar

Closure to “Orifice Spillway Aerator: Hydraulic Design” by V. V. Bhosekar, V. Jothiprakash, and P. B. Deolalikar

Cavity flow regime for spillway aerators

It is well known that the effect of air entrainment for cavitation damage controls is related not only to the air discharge into aerator devices but also the flow regime of the cavity below them. On the basis of the hydraulic characteristics of the flow, the aerator devices were for the first time classified. The theoretical considerations were performed about the jet length and cavity flow regime with the influencing factors. Comparing with the behavior of the flow through the aerator of discharge tunnels, the flow regimes of the cavity below spillway aerators were experimentally investigated, and the empirical expressions were presented to identify the conversions of the cavity flow regimes, including fully filled cavity, partially filled cavity, and net air cavity. Some issues of the design of the aerator devices were suggested in the present work.

Effects of tailwater depth on spillway aeration

Hydraulic structures such as spillways or weirs with their water-air controlling mechanisms are not only important for their structural properties but also for their effects on downstream ecology. Tailwater depth is an important factor affecting dissolved oxygen transfer and aeration rates of spillways. In this study, effects of tailwater depths and discharge values on the aeration efficiencies of laboratory-model smooth and stepped spillways were investigated. Changes of tailwater depths affect the dissolved oxygen transfer rates and aeration efficiencies are affected more than 100%, resulting in supersaturated dissolved oxygen concentrations. Tailwater depth is a significant parameter in the design of the spillway and the stilling basin of dams. Therefore, this value should be estimated correctly.

Orifice Spillway Aerator: Hydraulic Design

Orifice spillways are in vogue for the dams in the hilly regions where the spillway has to serve the dual function of flood disposal and flushing of sediment through the reservoir. Deep-seated orifice spillways are subjected to cavitation damage as the cavitation index drops below the critical cavitation index of 0.2 because of negative pressures on the profile and high flow velocities. Aerators are provided for mitigating cavitation damage. Design guidelines for aerator of orifice spillways are scanty and not reported much in the literature so far; thus, there still remains gray area in the field of spillway aerator design. The present study investigates the performance of an offset aerator with and without a ramp for deep-seated orifice spillway on a physical and numerical model. Performance of the aerator for varying discharges, heads, and gate openings is studied for varying cavity subpressures. Results with respect to jet length, cavity subpressure and air entrainment coefficients are presented in the form of nondimensional plots. From this study, the nondimensional jet length is in the range of 2 to 35 and increases as the cavity subpressure, approaches atmospheric pressure. It is also found that the air entrainment increases with increase in Froude number, ramp height and cavity pressure. From the present study results, equations for jet length and air entrainment coefficient are developed and are presented for the orifice spillway aerator.