Large-eddy simulation of transport of inert and chemically reactive pollutants over 2D idealized street canyons

Abstract

In view of the worsening air quality in the world, more concerns are focused on the

environment. This thesis uses the technique of CFD and develops the computer model

to investigate the wind and pollutant transport, as well as the chemistry of reactive

pollutants in idealized two-dimensional (2D) street canyons.

Three scientific questions are raised in this thesis. The first task is to find out the po-

sition with the most favorable pollutant removal along the ground level over 2D idealized

street canyon of different building-height-to-street-width (aspect) ratios (ARs). The di-

mensionless parameter, C, represents the pollutant removal performance. In the isolated

roughness regime, the two local maximum C locate at the reattachment point and the

windward corner. In the wake interference regime, C is peaked on the windward side. The

number of vertically aligned recirculations depends on the street depth in the skimming

flow regime. The sizes of the secondary recirculation upstream and downstream deter-

mine how the maximum C shifts from the street centre. After identifying the position of

peaked pollutant removal rate at the ground level, the emission source should be placed

with the highest constant C in order to remove the pollutants upward more quickly to

safeguard the street-level air quality.

After understanding the best pollutant removal in the street canyon of different ARs,

the second task is to find out what AR is the most favorable for the ventilation and

pollutant removal across the roof level. The three parameters, namely friction factor,

air exchange rate (ACH) and pollutant exchange rate (PCH), are introduced to quantify

the pressure difference to sustain the mean flow, the ventilation and pollutant removal,

respectively. The turbulence contributes more than 70% to the total ACH and PCH in

all the three flow regimes. By increasing the atmospheric turbulence in building geometry

as well as the surface roughness, the ventilation and pollutant removal performance can

be improved. The linear relation between the friction factor and ACH demonstrates the

larger resistance that in turn promotes the air exchange over the roof level.

The physical dispersion is studied; however atmospheric pollutants are seldom in-

ert but chemically reactive instead. The last task is to include the three common air

pollutants, NO, NO2 and O3, in the simple NOx ?O3 mechanism in terms of the photo-

stationary state and reaction rates. The Damkohler numbers of NO and O3, DaNO and

DaO3, are parameterized by the concentrations of the sources NO and O3. The normalized

mean and fluctuation NO, NO2 and O3 are separately considered. The integrated pho-

tostationary state (PSS) in the first canyon increases with DaO3 under the same DaNO.

The integrated PSS of the second to the twelveth street canyons are compared with each

case, the monotonic increase in the PSS from the second to twelveth canyon is perceived

in DaNO/DaO3 1, 0.03, 0.02, 0.001 and 0.000333. Further decreases the DaNO/DaO3 to

0.000143, 0.000125, 0.000118, 0.000111 and 0.0001, the PSS is found to be non-linear and

the trough appears in the fourth and fifth canyons.