2003 Overview of IST Group Results on the Sediment Benchmark 3 rd IMPACT Workshop Louvain-la-Neuve...

20032003

Overview of IST Group Results on the Overview of IST Group Results on the

Sediment BenchmarkSediment Benchmark

33rdrd IMPACT Workshop IMPACT Workshop

Louvain-la-NeuveLouvain-la-Neuve

University of Beira Interior

João Leal (UBI)João Leal (UBI)

Rui Ferreira (IST)Rui Ferreira (IST)

António Cardoso (IST)António Cardoso (IST)

António Almeida (IST)António Almeida (IST)



OBJECTIVES:OBJECTIVES:

Comment the IST numerical results

Perform a sensitivity analysis on the empirical

coefficients in order to improve the results


CONCEPTUAL MODELCONCEPTUAL MODEL

Closure equationsClosure equations


depth of the sheet-flow layerdepth of the sheet-flow layer(Sumer et al. 1996; Pugh & Wilson 1999)(Sumer et al. 1996; Pugh & Wilson 1999)

ch d

velocity in the sheet-flow layervelocity in the sheet-flow layer(Sumer et al. 1996)(Sumer et al. 1996)

3 442.57c fu C u



sediment transport ratesediment transport rate(Bagnold 1966)(Bagnold 1966)

*0 cG u

bed shear stressbed shear stress((Chézy equationChézy equation))

0 w fC u u

EMPIRICAL COEFFICIENTS

CONCEPTUAL MODELCONCEPTUAL MODEL

Coefficients evaluationCoefficients evaluation::

using Sumer et al. (1996) results and the non-dimensional fall velocityusing Sumer et al. (1996) results and the non-dimensional fall velocity

as the measure of similitude between the sediment

used by those authors and the PVC cylinders used in the UCL test

dsgww 1*

Bed material

(-) (mm) (cm/s) (-) (-) (-)

PVC cylinders 1.54 3.50 14.97 1.10 0.64 0.0155

s d w *w fC




Wilson’s (1987) theoretical relationWilson’s (1987) theoretical relation 10ch d 10

Levels at x= -0.50m

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




GOOD AGREEMENT

Levels at x= -0.25m

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




GOOD AGREEMENT

Levels at x= 0.00m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




BAD AGREEMENT

(the numerical model underestimates the scour hole)

Levels at x= 0.25m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




BAD AGREEMENT

(the numerical model underestimates the scour hole)

Levels at x= 0.50m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




BAD AGREEMENT

(the numerical model predicts no scour)

Levels at x= 0.75m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

IST

EXP

RESULTSRESULTS




BAD AGREEMENT

(the numerical model predicts no scour)

Extreme characteristics

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

distance [m]

tim

e [s

]

IST

EXP

RESULTSRESULTS




BAD AGREEMENT

(the numerical wave-front celerity is faster

than the experimental one)

COMMENTSCOMMENTS




Two major discrepancies were found between the UCL

experimental data and the IST numerical results

In the sections near the dam the scour is underestimated by

the numerical model

The numerical wave-front celerity is faster than the

experimental one

POSSIBLE SOLUTION: increase the coefficient of

Bagnold’s sediment transport formula

POSSIBLE SOLUTION: increase the friction coefficient fC

NOTE: should the initial water depth downstream the gate be non

negligible and the wave-front celerity would be slowed down

Levels at x= 0.75m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

] EXP

hd = 0.0001

hd = 0.0005

hd = 0.001

Influence hInfluence hdd




The increase of hd increases the

wave-front height

Limitation imposed by the closure equations




3 2

49

100c fu u C

(value adopted in the blind test)

In order to increase friction (Cf) we will need to use smaller values of

The experimental sheet-flow data by Sumer et al. (1996) and by Pugh & Wilson (1999) indicate that 6.6

which implies a new limit

10 0.0155fC

0.0289fC

Levels at x= 0.75m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

] EXP

beta = 10

beta = 8

beta = 6.6




As increases, the water levels increase in the wave-

front region while the sheet-flow height decreases.

The celerity is not influenced by

InfluenceInfluence

Levels at x= -0.25m

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

] EXP

qsi = 0.64

qsi = 1

qsi = 2




= 2 overestimates the erosion,

affecting the water level for

higher times

InfluenceInfluence

Levels at x= 0.25m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

] EXP

qsi = 0.64

qsi = 1

qsi = 2




= 2 gives better bed levels but

worst water levels

InfluenceInfluence

Levels at x= 0.00m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

he

igh

t [m

] EXP

Cf = 0.0155

Cf = 0.02

Cf = 0.025




The increase of Cf increases the scour hole, but the

numerical bed level is still much higher than the

experimental one

InfluenceInfluence fC

Levels at x= 0.75m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

] EXP

Cf = 0.0155

Cf = 0.02

Cf = 0.025




The increase of Cf allows the approximation between the

numerical and the experimental wave-fronts



0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

distance [m]

tim

e [s

] EXP

Cf = 0.0155

Cf = 0.02

Cf = 0.025




For higher Cf values the numerical

wave-front celerity is reduced and

approximates the experimental celerity


Levels at x= 0.00m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

EXP

initial

final




Final ResultsFinal Results




Levels at x= 0.75m

-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

time [s]

hei

gh

t [m

]

EXP

initial

final






0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

distance [m]

tim

e [s

] EXP

initial

final


CONCLUSIONSCONCLUSIONS

The major discrepancies between numerical and experimental

results can be attenuated by changing the value of some

parameters of the closure equations

The model always underestimate the scour hole that is

observed downstream the dam in the experiments

The IST numerical results are generally in good agreement with

the benchmark data




ENDEND

Date post:	24-Dec-2015
Category:	Documents
Upload:	clement-ford
View:	214 times
Download:	0 times

2003 Overview of IST Group Results on the Sediment Benchmark 3 rd IMPACT Workshop Louvain-la-Neuve...

Documents