Training on atmospheric dispersion in urban environments

4. Impact of buildings on flow and dispersion

Buildings and other structures disturb the flow of air. The urban airflow is highly complex, governed by the buildings and the speed and direction of the approaching flow. To describe how buildings impact flow in cities, an idealized flow around one or two simple buildings is first considered. In the real city the airflow patterns are more complex due to the presence of many buildings with different shapes and streets in different configurations.

Idealized flow around a single building is shown schematically in figure 4.1. There are three main flow zones:

  1. The displacement zone where the approaching air is deflected around and over the building,
  2. The relatively isolated cavity zone on the leeward side of the building, and
  3.  Downwind from the building, a highly disturbed area called the wake zone.
Flow around a single building

Figure 4.1: Air flow around a single building with the three main flow zones:
displacement zone, cavity zone and wake zone. [1]

The prevailing dispersion pattern depends upon both the release height and the receptor location.
Figure 4.2 shows a plume in the case of a release in the displacement zone of flow where the dispersion can be considered undisturbed by the building.

Flow in a displacment zone

Figure 4.2: Air flow in the displacement zone. [1]

Figure 4.3 shows a plume in the case of release in the wake zone, a region of downward mixing. This effect, known as building downwash, pulls the plume down to ground level and leads to elevated concentrations immediately downwind of the source.

Flow in the wake zone

Figure 4.3: Air flow in the wake zone. [1]

Figure 4.4 shows a plume in the case of release in the cavity zone, a region where the flow is highly turbulent and generally recirculating. The agent released is entrained and can be trapped in the immediate vicinity. There is an increase of vertical dispersion, enhancing the initial growth of the plume and the concentrations at ground-level.

Flow in a displacment zone

Figure 4.4: Air flow in the cavity zone. [1]

To understand the impact of buildings on flow, simple flows in idealized urban canyons have been extensively studied in wind-tunnel experiments, providing useful guidance. Figure 4.5 depicts a well- known configuration of flow over a regular array of identical buildings, in a cross-wind.

2D Flow regimes as function of width-to-height ratio

  Figure 4.5: 2D Flow regimes as function of width-to-height ratio [from Oke 1987]:

The nature of the flow is mainly determined by the ratio of the width between buildings (w) to the building height (h). For the case of long buildings, which can be considered as two-dimensional case, three flow regimes have been identified: isolated roughness regime, wake interference regime and skimming regime. More details on those regimes.

When the flow is three-dimensional, the effects of vertical side edges of the buildings are important. Figure 4.6 depicts an idealized airflow around an isolated building; showing recirculation vortices from the top and the lateral shear layers. In this case, the aspect ratio (w/h) is large enough that the flow reattaches to the building at the top and side.
Figure 4.7 relates the three-dimensional flow in the urban canyon, in the case of buildings with unequal heights, which is dependant on the ratio h1/h2. A Description of the complex flow is found here: urban-canyon flow characteristics.

Three dimensional flow around building

Figure 4.6: 3D airflow patterns in case of an isolated building, from Hosker 1984 [5].

3D flow in urban-canyon. In the case of unequal building heights

Figure 4.7: 3D flow in urban-canyon, from Britter and Hunt 1979 [6].

The following section describes atmospheric dispersion models in general.

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