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The Value of CFD Modeling in Designing a Hydro Plant
Posted Wed October 13, 2010 @08:13AM
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Application The use of CFD modeling to design a proposed hydroelectric facility allowed Manitoba Hydro to verify and adjust its physical model. By combining the use of both tools, utility engineers uncovered design changes that can save on construction costs.

By Kevin M. Sydor and Pamela J. Waterman
First published at HydroWorld.com

In 1911, when Canada's Commission of Conservation first started eyeing the Nelson River as a possible energy source, commission members probably were not thinking about mathematically analyzing flow issues. Since the 1950s, the vision of harnessing this river has become reality through the efforts of Manitoba Hydro, which has built five plants with a total capacity of 3,925 MW along the massive waterway. These plants began operating between 1961 and 1990.


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For more than a decade, Manitoba Hydro has tackled the complexity of plant design using FLOW-3D computational fluid dynamics (CFD) software from Flow Science. This software has been applied to assist in the hydraulic design of powerhouse intakes, spillway structures, stilling basins, and other regulating structures. For example, FLOW-3D was used to design the hydraulic components of the 200-MW Wuskwatim Generating Station, which is currently under construction. More recently, Manitoba Hydro has focused efforts on engineering design studies to aid in the decision to commit to building the proposed 675-MW Keeyask Generating Station on the Nelson River. If approved, this facility will be sited at a complicated location that includes rapids, multiple channels, and natural contours.

Building on previous use of FLOW-3D for initial design concepts, for this project the provincial utility performed an integrated study where the combined results from both simulations and physical modeling served to validate and improve each other's performance.

Studying the design of the Keeyask station
Keeyask will operate with seven generating units and rank as the fourth largest Manitoba Hydro plant. The reservoir will have an area of about 93 square kilometers. In May 2009, a Joint Keeyask Development Agreement was signed with four Canadian First Nations partners, outlining partnership arrangements for First Nations' participation in the project development. Once approved, construction is expected take seven to eight years. No decision has been made to go forward with development of this project, and no application has been made for regulatory review or approval.

Two phases of river diversion will be necessary to create temporary sections of dry land for the powerhouse and spillway construction. Manitoba Hydro conducted CFD simulations to estimate how water levels and velocities at various locations would change as physical conditions change during construction of the temporary cofferdams. The utility then compared results with those measured by LaSalle Consulting Group on both a 1/120 scale model of the cofferdam construction, diversion structure, river closure, and spillway and a 1/50 model of a section of the spillway. These model scales were selected taking into account the overall cost of model construction and the expected accuracy of the discharge and depth measurements within the model.

Nuances observed in operation of the 1/120 scale physical model led to its modification. Modifications included narrowing the tailrace channel to reduce overall rock excavation and slightly raising the invert level for the diversion structure. Details of these modifications were fed back as changes in the three-dimensional (3D) computer model representing boundary conditions within the CFD simulations. This detailed back-and-forth process not only predicted the behavior of the physical scale model within about 5 percent but also uncovered design changes that would save hundreds of thousands of dollars on construction costs.

By employing computer simulation in the early design stages for Keeyask, Manitoba Hydro verified agreement of the CFD model with physical model behavior, identified changes that reduced construction costs, and proved the benefits of fast, flexible changes during feasibility and pre-engineering design stages.

Modeling massive water flows
The Nelson River flows north-northeast through Manitoba, carrying a mean flow of more than 3,000 cubic meters per second (cms) at the Keeyask site, and draining Lake Winnipeg and its watershed into Hudson Bay. Churning along its 644-kilometer run, the river's huge volume and long drop make it an ideal source of hydroelectric power.

The proposed design for Keeyask incorporates a powerhouse with an outflow capacity of 4,000 cms and a south-channel spillway to manage various levels of overflow coming from upstream. The design flow for the spillway is 12,700 cms. It is critical to understand the flow rates, velocities, and water levels, as negative pressures potentially can result in cavitation damage to the concrete surfaces, and the individual piers must be built tall enough to contain the flow.

Manitoba Hydro had used FLOW-3D software to reproduce water-flow behavior at existing facilities. The hydrotechnical engineers again chose FLOW-3D to simulate the dynamic and steady-state flow conditions near the temporary construction dams and permanent structures. The goal was to assist in the planning stages for Keeyask, simulating 3D flow patterns for inflow and downstream that would provide guidance on spillway design, intake and tailrace channel designs, and river management during construction. Figure 1 shows the boundaries for the physical and FLOW-3D models. The CFD model was used to test various geometries and cofferdam layouts, and the optimized design was validated with the physical model.

Setting up and calibrating the CFD model
FLOW-3D is capable of simulating the dynamic and steady-state behavior of liquids and gases in one, two, or three dimensions. Simulations can account for free surface flows and handle transitions between subcritical and supercritical flow within a single model setup. These capabilities made the software ideal for evaluating the varied flow conditions expected in the spillway operation.

FLOW-3D subdivides the problem domain, as do other CFD analysis packages. However, this software applies a free gridding approach, which enables the user to change the mesh grids independent of the geometry. The software employs a technique, known as FAVOR™ (Fractional Area Volume Obstacle Representation), to perform accurate calculations even in the presence of complex geometries. These methodologies save significant time in setting up a simulation and in design variations.

