Marcela Politano

A multidimensional two-phase flow model for the total dissolved gas downstream of spillways

Elevated levels of dissolved gas in the spillway stilling basin, which are responsible for gas bubble disease in fish, constitute an important negative environmental effect of dams. Bubbles, entrained when a plunging jet impacts the tailwater pool, plunge beneath the surface and transfer mass to the liquid, causing an increase in the total dissolved gas (TDG) concentration. Most of the numerical studies onTDG downstream of spillways found in the literature are based on experimental correlations for the gas volume fraction.Abetter approach involves the use of a two-phase flowmodel. In this paper, a two-fluid model is used to calculate the gas volume fraction and velocity of the bubbles. A polydisperse model is used in which a Boltzmann transport equation predicts the bubble size distribution, to account for the different bubble sizes found in the flow downstream of spillways. The bubble mass is discretized considering groups of bubbles of variable mass, with the mass of the bubbles changing due to bubble/liquid mass transfer and pressure. A two-phase transport equation for the TDG is presented, whose source is the bubble/liquid mass transfer, which is a function of the gas volume fraction and bubble size distribution. Two-dimensional numerical results of TDG, gas volume fraction, bubble number density, and velocities are presented and discussed. The predictions of TDG downstream of a spillway are compared against field data in the stilling basin ofWanapum Dam, on the Columbia River.

Fish navigation of large dams emerges from their modulation of flow field experience.

Significance Whereas adult salmon swimming upstream through a ladder visibly illustrates the challenge a dam presents to fish returning home to spawn, the downstream passage of juveniles swimming toward the ocean is often a greater, although more unseen, challenge to their survival. Decades of work have identified many factors that affect fish behavior near dams, but why downstream passage structures may work well at one dam but not at another is poorly understood. We use a computer model to show that observed downstream passage patterns could result from a generic strategy that fish may use to navigate flow field obstacles. Our findings identify environmental and biological factors warranting further evaluation for sustaining native species amid economic development. Navigating obstacles is innate to fish in rivers, but fragmentation of the world’s rivers by more than 50,000 large dams threatens many of the fish migrations these waterways support. One limitation to mitigating the impacts of dams on fish is that we have a poor understanding of why some fish enter routes engineered for their safe travel around the dam but others pass through more dangerous routes. To understand fish movement through hydropower dam environments, we combine a computational fluid dynamics model of the flow field at a dam and a behavioral model in which simulated fish adjust swim orientation and speed to modulate their experience to water acceleration and pressure (depth). We fit the model to data on the passage of juvenile Pacific salmonids (Oncorhynchus spp.) at seven dams in the Columbia/Snake River system. Our findings from reproducing observed fish movement and passage patterns across 47 flow field conditions sampled over 14 y emphasize the role of experience and perception in the decision making of animals that can inform opportunities and limitations in living resources management and engineering design.

Water entrainment due to spillway surface jets

Strong flow entrainment has been observed downstream of spillways constructed with flow deflectors. This water entrainment has important environmental and ecological impacts because it improves the mixing of powerhouse and spillway flows, but may negatively impact fish migration or create adverse flow conditions. Most studies found in the literature attempt to explain this entrainment with turbulent mixing. Both reduced-scale hydraulic models and single-phase, isotropic RANS models grossly under-predict the degree of entrainment observed in prototypes. In this paper, an anisotropic model that accounts for the bubble volume fraction and attenuation of the normal velocity fluctuations at the free surface is presented. The model adequately predicts the main mechanisms causing water entrainment and compares well against experimental data for a round surface jet and for Brownlee Dam at model scale. It is shown that appropriate entrainment can only be captured if the turbulence anisotropy and the two-phase nature of the flow are modelled.

Investigation into the Total Dissolved Gas Dynamics of Wells Dam Using a Two-Phase Flow Model

Bubbles entrained by spilled water at hydroelectric projects increase the concentration of total dissolved gas (TDG), which may lead to gas bubble disease in fish. In this paper, the TDG dynamics downstream of Wells Dam are investigated using a two-phase flow model that accounts for the effect of the bubbles on the flow field. The TDG is calculated with a transport equation in which the source is the bubble/liquid mass transfer, a function of the gas volume fraction and bubble size. The model uses anisotropic turbulence modeling and includes attenuation of normal fluctuation at the free surface to capture the flow field and TDG mixing. The model is validated using velocity and TDG field data. Simulations under two plant operational configurations are performed to gain a better understanding of the effect of spill operations on the production, transport, and mixing of TDG. Model results indicate that concentrated spill releases create surface jets that result in the lowest TDG concentration downstream. On ...

A three-dimensional Thermal Model for McNary Dam

Water temperature is an important water quality parameter in aquatic ecosystems, impacting dissolved oxygen, chemical and biological reaction rates, and plant and animal mortality. In particular, during summer months, high temperatures in the forebay, gatewells and juvenile fish collection channel at McNary Dam lead to increased stress on fish population. A 3D hydrodynamic and heat transport model was developed to predict the water temperature on McNary Dam. The flow field was solved using an incompressible RANS solver for buoyant flows. The Boussinesq approach and a standard k � � model with wall functions were employed. The thermal model takes into account the energy sources due to solar radiation and the convective heat transfer at the free surface, which is function of the air temperature and wind velocity. The equations of the proposed thermal model were implemented into the commercial code FLUENT. The unsteady energy sources, boundary conditions at the free surface and inflows were programmed. A calibration for a typical day was conducted using measured temperature profiles and weather conditions. Comparison shows that the model reproduced general observed trends and daily fluctuations. The multidimensional temperature and velocities fields are presented and discussed.

