99thth INTERNATIONAL CFD INTERNATIONAL CFD CONFERENCE CONFERENCE

23-27 September 2002.23-27 September 2002.

Institute of Power TechnologyInstitute of Power Technology

MOSCOW, RussiaMOSCOW, Russia

NUMERICAL PREDICTION OF DISPERSION NUMERICAL PREDICTION OF DISPERSION CHARACTERISTICS OF AIR POLLUTANTS CHARACTERISTICS OF AIR POLLUTANTS IN IDEALISED URBAN STREET CANYONSIN IDEALISED URBAN STREET CANYONS

DEJAN MUMOVIC and JOHN M. CROWTHERDEJAN MUMOVIC and JOHN M. CROWTHER

BACKGROUNDBACKGROUND

Part IV of the Environment Act 1995 led to the Air Part IV of the Environment Act 1995 led to the Air Quality Strategy for England, Scotland, Wales and Quality Strategy for England, Scotland, Wales and Northern Ireland, (AQS), is revised and published in Northern Ireland, (AQS), is revised and published in January 2000. The Strategy sets standards and January 2000. The Strategy sets standards and objectives for the main air pollutants of concern. objectives for the main air pollutants of concern.

Where one or more of the prescribed air quality Where one or more of the prescribed air quality objectives is not likely to be met by set dates objectives is not likely to be met by set dates between 2004 and 2009, Local Authorities must between 2004 and 2009, Local Authorities must declare an Air Quality Management Area (AQMA) declare an Air Quality Management Area (AQMA)

According to the latest report on the state of air quality, According to the latest report on the state of air quality, Glasgow, as the biggest city in the Scotland, has areas at risk of Glasgow, as the biggest city in the Scotland, has areas at risk of

exceeding existing or proposed new air quality objectivesexceeding existing or proposed new air quality objectives..

INTRODUCTIONINTRODUCTION

The governing equation set is Reynolds-averaged in The governing equation set is Reynolds-averaged in Cartesian coordinates and includes the horizontal Cartesian coordinates and includes the horizontal momentum equation, the vertical momentum equation, momentum equation, the vertical momentum equation, the mass continuity equation, and the transport the mass continuity equation, and the transport equation for pollutant concentration. equation for pollutant concentration.

The aim of the present study is to develop a three-The aim of the present study is to develop a three-dimensional numerical model based on the Reynolds dimensional numerical model based on the Reynolds averaged Navier-Stokes equations, coupled with the averaged Navier-Stokes equations, coupled with the following turbulence models:following turbulence models:Standard k-Standard k- turbulence model turbulence modelRNG turbulence modelRNG turbulence modelChen-Kim k-ε turbulence modelChen-Kim k-ε turbulence modelTwo scale turbulence modelTwo scale turbulence model

SETTINGSSETTINGS

The figure shows the The figure shows the computational street computational street canyon configuration of canyon configuration of Hope Street in Glasgow, Hope Street in Glasgow, for for B/H=B/H=1, where 1, where BB is the is the street canyon width, and street canyon width, and is equal to is equal to BB= 20m; = 20m; HH is is the building height, and the building height, and is determined as is determined as HH= = 20m. The length of the 20m. The length of the street is street is L= L= 180 m. 180 m.

RESULTS:RESULTS:

Both the experimental Both the experimental and calculated results and calculated results show that the magnitude show that the magnitude of pollutant of pollutant concentrations on the concentrations on the leeward side of the up leeward side of the up wind building is much wind building is much larger, up to four times larger, up to four times than the windward side than the windward side of the down wind of the down wind building. building.

It can be noted that for all cases, It can be noted that for all cases, a vortex was formed within the a vortex was formed within the street canyon, characterised by street canyon, characterised by updrafts near the upwind updrafts near the upwind buildings and downdrafts near buildings and downdrafts near the downwind buildings.the downwind buildings.

