Investigation of wall-bounded turbulence over sparsely distributed roughness

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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

ected by the
surface condition, at su

cient distance from the wall. Similarity only holds if
the Reynolds Number is su

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
.