The CFD model covered an area about 3 kilometers by 2 kilometers, with boundary conditions set by a velocity boundary to control flow into the upstream end and a continuative outflow boundary at the downstream end. Designers imported 3D solid AutoCAD files of geometric objects such as piers, abutments, spillway structure, and cofferdams to represent physical boundaries, then defined the flow parameters.

model boundaries

To accommodate the characteristics of the river rapids and the expected range of flow rates churning through the parallel sections of the spillway, the CFD model was set up to include the effects of turbulence. For each simulation, the RNG turbulence model was used. Such an approach can take more memory than other methods but was chosen for its computational speed.

The input data requirements for the CFD model were river bathymetry throughout the river reach; upstream velocity data to control flow into the upstream end of the model; and geometric representations of the cofferdams, spillway structure, abutments, and piers.

The mesh was set up with Cartesian coordinates and used nested blocks to refine the grid in areas that required finer meshing to capture fine variations in detail across the geometry. Grid spacing in the area around the spillway structure was set at 1 meter by 1 meter by 1 meter, which was necessary to include the shapes of the spillway piers, abutments, and spillway crest. The model was calibrated against stage-discharge rating curves at gauge locations where measurements were available.

Goals for the simulation included estimating the discharge capacities, water levels, velocities, and flow patterns at various points on the construction timeline and for different positions of the spillway gates. These calculated values are important for choosing the size of stone needed for rockfill during the cofferdam construction. Individual boulders must be large enough to resist the drag forces that work to move them out of place and downstream at all stages of construction.

Physical modeling
With hydro plant design, too much is at stake not to get it right the first time. Building a scaled physical model of critical flow areas is still necessary to verify capacities, pressures, velocities, and spillway gate behavior (fully and partially opened) at critical topographic points for different discharge rates. Manitoba Hydro asked LaSalle Consulting Group to build two such models – one comprehensive layout at 1/120 scale and one partial spillway model with two full bays and two half bays at 1/50 scale.

Technicians pumped water into the model at four different rates, representing flows from 1,600 cms to 6,100 cms. Manually read point-gauges yielded water-level readings at three locations in the river channel, three locations in the spillway channel, and one point downstream. Velocity measurements were taken at two depths with a Nixon current meter at several locations, and overall flow conditions were monitored through digital time-lapse photography of test floats introduced upstream.

comparison of velocity

Integrated modeling results
The CFD simulations for rock sizing predictions came in slightly conservative when looking at the force/movement relationship of sizes in the physical model. However, initial water-level data curves showed good agreement between the simulation and physical model behavior, setting the stage for further testing. As the cofferdam was slowly constructed in the physical model, subsequent water-level measurements showed that the CFD simulation accurately predicted the required freeboard under construction design flood conditions.

After cofferdam construction was complete, physical testing showed that flow control was taking place at the channel entrance, instead of at the control structure, resulting in higher upstream water levels than desired. The CFD model, which was used to optimize the initial design carried out with one-dimensional backwater models, showed a similar effect. The physical model was reconfigured to lower the approach channel entrance, reflecting the elevations used in the CFD model. With the channel entrance excavated to a lower level, excavation along the left bank was only necessary at a smaller area near the entrance. Additional testing confirmed that the invert of the diversion structure could be raised by 0.6 meter, shifting channel control to the diversion structure without having a negative effect on upstream water levels. These changes were fed back into the CFD model, which generated similar results.

Working with the larger scale, partial model of the spillway, tests were also made to verify rating curves under partial gate openings, water pressures, and gate area behavior. With the diversion sluiceway (two of the gates) fully open, the difference between simulated and measured results was 0.85 percent or less. With the complete spillway structure in use and fully open, the difference in results was 5.25 percent or less. Overall, velocity agreement between simulation and model operation was +/- 0.5 meter per second, and water level agreement was +/- 0.3 meter (see Figures 2 and 3).

structural verification

Conclusions
Manitoba Hydro has found that CFD modeling offers multiple benefits for planning the construction and operation of the Keeyask station, as well as future plants such as the 1,250-MW Conawapa Generating Station proposed for the Nelson River. It is possible to quickly test various alternatives, providing a very cost-effective means to optimize the overall hydraulic design of the project. It also is possible to use CFD analysis to evaluate complicated design issues very early in the design process.

Not only was there good agreement between the results of the two approaches, but combining the FLOW-3D simulations with scale-model testing offered an iterative path to improving the validity of both design options. Further, the simulation lets users easily and quickly extract velocity, water level, and flow rates anywhere within the CFD model domain, as opposed to the limited values that could practicably be obtained with physical gauges and time-lapse photography.

The use of CFD simulation prior to and in conjunction with physical modeling is giving Manitoba Hydro the flexibility to change conceptual details and configurations long before digging the first shovel-full. Computer simulation also allowed the design to advance in stages, reducing the effect of changes on the physical model.

Physical modeling cost $500,000. For the CFD simulation work, cost for the FLOW-3D software, staff time, and consulting fees was $150,000.

Reference
Teklemariam, E., B. Shumilak, D. Murray, and G.K. Holder, "Combining Computational and Physical Modeling to Design the Keeyask Station," Hydro Review, Volume 27, No. 4, pages 64-72.

Kevin Sydor, P.Eng., is the section head of hydrotechnical and oceanographic studies for water resources engineering at Manitoba Hydro in Canada. He conducted the computational fluid dynamics modeling studies for this project. Pamela Waterman is an engineer and technical editor with EngineeringInk in the U.S.

This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.

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