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

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: Stilling basin:
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 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 cells ca cells

26. Grid nr. 9 includes the stilling basin though 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
Water reservoir head vs. flow discharge; Q=CBH3/2 where 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)
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

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

51. Total pressure on the basin end sill - location of investigation points

52. Total pressure on the basin end sill
Location
Pressure on
upstream face
(kPa)
Downstream
face (kPa)
Resultant
pressure
EXP
Results 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

53. Velocity profile above end sill right under the water surface

54. 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

55. 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

56. What more can be done? - e.g. time dependent calculations

57. Thank you!