Modelling Pollution Dispersion in Urban Areas

Silvana Di Sabatino

Universita’ di Lecce, Dipartimento Scienza dei Materiali, Via Arnesano, 73100 Lecce (I)

University of Cambridge, Engineering Department, Trumpington Street, Cambrige, (UK)

INCREASING URBANISATION

• MOBILITY

• WIND ENVIRONMENT / LOADING

/ PEDESTRIAN / COMFORT

• URBAN CLIMATOLOGY

• URBAN AIR QUALITY

• INDOOR AIR QUALITY/VENTILATION

• ENERGY USAGE

• WEATHER MODELLING

• HAZARDOUS RELEASES

SPATIAL SCALE OF THE PROBLEM

REGIONAL up to 200km-larger

CITY up to 20km

NEIGHBOURHOOD up to 2km

STREET (CANYON) up to 0.2km

POLLUTANT SOURCE e.g. TAILPIPE

0.0001km

WE SEEK GENERALISED SOURCE-RECEPTOR

RELATIONSHIPS IN THE FORM OF MODELS

STREET (CANYON) SCALE

• WHERE THE PEOPLE AND (THE EMISSIONS)

ARE

• DIRECT CFD/LES IS PRACTICABLE

• OPERATIONAL MODELLING IS TYPICALLY

BASED ON A MORE IDEALISED

RECIRCULATING VORTEX DRIVEN BY A

SHEAR LAYER

• EXCHANGE BETWEEN THE CANYON AND

FLOW ABOVE IS IMPORTANT DIFFICULTIES

IN CRITICAL SITUATIONS WHEN THE WIND

SPEED IS LOW; TRAFFIC PRODUCED

TURBULENCE BECOMES IMPORTANT

-0 .7 5 -0 .5 0 -0 .2 5 0 .0 0 0 .2 5 0 .5 0 0 .7 5

x / H

0 .0

0 .5

1 .0

1 .5

z/H

-0 .7 5 -0 .5 0 -0 .2 5 0 .0 0 0 .2 5 0 .5 0 0 .7 5

x / H

0 .0

0 .5

1 .0

1 .5

z/H

2222 5.05.0 twtvtuttTKE

dzdydxV t

tmt 1

22 2

1 hvCvNP D

Nnumber of vehicles producing turbulence (dimensionless);

DCaverage drag coefficient of the vehicles;

v vehicle speed;

h geometrical length scale of the vehicles (e.g. Awith A = frontal area of

the vehicle; 2h must be the area used in defining the drag coefficients)

fluid density

TRAFFIC-PRODUCED TURBULENCE MODEL

The production of turbulent kinetic energy per unit mass tP is

t

D

t

D

t V

hvCN

V

hvCvNP

2322

2

1

2

1

.

lcmt3

1,

The general form for the dissipation of turbulent kinetic energy per unit mass is given by

l length scale used to model the dissipation of turbulent kinetic energy; i.e.

the dissipation length scale, and

1c dimensionless constant.

t

Dmt V

vlhCNc

32

23

where 112 )2( cc .

Light traffic density - no flow interaction among the vehicles

1N3hVt hl2 2/3 2

3wt Dc C v

cctwwt VVN 22

3hVw cc SLV

HWSc where is the cross-section area in the canyon in which TPT is active

describes turbulence averaged over the traffic layer

describes turbulence averaged over the whole canyon

hWSc

HWSc

the number of vehicles can be expressed as 

vv nLLLN /

 

cDvct S

hvCnc

323/2

42

Intermediate traffic density - interaction between the vehicle wakes

vnLN

cct SLVV

hl

3/2

223/2

52 )(

cDvct S

hvCnc

Large traffic density – strong flow interaction among the vehicles

vv nLl /1

3/2

3423/2

62

cDct S

hvCc

Comparison with Experimental Data

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

y / H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

z/H

-0 .25

-0 .2

-0 .15

-0 .1

-0 .05

0

0.05

0.1

0.15

0.2

0.25

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

y / H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

z/H

-0 .02

-0 .016

-0 .012

-0 .008

-0 .004

0

0.004

0.008

0.012

0.016

0.02

m/s12tv -1m20vn

20-60 km/h Full scale

222 5.0 vuw

222 )/()/(2/3/ tvtutt vvv

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5

y / H

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

z/H

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

019.0/ 2 tct v

252 m105 plateAh 23 m106.325.0 WHSc

53/2

23/2

5

2

018.0)( cS

hCnc

v cDv

t

ct

Estimated normalised TKEt scale in the central plane of an idealised

street canyon without external wind-driven flow.

Conclusions

• Traffic produced turbulence (TPT) is important for estimation of pollution concentration levels in streets;

• A theoretical framework for parameterisation of turbulent transport by traffic induced air motions in street canyons is established;

• The analysis distinguishes between three traffic flow conditions: (i) light traffic conditions (isolated vehicles, non-interacting vehicle wakes); (ii) moderate traffic conditions corresponding to non-isolated (interacting) vehicle wakes; and (iii) heavy (congested) traffic conditions characterised by strongly interacting wakes;

• The analysed experimental data give indications of different scaling laws for TPT and resulting concentration fields under different traffic conditions.