Spillway Aerator Research Articles

Discussion of “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators” Appears in the Canadian Journal of Civil Engineering 36(5): 826-838.

The Authors presented the outputs of some numerical modelling of a spillway aerator conducted using a commercial package (Aydin and Ozturk 2009). A spillway aeration device is located on the spillway bottom and sometimes on the sidewalls. It is designed to deflect high-velocity flow away from the chute surface (and walls) to artificially introduce air into the flow and protect the spillway invert from cavitation erosion. The main flow regions are: (a) the approach flow region, which characterizes the initial nappe flow conditions; (b) the transition region, which coincides with the length of the deflector; (c) the aeration region; (d) the impact point region; and (e) the downstream flow region (Chanson 1989a, 1989b; Kramer et al. 2006). A key feature of the flow abovestream and downstream of an aerator is the strong mixing of air and water yielding a complicated high-velocity airnwater flow. Herein, it is argued that the validation and certification of the commercial CFD package were ill-performed and that the physical modelling technique based upon a proper dimensional analysis remains the basic design tool.In summary, we believe that the validation and certification of the Authorsr numerical results were improper. These should be conducted on the detailed airnwater flow properties of the flow abovestream and downstream of the spillway aerators, and several extensive datasets are freely accessible (e.g., Chanson 1988; Kramer 2004; Toombes 2002). The physical modelling and theoretical calculations remain the basic tools of the design engineers to size the spillway aeration devices on a large spillway, although the physical model dimensions must be carefully selected to address the potential scale effects (e.g., Kramer 2004; Chanson 2009).

Discussion of “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators” by M.C. Aydin and M. OzturkAppears in the Canadian Journal of Civil Engineering 36(5): 826-838.

Discussion of “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators” by M.C. Aydin and M. OzturkAppears in the Canadian Journal of Civil Engineering 36(5): 826-838.

Reply to Discussion by M. Pfister on “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators”Appears in the Canadian Journal of Civil Engineering, 36(5): 826-838.

In our study, we compared the results of computational fluid dynamics (CFD) using FLUENT software with a small-scale physical model including considerable scale effects and some prototype data only in terms of the air entrainment rates. The verifications of the CFD analyses were based on Richardson extrapolation and other methods in the study. It is noted that the study should be considered in the above framework. In FLUENT, three different Euler–Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. Spillway aerator flows is a free-surface flow and a multiphase flow at the same time. For free-surface flow and large bubble flow, the best multiphase model is VOF, but the VOF model is not appropriate for air–water mixture and small bubble flow. Since there are small bubbles and air–water mixture in spillway flows downstream of an aerator, we suppose that the ASM model is more suitable for modeling of aeration. FLUENT also recommends using the Algebraic Slip Mixture (ASM) model for bubble flows as aeration. It is stated in the tutorial of FLUENT 5.3 (FLUENT 2000) that ASM model can be used at all values of volume fraction. However, it is true that it restricts to relatively small bubbles and low volume fraction when primary phase density is much smaller than the secondary phase density for gas–liquid flows. Among available multiphase models, we think that the ASM model is the most suitable for both free surface and air–water mixture flows since flow aerated by an aerator includes small bubbles. Low gas (air) fraction was only an assumption in our study. Turbulence model

Reply to discussion by Chanson and Lubin on “Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators” Appears in the Canadian Journal of Civil Engineering, 36(5): 826-838.

In our study, we compared the results of computational fluid dynamics (CFD) using FLUENT software with a small-scale physical model including considerable scale effects and some prototype data only in terms of the air entrainment rates. The verifications of the CFD analyses were based on Richardson extrapolation and other methods in the study. It is noted that the study should be considered in the above framework. In FLUENT, three different Euler–Euler multiphase models are available: the volume of fluid (VOF) model, the mixture model, and the Eulerian model. Spillway aerator flows is a free-surface flow and a multiphase flow at the same time. For free-surface flow and large bubble flow, the best multiphase model is VOF, but the VOF model is not appropriate for air–water mixture and small bubble flow. Since there are small bubbles and air–water mixture in spillway flows downstream of an aerator, we suppose that the ASM model is more suitable for modeling of aeration. FLUENT also recommends using the Algebraic Slip Mixture (ASM) model for bubble flows as aeration. It is stated in the tutorial of FLUENT 5.3 (FLUENT 2000) that ASM model can be used at all values of volume fraction. However, it is true that it restricts to relatively small bubbles and low volume fraction when primary phase density is much smaller than the secondary phase density for gas–liquid flows. Among available multiphase models, we think that the ASM model is the most suitable for both free surface and air–water mixture flows since flow aerated by an aerator includes small bubbles. Low gas (air) fraction was only an assumption in our study. Turbulence model

