1994). The atmospheric flow in inner urban canopies is complex and its study is
applied mainly to specific situations which are not directly generalized (Arya 1988).
Most of the studies carried out to date are done in a wind tunnel using models of
individual buildings including prototypes.
The areas of accelerated flow correspond to higher mean wind velocities.
Separation zones correspond to lower mean velocity zones and greater turbulence,
and the flow in the center of the vortices is intense, strongly turbulent, and periodic
in nature. As an example of a practical application, above mentioned, the localization of wind turbines requires information in relation to areas where accelerated
flow predominates or where wind velocity is not significantly diminished. In this
context, street and buildings microenvironments are also important for the safety
and well-being of inhabitants.
The pattern of airflow over a rectangular isolated building with a flat roof
positioned perpendicular to the wind (with its upwind side at 90° to the flow) has
four zones namely, background, displacement, cavity, and wake (Fig. 5.3). In the
displacement zones, flow over an edge at the back of an obstacle leads to a wake
flow in an opposite direction.
The displacement separation on an obstacle edge (e.g., a building) is stationary,
independent of the Reynolds number (for Re > 10
4 ), and occurring at a fixed
location, thereby facilitating experimental modeling studies in terms of scaling the
obstacles that make up the urban canopy (Arya 1988). The displacement zone
(Fig. 5.3) corresponding to an unperturbed flow, wraps around the building, the
wake, and the cavity region. The streamlines in this zone are diverted upwind and
laterally, wrapping around the developing wake (Arya 1988). After passing the
building, the separation zone follows a downward direction, attenuates, and disappears downstream.
After contact with the building wall, the streamlines flow upwards to the top,
downward, or laterally around the building (Fig. 5.4). The maximum pressure
occurs in the central area of the windward wall, where the air velocity reaches a
stagnation point, and the pressure decreases according to Bernoulli's equation
(Annex A2). If the building is rectangular, there is flow separation at the top and
sides. These areas are subject to suction and flow reversal, causing recirculation of
Wake boundary
Cavity boundary
Downwind stagnation
point
Cavity
Background
flow
Wake
Fig. 5.3 Schematic of flow areas around a two-dimensional building with sharp edges (after Arya
1988)
5.4 Flow in Urban Areas
139
applied mainly to specific situations which are not directly generalized (Arya 1988).
Most of the studies carried out to date are done in a wind tunnel using models of
individual buildings including prototypes.
The areas of accelerated flow correspond to higher mean wind velocities.
Separation zones correspond to lower mean velocity zones and greater turbulence,
and the flow in the center of the vortices is intense, strongly turbulent, and periodic
in nature. As an example of a practical application, above mentioned, the localization of wind turbines requires information in relation to areas where accelerated
flow predominates or where wind velocity is not significantly diminished. In this
context, street and buildings microenvironments are also important for the safety
and well-being of inhabitants.
The pattern of airflow over a rectangular isolated building with a flat roof
positioned perpendicular to the wind (with its upwind side at 90° to the flow) has
four zones namely, background, displacement, cavity, and wake (Fig. 5.3). In the
displacement zones, flow over an edge at the back of an obstacle leads to a wake
flow in an opposite direction.
The displacement separation on an obstacle edge (e.g., a building) is stationary,
independent of the Reynolds number (for Re > 10
4 ), and occurring at a fixed
location, thereby facilitating experimental modeling studies in terms of scaling the
obstacles that make up the urban canopy (Arya 1988). The displacement zone
(Fig. 5.3) corresponding to an unperturbed flow, wraps around the building, the
wake, and the cavity region. The streamlines in this zone are diverted upwind and
laterally, wrapping around the developing wake (Arya 1988). After passing the
building, the separation zone follows a downward direction, attenuates, and disappears downstream.
After contact with the building wall, the streamlines flow upwards to the top,
downward, or laterally around the building (Fig. 5.4). The maximum pressure
occurs in the central area of the windward wall, where the air velocity reaches a
stagnation point, and the pressure decreases according to Bernoulli's equation
(Annex A2). If the building is rectangular, there is flow separation at the top and
sides. These areas are subject to suction and flow reversal, causing recirculation of
Wake boundary
Cavity boundary
Downwind stagnation
point
Cavity
Background
flow
Wake
Fig. 5.3 Schematic of flow areas around a two-dimensional building with sharp edges (after Arya
1988)
5.4 Flow in Urban Areas
139