Investigation of wall

bounded turbulence
over sparsely distributed roughness
M. Placidi, B. Ganapathisubramani and M. Tan
Faculty of Engineering and the Environment
mp1r10@soton.ac.uk
Fluid Structure
Interactions (FSI)
Research Group
Acknowledgement
:
This
project
is
supported
by
Lloyd
Registered
Educational
Trust
FSI Away Day 2012
Background
•
Sparse
distribution
of
surface
roughness
is
found
in
abundance
in
natural
environments
and
in
a
variety
of
engineering
applications
.
•
Barnacle
growth
on
ship’s
hull
(Figures
1
and
2
)
and
Atmospheric
Boundary
Layer
(Figure
3
)
are
classical
examples
.
References
:
•
Jimenez,
J
.
2004
Turbulent
flows
over
rough
walls
.
Annual
Review
of
Fluid
Mechanics
36
(
1
),
173
–
196
.
•
Townsend,
A
.
A
.
1976
The
structure
of
turbulent
shear
flow
;
•
Krogstad,
P
.
A
.
&
Antonia,
R
.
1999
Surface
roughness
effects
in
turbulent
boundary
layers
.
Experiments
in
Fluids
27
,
450
–
460
,
10
.
1007
.
•
Ganapathisubramani,
B
.
&
Shultz,
M
.
P
.
2011
Turbulent
boundary
layer
structure
over
sparsely
distributed
roughness
pp
.
1
–
6
.
•
Volino,
R
.
J
.
,
Schultz,
M
.
P
.
&
Flack,
K
.
A
.
2007
Turbulence
structure
in
rough
and
smooth
wall
boundary
layers
.
Experiments
in
Fluids
592
,
1
–
31
.
•
Grimmond,
C
.
S
.
B
.
&
Oke,
T
.
R
.
1998
Aerodynamic
properties
of
urban
areas
derived,
from
analysis
of
surface
form
.
Journal
of
Applied
Meteorology
38
(
9
),
1262
–
1292
.
Future
Work
•
Analyzing
the
effect
of
u
rban

like
roughness
on
the
turbulence
structure
;
•
Considering
the
problem
of
s
tep
change
in
roughness
;
•
Studying
the
effect
of
roughness
on
p
ollutant
dispersion
.
Experimental
Facilities
and
Method
3x2 Wind tunnel
•
0.9 x 1.35 x 4.5 m long test section
•
Velocity range = 0.1
–
25 m/s
•
Slightly favorable pressure gradient
•
PIV “friendly”
Lego® Elements
•
cylindrical and rectangular 2x2 bricks
•
Staggered arrays
Aims
and
Objectives
•
Systematic
study
of
the
effect
of
Frontal
and
Plan
solidity
on
the
structure
of
the
turbulence
;
•
Evaluate
the
importance
and
the
effects
of
additional
parameters
on
the
turbulence
structure
;
•
Investigate
the
validity
of
Townsend’s
similarity
Hypothesis
in
the
sparse
regime
.
Regular
roughness
can
be
characterized
using
two
parameters
:
1.
Frontal Solidity λ
F
=A
F
/A
T
(which is the ratio of the total projected
frontal area of the roughness per unit wall parallel area);
2.
Plan Solidity λ
P
=A
P
/A
T
(which is the ratio of the total plan area of the
roughness per unit wall parallel area).
Roughness
Characterization
•
Jimenez
(
2004
)
pointed
out
that
while
boundary
layers
over
surfaces
with
a
dense
distribution
of
roughness
elements
have
been
the
object
of
numerous
studies,
the
effects
of
sparse
roughness
has
been
poorly
investigated
(Figure
4
)
.
Figure
1
:
Accumulation
of
marine
growth
(barnacles)
on
the
ship’s
hull
.
©
2009
Igor
Sterzhantov
.
Figure 2: Maintenance of ship
hull in situ.
ShipHullPerformance.org.
Figure 3: Urban boundary layers and turbulence in
the urban canopy. Princeton University.
Figure
4
:
Equivalent
sand
roughness
for
various
surfaces
versus
the
frontal
solidity
.
Jimenez
(
2004
)
.
Figure 5: Frontal and Plan Solidity.
Grimmond and Oke (1998).
Figure
6
:
Experimental
set
up
Experimental
Technique
Particle
Image
Velocimetry
(PIV)
•
PIV
is
based
on
the
measurement
of
the
displacement
of
small
tracer
particles,
that
are
released
into
the
fluid
;
•
The
tracer
particles
are
illuminated
within
a
thin
light
sheet
generated
from
a
pulsed
light
source
;
•
the
light
scattered
by
the
particles
is
recorded
onto
two
subsequent
image
frames
by
a
CCD
camera
;
•
Cross

correlating
the
particle

image
patterns
in
small
interrogation
windows,
between
the
first
and
second
image
frame,
allows
the
evaluation
of
the
displacement
hence,
the
local
fluid
velocity
.
Figure
7
:
Schematic
of
a
typical
PIV
measurements
system
.
Scarano
2010
Townsend’s
Similarity
Hypothesis
•
Townsend (1976)
stated that the turbulence structure is una
ﬀ
ected by the
surface condition, at su
ﬃ
cient distance from the wall. Similarity only holds if
the Reynolds Number is su
ﬃ
ciently high and if the mean height of the
roughness elements is small, compared to the boundary layer thickness. Our
understanding of rough walls heavily relies on this similarity.
•
Krogstad
&
Antonia
1999
,
Volino
et
al
.
2007
and
Ganapathisubramani
&
Schultz
2011
have
shown
evidence
of
lack
of
similarity
for
rough
walls
characterized
by
sparse
roughness
.
Figure
8
:
Classical
PIV
image
.
Streamwise
(x,y)
plane
.
Colorbar
shows
mean
streamwise
velocity
U
.
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