Table 1: Time consumed for numerical calculation under four different turbulence models and two different grids

Hours:minutes

Standard k-ε

RNG k-ε Chen-Kim k-ε Two scale

Coarse grid

0:22 0:23 0:24 0:52

Refined grid

0:49 0:51 0:53 1:50

The figure shows, that on the leeward side, the pollutant The figure shows, that on the leeward side, the pollutant concentrations decrease exponentially from the street concentrations decrease exponentially from the street surface to the roof of the up wind building for all tested surface to the roof of the up wind building for all tested

turbulence models.turbulence models.

0

5

10

15

20

25

0.000015 0.000020 0.000025 0.000030 0.000035 0.000040

Standard k- model RNG k- model Kim-Chen k- model Two-scale model

Concentration, c(kg/kg)

He

igh

t, Y

(m)

The figure shows the horizontal wind velocity at the The figure shows the horizontal wind velocity at the leeward face of the upwind building predicted by the leeward face of the upwind building predicted by the developed model coupled with the four different developed model coupled with the four different turbulence models, and compared with the experimental turbulence models, and compared with the experimental

data.data.

0

5

10

15

20

25

-0.05 -0.04 -0.03 -0.02 -0.01 0.00 0.01

Standard k- model RNG k- model Chen-Kim k- model Two scale model

Horizontal Velocity, w1(m/s)

Hei

ght,

Y(m

)

Dependency of the dispersion Dependency of the dispersion of the pollutants on the aspect of the pollutants on the aspect ratio for the chosen RNG k-ε ratio for the chosen RNG k-ε model is given in the figure. model is given in the figure. The air flow characteristics are The air flow characteristics are compared with previous compared with previous research findings (Hassan and research findings (Hassan and Crowther, 1999) and highly Crowther, 1999) and highly satisfactory agreement is satisfactory agreement is

obtained.obtained.

0

5

10

15

20

25

0 10 20 30 40

Aspect ratio: H/W=2 H/W=1 H/W=0.5

Concentration, c[ppm]

Hei

ght,

Y[m

]

The set of tests conducted to examine the influence of grid The set of tests conducted to examine the influence of grid refinement on the dispersion of the pollutants shows that refinement on the dispersion of the pollutants shows that working domain is fairly scaled if 40 to 60 cm is represented working domain is fairly scaled if 40 to 60 cm is represented by single cell near pavementby single cell near pavement..

0

5

10

15

20

25

0.000000 0.000005 0.000010 0.000015 0.000020 0.000025 0.000030 0.000035 0.000040

RNG k- refined model RNG k- coarse model

Concentration, c(kg/kg)

Hei

ght,

Y(m

)

The sample intake was situated 2.5 m above pavement The sample intake was situated 2.5 m above pavement and 0.5 m from the kerb. When the wind was from the and 0.5 m from the kerb. When the wind was from the east (90º) the sampler was on the leeward face. east (90º) the sampler was on the leeward face.

The figure shows the comparison between the The figure shows the comparison between the predicted and measured CO concentrations at the predicted and measured CO concentrations at the leeward and the windward faces.leeward and the windward faces.

0 2 4 6 8 10 12 140

2

4

6

8

10

12

14

16

18

20

Pre

dict

ed C

once

ntra

tion

[ppm

]

Measured concentration [ppm]

The reasonable agreement between the calculated and The reasonable agreement between the calculated and observed concentrations, proves that PHOENICS, as a observed concentrations, proves that PHOENICS, as a general CFD code, is capable to predict the pollutant general CFD code, is capable to predict the pollutant distributions emitted by vehicle within an urban street distributions emitted by vehicle within an urban street canyon.canyon.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

D. Mumovic wishes to thank to The Lord Provost D. Mumovic wishes to thank to The Lord Provost of Glasgow, Mr. Alex Mosson, for a warm of Glasgow, Mr. Alex Mosson, for a warm

welcome and encouraging discussions. The welcome and encouraging discussions. The authors wish to thank to Mr. Peter Spalding for authors wish to thank to Mr. Peter Spalding for his overwhelming help, without whom this study his overwhelming help, without whom this study

would be impossible.would be impossible.

99thth INTERNATIONAL INTERNATIONAL CFD CONFERENCECFD CONFERENCE

23-27 September 2002.23-27 September 2002.Institute of Power TechnologyInstitute of Power TechnologyMOSCOW, RussiaMOSCOW, Russia