1.
Introduction
Density
or
gravity
currents
are
predominantly
slender
flows
that
are
impelled
through
density
differences,
established
either
by
salinity,
temperature
or
particles
in
suspension
within
a
fluid
[
1
]
.
The
latter,
currents
where
fine
sediments
in
suspension
are
accountable
for
the
extra
density
are
titled
turbidity
currents
.
Some
of
the
reported
outcomes
of
these
stratified
flows
in
large
deep
reservoirs
comprise,
among
other
consequences,
inlet
and
bottom
outlet
structures
blockage
and
storage
capacity
lessening
[
2
],
and,
at
broader
scales,
water
quality
and
biodiversity
degradation
[
3
]
.
In
virtue
of
multiple
water
bodies
stressors
(e
.
g
.
climate
change,
increasing
population,
among
others),
the
loss
of
storage
in
reservoirs
caused
by
turbidity
currents
is
currently
a
topic
of
forceful
scientific
research
([
4
],
[
5
],
among
others)
.
In
this
context,
the
use
of
Computational
Fluid
Dynamics
(CFD)
tools
as
an
appliance
of
water
bodies
monitoring
programs
is
undoubtedly
of
foremost
importance
.
The
main
objective
of
this
study
is
to
validate
a
CFD
code
(ANSYS

CFX)
applied
to
the
simulation
of
the
interaction
between
turbidity
currents
and
passive
retention
systems,
designed
to
induce
sediment
deposition
.
To
accomplish
the
proposed
objective,
laboratory
tests
were
initially
conducted
using
a
straightforward
obstacle
configuration
exposed
to
the
passageway
of
a
turbidity
current
.
Afterwards,
the
experimental
data
was
used
to
build
a
benchmark
case
to
validate
the
3
D
CFD
software
ANSYS

CFX
.
Mitigation
of
turbidity
currents
in
reservoirs
with
passive
retention
systems
:
validation
of
CFD
modeling
6
.
Conclusions
In
this
work
ASM
fulfillment
for
modeling
turbidity
currents
was
assessed
.
A
part
its
demanding
computational
requirements
and
even
though
the
numerical
solution
represents
a
fairly
reasonable
prediction
of
the
observed
flow
,
results
reveal
,
at
the
present
stage
,
its
inadequacy
to
describe
a
number
of
aspects
:
i)
the
velocity
maximum
is
under

predicted
;
ii)
a
sharp
difference
is
observed
at
the
near
wall
region
and
iii)
a
significant
discrepancy
between
experimental
and
numerical
suspended
sediment
concentration
outcomes
is
verified
downstream
the
obstacle
.
I
ssues
i)
and
ii)
may
be
due
to
a
variety
of
factors,
namely
the
grid
density
and
the
goodness
of
the
choice
of
the
turbulence
model
and
its
parameters,
and
issue
iii)
to
the
overprediction
of
sediment
deposition
upstream
the
obstacle
.
In
the
near
future,
the
impact
of
these
uncertainties
will
be
investigated
by
means
of
a
systematic
sensitivity
analysis
.
References
[
1
]
Simpson
,
J
.
E
(
1999
)
.
Gravity
Currents
:
In
the
Environment
and
the
Laboratory
.
Cambridge
University
Press
.
[
2
]
Oehy
,
C
.
D
.
,
Schleiss
,
A
.
J
.
(
2007
)
.
Control
of
turbidity
currents
in
reservoirs
by
solid
and
permeable
obstacles
.
Journal
of
Hydraulic
Engineering,
133
(
6
),
637

648
.
[
3
]
Chung,
S
.
W
.
,
Hipsey
,
M
.
R
.
,
Imberger
,
J
.
(
2009
)
.
Modeling
the
propagation
of
turbid
density
inflows
into
a
stratified
lake
:
Daecheong
Reservoir,
Korea
.
Environmental
Modelling
and
Software,
24
(
12
),
1467

1482
.
[
4
]
Alves
,
E
.
,
Ferreira,
R
.
M
.
L
.
,
Cardoso,
A
.
H
.
(
2010
)
.
One

dimensional
numerical
modeling
of
turbidity
currents
:
hydrodynamics
and
deposition
.
River
Flow
2010
–
International
Conference
on
Fluvial
Hydraulics,
Braunschweig
,
Germany,
8

10
September
2010
,
1097

1104
.
[
5
]
Rossato
,
R
.
,
Alves
,
E
.
(
2011
)
.
Experimental
study
of
turbidity
currents
flow
around
obstacles
.
7
th
International
Symposium
on
Stratified
Flows,
Rome,
Italy,
22
–
26
August
2011
.
[
6
]
Ansys
,
CFX

