Revision as of 09:04, 12 May 2023

Pollutant transport between a street canyon and a 3D urban array as a function of wind direction and roof height non-uniformity

Measurement data/results

Results

Figs. 1 and 2 show the mean dimensionless total pollution fluxes through the top (at z/H = 0.6) and side openings of the studied street canyons for the vertical (90°) and oblique (45°) wind directions, respectively. The mean dimensionless total vertical pollution fluxes were calculated for the top opening as

${\frac {\overline {c^{*}w}}{U_{ref}}}={\frac {\overline {\frac {cU_{ref}HLw}{Q}}}{U_{ref}}}={\frac {HL}{Q}}{\overline {cw}}$ , (1)

where c* is the instantaneous dimensionless concentration, c* is the instantaneous concentration in ppm, w is the instantaneous vertical velocity component in m s^-1, U_ref is the reference velocity in m s^-1(here the freestream velocity), H is the reference height in m (here the height of the building of uniform roofs), L is the length of the line source in m, Q is the volumetric flow of ethane from the line source in ml s^-1, and the overbar denotes the time averaging. Similarly, the mean dimensionless total latera pollution fluxes were calculated for the lateral opening as

${\frac {\overline {c^{*}v}}{U_{ref}}}={\frac {\overline {\frac {cU_{ref}HLv}{Q}}}{U_{ref}}}={\frac {HL}{Q}}{\overline {cv}}$ , (2)

where v is the lateral velocity component.

Fig. 1a shows that in the case of the uniform roof, the pollutant is transported up the leeward wall and down the windward wall due to the vertical recirculation in the middle of the street canyon. Near the lateral ends of the street canyon, the pollutant is transported by horizontal recirculation, also known as corner vortex. This type of transport can also be observed at the lateral openings of the street canyon. However, in the case of the uneven roof heights, these vortices are either absent altogether, as in the case of the right uneven street canyon (Fig. 1b), or they are enhanced, as in the case of the left uneven canyon (Fig. 1c). Especially near the right lateral end (seen from the downstream) of the left non-uniform canyon, there is a very strong horizontal recirculation that traps the pollutant also at the windward wall (se y = H in Fig. 1c). It can also be seen from Fig. 1 that both the uniform and left non-uniform street canyons have a higher re-emission (negative total pollutant flux) of the pollutant than the right non-uniform canyon and are therefore more poorly ventilated.

Figure 1: Mean dimensionless total pollution flux fields (coloured contours) through the top (c*w/U_ref in the middle) and lateral (c*v/U_ref, at the sides) of the a) uniform (A1-R) and non-uniform b) right (A2-R) and c) left (A2-L) street canyons for the perpendicular wind direction. The positive and negative values of the fluxes represent the outgoing and incoming flux from/into the canyon, respectively. The grey contour represents the dimensionless height (z/H) of the buildings, where H is the mean urban height.

When the wind blows at a 45° angle, a spiral vortex is created in the street canyons, carrying the pollutant up the leeward wall and down the windward wall (Fig. 2). But the non-uniformity of the roof height also influences the development of this pollutant transport here. The best ventilated street canyon is now the left non-uniform one, since less pollutant is transported into the canyon through the lateral opening (compare the negative pollutant flows for the left openings of the street canyons).