2.11 Climatic Urban Design Strategies
2.11.2 Wind Effect
89 The researcher compared two locations, traditional and contemporary areas, in Greece. The researcher found that the compacted traditional pattern provides more desired outdoor shading comparing with the contemporary area (Figure 2.48). Furthermore, the author illustrated that street pattern, buildings configuration and latitude are some of the factors that contribute to the canyon solar access.
Figure 2.48: The shading effect on traditional area (up) and contemporary (down) in summer and winter (Andreou, 2014)
90 The airflow behaviour in the canyon can be studied through the effect of the two layers of the urban space. The Urban Boundary Layer (UBL), which is above the buildings height, and the Urban Canopy Layer (UCL), which represents the space within the canyon between the buildings. The UCL represents the pedestrian level of the canyon and it is isolated or blocked by the above UBL (Oke, 1988). The air flow is slow in UCL by the effect of the obstruction objects with the canyon, such as buildings, trees, transportation vehicles.
On the other side, the air flow with the UBL layer is stronger and it is affected by the canyon orientation and the H/W ratio (Oke, 1988). The orientation of the canyon is a very important element in climatic urban design. The wind effect can be adopted to reduce the canyon air temperature and UHI consequently. Ali-Toudert (2006) proved that a canyon extending along a N-S direction is more efficient in releasing heat comparing with the E-W direction. Cao, Li and Meng(2015) explored the effect of the canyon orientation on the air velocity and UHI.
The researcher studied eight angles of canyon orientation in Guangzhou, China. The researchers concluded that for a specific location on the earth, a canyon should be designed with a specific angle to increase the wind speed (Figure 2.49). The increase in wind speed
will consequently reduce the average temperature and the UHI effect in the canyon (Figure 2.50). Furthermore, the H/W ratio plays a significant role in forming the air flow
within the canyon. The airflow within the canyon can be categorised into three types:
1) isolated roughness flow, 2) the weak influence flow, and 3) the skimming flow (Oke, 1988). The canyon geometry and the H/W ratio are the influencing factors in transition
between these types of flow (Figure 2.51).
91 Figure 2.49:The relationship between the air velocity and canyon angles (Cao, Li and Meng, 2015)
Figure 2.50: The percentage of urban heat island UHI intensity and canyon angles (Cao, Li and Meng, 2015)
In the uniform or shallow canyon, the isolated flow takes place between the buildings.
Increasing the H/W ratio will cause a vortex circulation in the space between the buildings, and the airflow will be slower in the deep canyon. Further increase in H/W ratio creates more vortex and leads to the skimming flow (Figure 2.51).
In the same context, Priyadarsini and Wong (2005) proved the effect of building height diversity on the canyon microclimate, particularly, on airflow and wind speed parameters.
92 The researchers found that placing any block of high-rise buildings within mid-rise buildings will increase and enhance the air movement and velocity within the canyon. They also found that this change in air movement will enhance the air temperature within the canyon, and by placing more than one high-rise building the wind velocity can be increased by 90 % and the air temperature can be decreased by 1°C (Figure 2.52). A separate research was conducted to identify methods to enhance the street or canyon sustainability conducted by (Chan et al., 2001).The researchers proved that the variation in building heights can provide better ventilation, and can enhance the urban microclimate, but they recommended that the H/W ratio should not exceed five in order to keep the microclimate within preferable levels.
Figure 2.51: The effect of canyon H/W ratio and the three types of air flow within the UBL (Oke, 1988)
93 Furthermore, Yang, Qian and Lau (2013) studied the urban form effect on wind velocity and the canyon ventilation in Shanghai, China. The researchers conducted their study on ten high-rise residential areas and concluded that increasing the SVF by 10 % would increase wind velocity by 7% - 8%. Moreover, the researchers stated the importance of a diverse wind instead of a uniform wind for people’s outdoor comfort and activities.
Figure 2.52: The effect of high-rise block within midrise blocks on enhancing air movement (Priyadarsini and Wong, 2005)
The buildings configuration has been explored in a number of studies in order to find the effect of buildings arrangement on wind speed and airflow behaviour in the canyon.
Santamouris (1999) found that arranging buildings in a grid configuration and creating parallel, strait canyons, will promote the airflow within the canyon (Figure 2.53).
94 Moreover, the researches claimed that rearranging the buildings with a winding configuration is suitable for hot or cold stressful climates. This configuration decreases the speed of the wind and reduces the air velocity within the canyon (Figure 2.53).
Figure 2.53: Airflow in the parallel and strait buildings configuration (Santamouris, 1999) However, the prevailing wind can be adopted to enhance the thermal performance of the built environment on both the outdoor and indoor levels. The effect of canyon geometry and buildings configuration on potential of wind speed requires more investigation. Recently, the progress in simulation software provides a good opportunity for this type of researchers.
2.12 Summary of the Previous literatures on Urban Geometry and Gap Identification