Computational Modeling of Flow over a Spillway In Vatnsfellsstífla Dam in Iceland Master’s Thesis...
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Transcript of Computational Modeling of Flow over a Spillway In Vatnsfellsstífla Dam in Iceland Master’s Thesis...
Computational Modeling of Flow over a Spillway
In Vatnsfellsstífla Dam in Iceland
Master’s Thesis PresentationChalmers 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
Method1. 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
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
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
Grid nr. 9: different types of mesh; consisting of both hexahedron cells and tetrahedron cells
ca. 498 000 cells
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.
0i iu
0
1i i j j iD u p u
0q
1q
0 1q
Water reservoir head vs. flow discharge; Q=CBH3/2
where Q= flow discharge, C= discharge coefficient, B = length of crest, H=head
Pressure on two baffles in the first row (deviations from experimental results in
parantheses)
BafflePressure 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
Total pressure on the basin end sill- a view under the water surface in the
downstream end of the basin
Location Pressure onupstream face(kPa)
Pressure onDownstreamface (kPa)
Resultantpressure (kPa)
EXPResults(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
Total pressure on the basin end sill
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