Verification and validation of a computational fluid dynamics (CFD) model for air entrainment at spillway aerators

The results of computational fluid dynamics (CFD) modeling obtained by using FLUENT software with respect to the air entrainment at spillway aerators are compared with data from a physical model study, as well as the results of some empirical equations and prototype observations presented by other investigators. The air-entrainment rates obtained from the CFD analyses are in reasonable agreement with the prototype data and the values calculated from empirical equations, and are better than the physical model data, which include considerable scale effects. The numerical verification procedure in this study is based on the American Society of Mechanical Engineers (ASME) editorial policy statement, which provides a framework for CFD uncertainty analysis. Thus, validation of the CFD is discussed within the scope of this study.

Verification of a 3-D Numerical Model for Spillway Aerator

In this study, the results of CFD obtained by using Fluent with respect to the air entrainment at spillway aerators are compared to the data of the physical model study and the results of some empirical equations presented by other investigators. The air entrainment rates obtained from the CFD analyses are in reasonable good agreement with the values calculated by the empirical equations. However, the CFD results are better than the physical model data including considerable scale effects. The numerical verification procedure in this study is based on the ASME editorial policy statement, which provides a framework for computational fluid dynamics uncertainty analysis. So, the validation of the CFD model was discussed in this scope.

TECHNICAL NOTES Air uptake along the lower nappe of a spillway aerator

TECHNICAL NOTES Air uptake along the lower nappe of a spillway aerator

High-speed jet flows over spillway aerators

The characteristics of high-speed air-entrained jet flow over a spillway aerator were investigated using experimental data. Three different ramp designs (as well as the case of no ramp) were tested in a channel where the bed slope was adjusted to 0, 0.17, and 0.57, respectively. The effects of aerator geometry and flow condition on the air-entrainment process within the water jet and on the pressure fluctuations in the impact region are the main focus of the investigation. An extensive data analysis was performed for data obtained from this study and from a similar laboratory study conducted by another investigator. Experimental relations were derived for nondimensional parameters of jet length, aerator cavity subpressure number, and aeration rates through the lower and upper nappes of the water jet. Scale effects for lower nappe aeration and aerator cavity subpressure number were considered and a good correlation was determined with prototype aerator measurements of Emborcaçao, Foz do Areia, and Keban dams for lower nappe aeration and Guri Dam for aerator cavity subpressure numbers. An aerator without a ramp may be subjected to excessive pressure fluctuations at the impact region due to insufficient air entrainment. Key words: air entrainment, aerator, scale effect, jet length, cavity subpressure.

AIR CONCENTRATION DISTRIBUTION DOWNSTREAM OF SPILLWAY AERATORS

ABSTRACT Laboratory experiments on the distribution of air concentration downstream of spillway aerators have been reported. These data along with those of earlier investigators on different shapes of aerators have been analysed to develop relationships for concentration distribution in terms of geometry and approach flow conditions. The analysis has also been used to evolve an approximate criterion for the spacing of aerators.

Predicting the filling of ventilated cavities behind spillway aerators

Cavitation damage to spillway surfaces may be prevented with the use of aeration devices. These serve to introduce air into the layers close to the channel bottom in order to reduce cavitation erosion. Under some circumstances, the aerator can be drowned out, will cease to protect the spillway surface, and act potentially as a cavitation generator. This article analyses the conditions of filling. Then experimental data for ten aerator geometries are reviewed. Depending upon the aerator geometry, the cavity filling occurs when the Froude number is less than a critical value or when the ratio of the flow depth over the total offset is larger than a characteristic value.

Mathematical modelling of air-water mixtures in open channels

A mathematical model was developed to predict the air concentration profiles in the deaeration zone below a spillway aerator by solving the conservation equations of mass and momentum. The turbulent diffusion coefficient was calculated by a k-∊ model and the air-water mixture was treated as a stratified flow. To test the mathematical model a series of experiments was conducted. Air was injected from the bed of a flow channel and air concentration profiles were measured at various downstream sections. From the comparison of the computer predictions with the experimental data it was concluded that the present analytical approach was satisfactory for practical purposes and showed promise for further development.

