Computational Modeling of Flow over a Spillway In Vatnsfellsst

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

• Masters Thesis Presentation
• Chalmers University of Technology
• 2007 02 - 02

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Presentation Schedule

• Introduction and background
• Method
• Theory
• Results
• Conclusions and future work

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Vatnsfellsvirkjun hydroelectric scheme from above
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The spillway at Vatnsfell from below
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The spillway at Vatnsfell the crest
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The splitter wall and cover from above
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The chute cover from below
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The spillway and the stilling basin
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Layout chute, bottom outlet and stilling basin
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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

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If neither splitter wall nor chute cover...
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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

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Background and goals

• In 1999 Vattenfall in Sweden did hydraulic
  experiments for the spillway with a 130 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

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Vattenfalls hydraulic model
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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

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Method

1. Identify the computational domain to be modeled
   (according to the goals!)
2. Draw the computational domain in 3D in Autodesk
   INVENTOR
3. Import the geometry into the mesh making software
   GAMBIT and divide the computational domain into
   computational cells of different size in GAMBIT
4. Import the mesh into the CFD-solver FLUENT, set
   up the numerical model, compute and monitor the
   solution
5. Postprocessing with FLUENT and MATLAB examine
   the results and consider revisions to the model

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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

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Computational domain nr. 1
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Computational domain nr. 2
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Computational domain nr. 3
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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
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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

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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

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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

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Grid nr. 9 different types of mesh consisting
of both hexahedron cells and tetrahedron cells
ca. 498 000 cells
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Grid nr. 9 includes the stilling basinthough
coarse in view of the size of the computational
domain
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Grid nr. 9 includes a simplified rock rip-rap
downstream the basin
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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-e)
• Initial solution (nB steady flow)
• Convergence criteria

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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 aq 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.
  

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Main results!Comparison to the experimental
results
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Water reservoir head vs. flow discharge
QCBH3/2where Q flow discharge, C discharge
coefficient, B length of crest, Hhead

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Discharge coefficient (C) vs. flow discharge
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Water level along the chute sidewalls
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Pressure on the chute bottom location of
investigation points
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Pressure on the chute bottom point A 23
deviation from exp-results
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Pressure on the chute bottom point B 16
deviation from exp-results
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Pressure on the chute bottom point C 9
deviation from exp-results
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Water surface in the stilling basin
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Water surface in the stilling basin
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Water surface in the stilling basin
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Water level in the left upstream corner of the
stilling basin
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Volume fraction of water in the basin
(longitudinal profile) determines the water
level
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Velocity contours in the spillway and the
stilling basin
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Velocity vectors in the stilling basin
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Pressure on the baffles in the first baffle row
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Pressure on two baffles in the first row
(deviations from experimental results in
parantheses)
Baffle Pressure on upstream face (kPa) Pressure on downstream face (kPa) Resultant pressure (kPa)
B1CFD_case 9 151 18 133 (53 dev.)
B1CFD_case 6 155 1 154 (46 dev.)
B1EXP 272 -14 286

B2CFD_case 9 199 - 2 201 (16 dev.)
B2CFD_case 6 200 -11 211 (11 dev.)
B2EXP 233 -5 238
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Static pressure in the stilling basin
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Dynamic pressure in the stilling basin
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Total pressure in the stilling basin
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Total pressure on the basin end sill- a view
under the water surface in the downstream end of
the basin
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Total pressure on the basin end sill - location
of investigation points
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Total pressure on the basin end sill
Location Pressure on upstream face (kPa) Pressure on Downstream face (kPa) Resultant pressure (kPa) EXP Results (kPa)
K 32.4 29.2 3.2 2.5
L 35.9 34.3 1.6 8.7
M 31.3 26.6 4.7 3.7
N 29.3 26.2 3.1 0.3
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Velocity profile above end sillright under the
water surface
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Main results - summary

• Good agreement is reached between the experiments
  and CFD calculations for the following aspects
• head vs. discharge capacity (QCBH3/2)
• 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

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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

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What more can be done?- e.g. time dependent
calculations
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Thank you!