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# Computational Modeling of Flow over a Spillway In Vatnsfellsstífla Dam in Iceland PowerPoint PPT Presentation

Computational Modeling of Flow over a Spillway In Vatnsfellsstífla Dam in Iceland. Master’s Thesis Presentation Chalmers University of Technology 2007 – 02 - 02. Presentation Schedule. Introduction and background Method Theory Results Conclusions and future work.

Computational Modeling of Flow over a Spillway In Vatnsfellsstífla Dam in Iceland

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## Computational Modeling of Flow over a SpillwayIn Vatnsfellsstífla Dam in Iceland

Master’s Thesis Presentation

Chalmers University of Technology

2007 – 02 - 02

### Presentation Schedule

• Introduction and background

• Method

• Theory

• Results

• Conclusions and future work

### The spillway – characteristics

• Function: cope with accidental flooding

• Height above stilling basin bottom: 27.5 m

• Lenght of spillway crest: 50 m

• Equipped with a splitter wall and cover to prevent overtopping of the chute sidewalls

• The velocity of the water is above 20 m/s (=72 km/hour!) where it flows into the stilling basin

### The stilling basin – characteristics

• Function: Decrease flow velocity in order to decrease risk for erosion in the river wally downstream the basin

• Equipped with 28 energy dissipating baffles (height from 1.5 to 2.0 m)

• Length ca. 33 m and the width increasing from 22 m in the upstream part to 33 m in the downstream part, depth ca. 7 m

• Downstream the stilling basin is a 35 m long rock rip-rap made of rocks with diameter of

0.4 – 1.2 m

### Background and goals

• In 1999 Vattenfall in Sweden did hydraulic experiments for the spillway with a 1:30 model

• In the experiments flow was investigated over the spillway, through the bottom outlet and in the stilling basin

• Goals of the present study:

• investigate flow over the spillway and in the stilling basin with computational methods (CFD)

• compare CFD-results with experimental results

### Aspects

• Spillway:

• water head in the reservoir vs. the discharge capacity of the spillway

• Water level along the chute sidewalls

• Pressure acting on the chute bottom

• Stilling basin:

• Water level

• Pressure acting on the baffles and the end sill

• Flow velocity out of the basin

### Method

• Identify the computational domain to be modeled (according to the goals!)

• Draw the computational domain in 3D in Autodesk INVENTOR

• Import the geometry into the mesh making software GAMBIT and divide the computational domain into computational cells of different size in GAMBIT

• Import the mesh into the CFD-solver FLUENT, set up the numerical model, compute and monitor the solution

• Postprocessing with FLUENT and MATLAB; examine the results and consider revisions to the model

### The computational domain

• Three different domains:

• One for head vs. flow discharge

• One for water level and pressure in the spillway chute

• One for water level, pressure and flow velocity in the stilling basin

• Why different domains?

• to spare computational power and get more precise results

### Computational domain nr. 3

Grids nr. 1 – 7 as seen from above- one grid for each of the seven different cases with flow discharge of 50 – 350 m3/s, ca. 653 000 cells/grid

### Cut through grids nr. 1 and 7 in the downstream end of the reservoir by the spillway crest – different water levels

• Grid to the left: designed for flow discharge of 50 m3/s

• Grid to the right: designed for flow discharge of 350 m3/s

### Grid nr. 8: finer in the chute than grids nr. 1 – 7, ca. 1393 000 cells

• The mesh in the spillway bottom

• To the left: mesh 7 which is NOT specifically designed to investigate pressure and water level in the spillway chute

• To the right: mesh 8 which is specifically designed to investigate pressure and water level in the spillway chute

### Mesh nr. 8: finer in the chute than meshes nr. 1 - 7

• The grid perpendicular to the splitter wall

• To the left: mesh 7 which is NOT specifically designed to investigate pressure and water level in the spillway chute

• To the right: mesh 8 which is specifically designed to investigate pressure and water level in the spillway chute

### Setting up the numerical model

• Define

• Material properties (air, water, concrete)

• Boundary conditions (inlet, outlet, walls,

air pressure,...)

• Operating conditions (air pressure, gravity, temperature...)

• Turbulence model (standard k-ε)

• Initial solution (nB: steady flow)

• Convergence criteria

### Theory – equations of motion and the VOF method

• The continuity equation for incompressible flow:

• The momentum equation for incompressible flow:

• VOF method in FLUENT

• assumes that the two phases (air and water) are not interpenetrating

• denoting αq as the volume fraction of the q-th phase three possibilities for a given cell can be noted:

• i) : the cell is empty of the q-th phase,

• ii) : the cell is full of the q-th phase,

• iii) : the cell contains the interphase between the q-th phase and one or more phases.

## Main results!Comparison to the experimental results

### Main results - summary

• Good agreement is reached between the experiments and CFD calculations for the following aspects:

• head vs. discharge capacity (Q=CBH3/2)

• pressure in the spillway chute

• flow velocity above the basin end sill

• Worse agreement is reached for:

• pressure on baffles in the upstream end of the basin

• water depth along chute sidewalls and in the left upstream corner of the basin

• pressure on the basin end sill

### Future work – what might to be done better or added?

• Calculate the flow through the bottom outlet

• Better resolve the turbulent boundary layers close to walls

• finer mesh

• more computational power

• even parallel processing