Model development in OpenFOAM to predict spillway jet regimes

Hydropower is the most important renewable energy source. Though hydropower provides abundant benefits, dams have also altered natural flow conditions affecting fish habitat. Elevated total dissolved gas (TDG) can result in gas bubble disease in affected fish. TDG production depends on the gas volume fraction and bubble depth in the tailrace, which are a function of spillway jet regimes. This paper presents a model developed in OpenFOAM to predict spillway jet regimes. The model utilizes the volume of fluid method to capture the dynamic free surface. A Large Eddy Simulation model, together with Detached Eddy Simulation, was used for turbulence closure. The model adequately reproduced jet regimes observed in a reduced-scale laboratory model. Differences in jet regimes predicted at reduced and prototype scales were observed. Results suggest that turbulence, not scaled in the laboratory model, plays an important role in the flow characteristics downstream of spillways.

Assessment of ice plugging of a cooling water intake by a numerical model

American Electrical Powers Conesville plant had historically prevented ice buildup at a water intake by routing warm water from a once-through cooling system to the intake. In 2012, this system was retired and warm water was no longer available for de-icing. A numerical study was conducted to evaluate design alternatives to reduce ice transport to the intake. An Eulerian–Lagrangian model was used to predict the hydrodynamics and ice trajectories. A porous media model accounted for the ice accumulation in the trash racks. The model was capable of predicting the observed flow pattern. Predicted velocities compared well with measurements using large-scale particle image velocimetry (LSPIV). According to the model, sediment control vanes, placed in front of the intake to alleviate sediment ingestion, favour the ice transport to the intake. Based on model results, a mitigation plan to be implemented during several phases was elaborated.

Water Entrainment and Mixing due to Spillway Discharges

Many spillway designs form surface jets that discharge in relatively large reservoirs. It has been often observed that strong liquid entrainment is caused by these surface jets. Velocity measurements at both model and prototype scale for Wanapum Dam, Washington, and at model scale for Brownlee Dam, Idaho, indicate that the jet configuration and strength affect the entrainment. This water entrainment can have important environmental and ecological impacts. In the abovementioned power dams, water coming from the powerhouse is attracted to the spillway, causing beneficial phenomena. The Total Dissolved Gas (TDG) produced in the spillway is diluted with low TDG water coming from the powerhouse. In addition, low Dissolved Oxygen (DO) waters from the powerhouse are rapidly oxygenated by mixing with the aerated spillway discharge. The mechanisms causing this entrainment are poorly understood. Most of the studies found in the literature explain the entrainment with turbulent mixing. Aeration and free-surface instabilities contribute to modify the turbulence structure and have an effect on the entrainment. Standard RANS numerical models fail to predict the degree of entrainment observed experimentally. In this work a study of the entrainment in free surface jets, with the objective of reveal the reasons for the poor performance of RANS solvers in predicting water entrainment, was carried out. A numerical model was developed to resolve the free surface using the commercial code FLUENT. A submerged round jet parallel to the free surface was modeled and the effect of turbulence models was analyzed. Following a brief description of the models, the paper discusses the mechanisms involved in entrainment in spillways by comparing models with available experimental results.

The Iowa Watersheds Project: Iowa's prototype for engaging communities and professionals in watershed hazard mitigation

ABSTRACT After more than a century of intensive changes in the states agricultural watersheds, repeated record floods motivated Iowa to innovate in its flood recovery and disaster mitigation efforts following the 2008 floods. The state created the Iowa Flood Center (IFC) and authorized the creation of Watershed Management Authorities. With enhanced funding from the federal government for disaster recovery, the groups collaborated in four watersheds as part of the Iowa Watersheds Project. The Watershed Management Authorities brought together governmental organizations and local stakeholders, developed watershed plans, defined potential flood and nutrient reduction projects, identified willing landowners for project locations in one subwatershed, and hired consultants to design and build the projects. The IFC coordinated efforts across the four watershed areas, provided hydrologic assessments for each, carried out monitoring and modelling to evaluate the performance of all the constructed projects, and developed water resources information systems to support ongoing activities. The Iowa Watersheds Project adopted a watershed systems approach, and the resulting synergy between engaged communities and technical professionals demonstrates avenues for implementing elements of an Integrated Water Resources Management framework in a highly decentralized setting.

Numerical modelling of the hydrodynamics and total dissolved gas downstream of Hells Canyon Dam

Abstract A computational fluid dynamics model was developed to predict the flow pattern and total dissolved gas (TDG) distribution downstream of Hells Canyon Dam on the border of Idaho and Oregon, USA. Free surface simulations in the tailrace were carried out to predict spillway jet regimes with and without deflectors. Model results agreed with deflector performance curves obtained in a 1:48 laboratory scale model. The distribution of TDG was predicted with a mixture model that accounts for the two-phase flow in the tailrace and the mass transfer between bubbles and water. Attenuation of the turbulence at the free-surface and the effect of the bubbles on the turbulence was included to capture water attraction towards spillway surface jets, which affects the amount of water exposed to bubbles and downstream TDG dilution. The model takes into account changes in bubble size with dissolution and pressure. Good agreement was obtained between predicted TDG and available measurements in the field.