Solver
Theory
Guide
(
2009
)
.
[
7
]
W
.
Rodi
(
1993
)
.
"Turbulence
models
and
their
application
in
hydraulics

a
state
of
the
art
review",
International
Association
for
Hydraulic
Research,
Delft,
3
rd
edition
1993
,
Balkema
.
[
8
]
B
.
Launder,
B
.
Sharma
(
1974
)
.
Application
of
the
energy

dissipation
model
of
turbulence
to
the
calculation
of
flow
near
a
spinning
disc,
Letters
in
Heat
Mass
Transfer,
1
,
131
–
138
.
[
9
]
Gerber,
G
.
,
Diedericks
,
G
.
,
Basson
,
G
.
R
.
(
2011
)
.
Particle
image
velocimetry
measurements
and
numerical
modeling
of
a
saline
density
current
.
Journal
of
Hydraulic
Engineering,
137
(
3
),
333
–
342
.
[
10
]
Huang,
H
.
,
Imran,
J
.
,
Pirmez
,
C
.
(
2005
)
.
Numerical
modeling
of
turbidity
currents
with
a
deforming
bottom
boundary
.
Journal
of
Hydraulic
Engineering,
131
(
4
),
283

293
.
Acknowledgements
This
study
was
funded
by
the
Portuguese
Foundation
for
Science
and
Technology
through
the
project
PTDC/ECM/
099485
/
2008
.
The
first
author
thanks
the
assistance
of
Professor
Moitinho
from
ICIST,
to
all
members
of
the
project
PTDC/ECM/
099485
/
2008
and
to
the
Fluvial
Hydraulics
group
of
CEHIDRO
.
Edgar A. C. Ferreira
1
, Elsa C. T. L. Alves
1
and Rui M. L. Ferreira
2
1
Hydraulics and Environment Department
–
LNEC, Portugal (email: edgaracf@civil.ist.utl.pt; ealves@lnec.pt )
2
CEHIDRO
–
IST
–
TULisbon, Portugal (email: ruif@civil.ist.utl.pt)
2
.
Experimental
Facilities
and
Instrumentation
The
experiments
were
performed
at
LNEC
in
a
16
.
45
m
long,
0
.
30
m
wide
and
0
.
75
m
maximum
height
flume
with
its
bottom
devised
to
simulate
hyperpycnal
turbidity
currents
in
reservoirs
.
During
the
experiments,
longitudinal
velocities
were
measured
in
five
control
sections
with
an
Ultrasound
Velocity
Profiling
(UVP)
system
and
suspended
sediment
concentration
profiles
were
collected
at
two
control
sections
using
syphon
probes
(
Figure
1
)
.
T
he
experimental
conditions
for
the
present
study
case
are
summarized
in
Table
1
.
4
.
Initial
and
Boundary
Conditions
The
numerical
solution
of
the
governing
equations
was
achieved
by
employing
the
Finite
Volume
Method
(Figure
2
)
.
At
the
inflow
section,
a
uniform
streamwise
velocity
distribution
and
a
low
level
of
turbulence
intensity
I
=
0
.
01
(I=
u
rms
/U
with
u
rms
being
the
root

mean
square
of
turbulent
velocity
fluctuations
and
U
the
mean
velocity)
were
imposed
.
At
the
outlet,
in
order
to
maintain
a
constant
domain
water
level
regardless
of
the
upstream
mass
flow
rate,
a
mixed
boundary
condition
was
attempted,
i
.
e
.
,
associated
to
an
orifice
at
atmospheric
pressure
a
significant
portion
of
the
outlet
was
prescribed
as
a
no

slip
smooth
wall
.
The
free
surface
was
modeled
as
a
free
slip
rigid
lid
and
in
the
bottom
and
lateral
walls
of
the
channel,
including
the
obstacle,
the
no

slip
condition
with
a
scalable
wall
function
formulation
was
introduced
.
Furthermore,
the
simulation
was
initialized
with
hydrostatic
conditions
.
Figure 1

Flow evolution of the turbidity current at inlet (top left) and outlet
(top right) and the sampling system below
Table
1

Experimental
conditions
(
h
obst
refers
to
the
obstacle
height,
d
50
to
the
medium
value
of
particles
diameter,
C
s
0
to
the
initial
sediments
concentration,
U
0
to
inlet
velocity
and
H
wat
to
the
initial
ambient
fluid
height)
.
The
downstream
side
of
the
barrier
is
located
8
.
25
meters
from
the
domain
final
section
.
3
.
The
Algebraic
Slip
Model
(ASM)
–
Theoretical
Background
The
numerical
simulation
of
the
turbidity
current
evolution
was
carried
out
using
the
ASM
approach
[
6
]
.
The
ASM
is
a
single

