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

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.

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

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  1. Computational Modeling of Flow over a SpillwayIn Vatnsfellsstífla Dam in Iceland Master’s Thesis Presentation Chalmers University of Technology 2007 – 02 - 02

  2. Presentation Schedule • Introduction and background • Method • Theory • Results • Conclusions and future work

  3. Vatnsfellsvirkjun hydroelectric scheme from above

  4. The spillway at Vatnsfell – from below

  5. The spillway at Vatnsfell – the crest

  6. The splitter wall and cover from above

  7. The chute cover from below

  8. The spillway and the stilling basin

  9. Layout chute, bottom outlet and stilling basin

  10. 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

  11. If neither splitter wall nor chute cover...

  12. 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

  13. 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

  14. Vattenfall’s hydraulic model

  15. 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

  16. 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

  17. 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

  18. Computational domain nr. 1

  19. Computational domain nr. 2

  20. Computational domain nr. 3

  21. 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

  22. 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

  23. 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

  24. 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

  25. Grid nr. 9: different types of mesh; consisting of both hexahedron cells and tetrahedron cellsca. 498 000 cells

  26. Grid nr. 9 includes the stilling basinthough coarse in view of the size of the computational domain

  27. Grid nr. 9: includes a simplified rock rip-rap downstream the basin

  28. 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

  29. 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.

  30. Main results!Comparison to the experimental results

  31. Water reservoir head vs. flow discharge; Q=CBH3/2where Q= flow discharge, C= discharge coefficient, B = length of crest, H=head

  32. Discharge coefficient (C) vs. flow discharge

  33. Water level along the chute sidewalls

  34. Pressure on the chute bottom – location of investigation points

  35. Pressure on the chute bottom point A: 23 % deviation from exp-results

  36. Pressure on the chute bottom point B: 16 % deviation from exp-results

  37. Pressure on the chute bottom point C: 9 % deviation from exp-results

  38. Water surface in the stilling basin

  39. Water surface in the stilling basin

  40. Water surface in the stilling basin

  41. Water level in the left upstream corner of the stilling basin

  42. Volume fraction of water in the basin (longitudinal profile) – determines the water level

  43. Velocity contours in the spillway and the stilling basin

  44. Velocity vectors in the stilling basin

  45. Pressure on the baffles in the first baffle row

  46. Pressure on two baffles in the first row (deviations from experimental results in parantheses)

  47. Static pressure in the stilling basin

  48. Dynamic pressure in the stilling basin

  49. Total pressure in the stilling basin

  50. Total pressure on the basin end sill- a view under the water surface in the downstream end of the basin

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