Computational modeling of flow over a spillway in vatnsfellsst fla dam in iceland
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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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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


Vatnsfellsvirkjun hydroelectric scheme from above


The spillway at Vatnsfell – from below


The spillway at Vatnsfell – the crest


The splitter wall and cover from above


The chute cover from below


The spillway and the stilling basin


Layout chute, bottom outlet and stilling basin


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


If neither splitter wall nor chute cover...


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


Vattenfall’s hydraulic model


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. 1


Computational domain nr. 2


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


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


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


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


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


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


Discharge coefficient (C) vs. flow discharge


Water level along the chute sidewalls


Pressure on the chute bottom – location of investigation points


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


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


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


Water surface in the stilling basin


Water surface in the stilling basin


Water surface in the stilling basin


Water level in the left upstream corner of the stilling basin


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


Velocity contours in the spillway and the stilling basin


Velocity vectors in the stilling basin


Pressure on the baffles in the first baffle row


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


Static pressure in the stilling basin


Dynamic pressure in the stilling basin


Total pressure in the stilling basin


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


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


Total pressure on the basin end sill


Velocity profile above end sillright under the water surface


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


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


Thank you!


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