phase
multi

component
simplified
model
which
basically
comprehends
a
mixture
conservation
equation
,
𝜕
𝜌
𝑚
𝜕
+
𝛻
∙
𝜌
𝑚
𝒖
𝑚
=
0
a
momentum
equation,
𝜕𝜌
𝑚
𝒖
𝑚
𝜕𝑡
+
𝛻
∙
𝜌
𝑚
𝒖
𝑚
⊗
𝒖
𝑚
=
−
𝛻𝑝
+
𝛻
∙
𝝉
𝑚
+
𝜌
𝑚
𝒈
and
,
additionally,
a
solids
mass
conservation
equation
𝜕𝜌
𝑚
𝑌
𝑝
𝜕
+
𝛻
∙
𝜌
𝑚
𝑌
𝑝
𝒖
𝑚
+
𝒖
𝑑𝑟𝑖𝑓𝑡
,
𝑝
−
𝜇
+
𝜇
𝑓
𝑡
𝜎
𝑝
𝛻
𝑌
𝑝
=
0
where
:
𝜌
𝑚
=
mixture
density
,
=
time
,
𝒖
𝑚
=
mixture
velocity
vector
,
𝑝
=
pressure
,
𝝉
𝑚
=
stress
tensor
,
𝒈
=
gravity
acceleration
vector,
𝑌
𝑝
=
sediments
mass
fraction,
𝒖
𝑑𝑟𝑖𝑓𝑡
,
𝑝
=
drift
velocity,
𝜇
=
dynamic
viscosity
(
8
.
899
×
10
−
4
𝑘𝑔
𝑚
−
1
−
1
)
,
𝜇
𝑓
𝑡
=
eddy
viscosity
and
𝜎
𝑝
=
Turbulent
Schmidt
number
.
Closure
for
the
Reynolds
stress
tensor
was
computed
through
the
Boussinesq
assumption
whilst
closure
for
the
Reynolds
flux
vector
was
calculated
via
the
flux

gradient
hypothesis
[
7
]
.
In
this
work,
one
made
use
of
a
buoyancy
modified
ƙ

ε
two

equation
model
(where
ƙ
represents
the
turbulent
kinetic
energy
and
ε
denotes
the
dissipation
rate
of
ƙ)
[
6
]
.
With
the
exception
of
buoyancy
turbulence
terms,
modeling
constants
were
adopted
from
Launder
and
Sharma
(
1974
)
proposal
[
8
]
.
Buoyancy
terms
incorporated
in
ƙ
and
ε
equations
were
estimated,
respectively
by
𝑃
ƙb
=
−
𝜇
𝑓
𝑡
𝜌
𝑚
𝜎
𝑝
𝑔
∙
𝛻
𝜌
𝑚
𝑃
εb
=
𝐶
3
∙
max
(
0
,
𝑃
ƙb
)
In
what
concerns
𝜎
𝑝
and
the
dissipation
coefficient
𝐶
3
no
universality
is
foreseeable
.
Quite
on
the
contrary,
several
authors
have
concluded
from
empirical
evidence
that,
on
the
one
hand,
the
eddy
viscosity
and
diffusivity
and
hence
the
turbulent
Schmidt
number
is
related
to
the
level
of
stratification
and
,
on
the
other
hand,
buoyancy
effects
in
the
ε
equation
can
have,
for
particular
cases,
an
irrelevant
role
in
the
turbulence
dynamics
.
Following
the
studies
of
[
9
]
and
[
10
],
in
this
work
it
has
been
assumed
𝜎
𝑝
=
1
.
3
and
𝐶
3
=
0
.
Figure
3

ASM
model
results
at
time
t
=
17
s
(left),
53
s
(center)
and
200
s
(right)
5
.
Results
The
spatio

temporal
turbidity
current
evolution
is
shown
in
Figure
3
.
In
regards
to
the
adequacy
of
the
ASM
approach,
experimental
and
numerical
results
were
likened
(Figures
4
and
5
)
.
Figure
2

Mesh
detail
of
the
CFD
model
in
the
obstacle
´
s
vicinity
Figure
4

Non

dimensional
time

averaged
streamwise
velocities,
where
Z
denot es
vert i cal
coordi nat es,
X
is
the
distance
to
domain
´
s
downstream
section
and
Z
0
.
5
is
an
outer
length
scale
defined
as
the
height
at
which
the
time

averaged
velocity
𝐔
is
equal
to
half
t he
maximum
t i me

averaged
velocity
??
%
max
Figure
5

Variation
of
suspended
sediment
concentration
with
water
depth
.
Experimental
profiles
were
obtained
during
a
brief
sampling
period
whilst
the
numerical
ones
were
acquired
at
t
=
216
s
(x
=
5
.
75
m)
and
at
t
=
186
s
(x=
10
.
25
m)
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