Closure to “ Air Entrainment by Spillway Aerators ” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Closure to “ <i>Air Entrainment by Spillway Aerators</i> ” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Discussion of “ Air Entrainment by Spillway Aerators ” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Discussion of “ <i>Air Entrainment by Spillway Aerators</i> ” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Aeration of a free jet above a spillway

A study of air entrainment above an spillway aerator is presented and discussed with a dimensional analysis. We conclude that similitude of air entrainment processes for spillway aerator is not possible between model and prototype. New informations on the aeration region are presented and an analytical solution of the upper nappe entrainment is developed.

Reduction of cavitation on spillways by induced air entrainment

Spillways for medium and high head dams may be exposed to high velocity flows and the associated destructive phenomenon of cavitation. Cavitation may occur at rough spots in the surface of the chute or tunnel, at local discontinuities in the finished surface such as construction joints, and at locations along critical flow profiles having significant deviations from design specifications. This paper addresses, on the basis of a review of the literature, a method for preventing or reducing cavitation damage on spillways through the use of spillway aerators. While the ability of induced or forced aeration to reduce or eliminate cavitation has been known for many years, it is only in relatively recent times that the aeration mechanism has been used to this advantage on spillways subject to high velocity flow. The recent application of aerators to spillway design is related, in part, to the trend toward higher head dams and larger design unit discharge rates. Design considerations and criteria for spillway aerators are presented in the paper, and the use of physical hydraulic models to make determinations of aerator performance is discussed. The intent of the paper is to provide a document with sufficient detail and scope to be useful as a first resource for spillway design practitioners. Key words: aerator design, air entrainment, cavitation, design criteria, high dam, model – prototype comparison, physical hydraulic model, spillway, spillway aeration.

Erratum: “Air Entrainment by Spillway Aerators” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Erratum: “Air Entrainment by Spillway Aerators” by Peter Rutschmann and Willi H. Hager (June, 1990, Vol. 116, No. 6)

Water‐Quality Impact Assessment for Hydropower

A methodology to assess the impact of a hydropower facility on downstream water quality is described. Negative impacts can result from the substitution of discharges aerated over a spillway with minimally aerated turbine discharges that are often withdrawn from lower reservoir levels, where dissolved oxygen (DO) is typically low. Three case studies illustrate the proposed method and problems that can be encountered. Historic data are used to establish the probability of low‐dissolved‐oxygen occurrences. Synoptic surveys, combined with downstream monitoring, give an overall picture of the water‐quality dynamics in the river and the reservoir. Spillway aeration is determined through measurements and adjusted for temperature. Theoretical computations of selective withdrawal are sensitive to boundary conditions, such as the location of the outlet relative to the reservoir bottom, but withdrawal from the different layers is estimated from measured upstream and downstream temperatures and dissolved‐oxygen profi...

Air Entrainment by Spillway Aerators

Aerators on spillways may be regarded as an effective protection against cavitational erosion. The air entrainment of aerators is governed by a number of independent parameters, that include the sl...

Etude des phénomènes d'entraînement d'air. Application aux évacuateurs de crue

Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices, called aerators. These serve to introduce air in the layers close to the channel bottom in order to reduce the cavitation damage.The performances of spillway aerator are studied with data obtained on a steep spillway model and high velocities (from 4 m/s up to 15 m/s). Air concentration and velocity profiles at various cross-sections above and downstream of the aerator are presented and analyzed. They provide a better understanding of the air entrainment processes occurring above a bottom aerator.A new analysis of the flow downstream of an aerator is presented and this contributes to a new method for the determination of aerator spacing.

Flow downstream of an aerator - aerator spacing

Cavitation erosion damage to spillway surfaces may be prevented with the use of aeration devices (aerators), introducing air in the layers close to the channel bottom. A study of the flow downstream of a spillway aerator was performed on the Clyde dam spillway model. The results are presented and a comparison between the free-surface aeration and the flow downstream of an aerator is developed. Such a method enables to compute the flow parameters at any point downstream of an aerator, in particular the air concentration near the spillway bottom. The computed results are compared with experimental data. This analysis contributes to a method for the determination of aerator spacing and aerator location.