Microclimate and outdoor thermal comfort are highly affected by urban geometry and buildings configuration, and the microclimate around a building is a significant factor in building energy performance, it represents the exchanging relationship between one building and its neighbouring buildings or a building and the surrounding natural environment.
A literature review as a method of study is conducted to identify the recent and previous studies related to the urban geometry and its effect on the built environment. The effect of urban geometry and design on the built environment can be evaluated through a number of micro climate parameters. Many studies have examined air temperature as an environmental parameter that express the variation in urban design variables, such as building configuration, building external material and street surface albedo. The most related studies and publications
on urban planning and design shows that building configuration, canyon H/W, building / canyon orientation and materials are the most urban parameters relevant and
51 responsible for the microclimatic changes around a building (Todhunter, 1990; Arnfield and Mills, 1994).
These parameters directly affect the, solar access, air temperature, airflow and speed at the pedestrian level, and consequently the urban microclimate (Nakamura and Oke, 1988;
Arnfield and Mills, 1994). Nazarian and Kleissl (2015) studied two of the urban design variables: surface material and building geometrical design, the researchers used the 'ANSYS/FLUENT 14.5' software for adopting 'Computational Fluid Dynamics (CFD)' simulation in order to find out the effect of these two design factors on urban street environment. The researchers examined the effect of ground surface materials, including concrete and asphalt, on a number of environmental parameters. For this aim, the researchers developed a model for urban configuration to be simulated; the model has a fixed height and length blocks dimensions equals to (D=1) and used it to estimate the outdoor comfort in the street ‘canyon’. The authors used the weather condition of Southern California on a clear summer day and validated the results against the actual field measurements of air temperature and wind speed (Figure 2.17).
Figure 2.17: The 3D developed model and the solar heat flux in W/m² (Nazarian and Kleissl, 2015)
52 The researchers explored the effect of urban geometry by simulating four scenarios with respect to H/W or aspect ratio in order to find the effect of urban density on the mentioned outdoor environmental parameters of air temperature and wind speed. The researchers claimed that a number of studies adopted the ANSYS/FLUENT (CFD) to simulate the turbulence of air flow on the street scale but these studies did not investigate the effect of three dimensional mass on air flow movement, and the changes in air turbulence that could be caused by constructing this mass or development. The researchers examined the effect of four simulated cases of H/W aspect ratio on the alter configuration and the movement of air flow of each case, and they illustrated the variation in diurnal temperature for the building’s outdoor wall and ground according to air movement variation. The researchers concluded that a high H/W ratio causes a small vortex and decreases the wind velocity which reduces the buildings adjacent ground, wall and roof temperature, and this reduction in air temperature is varied according to ground and wall materials. Furthermore, many studies explored the thermal performance of the built environment and the effect of urban developments on urban spatial environment.
A recent publication that studied the effect of urban geometry factors on the environmental parameters of the built environment is a study published by Andreou (2014).The researcher presented the urban geometry effect in the Mediterranean area. The researcher studied the effect of some urban parameters, such as urban geometry, street pattern and orientation on solar access and shading. The author compared between two locations, traditional and contemporary areas in Greece, and selected the canyon to study the effect of urban variables.
This included street orientation and building H/W ratio and analysed the effect of these factors on the buildings’ solar gains.
53 The two selected sites in Greece represent a contemporary and traditional pattern in the city, the researcher conducted a comparison between these two sites, both sites have the same street axis orientation but they are varied in H/W ratio. The traditional site has a higher H/W ratio which is between 4 and 2, compared with 0.9 to 0.7 in the contemporary site, and both sites have the same latitude of 37ᵒN. Andreou (2014) also identified the weather of this area as high solar intensity weather where shading is desired during summer while a limited level of solar access and solar gain is preferred in winter. The Autodesk Ecotect software was used in this study for case study simulation, and the researcher justified the use of this software as it is successful in simulating the shading and solar access on an urban level. Further to the shading assessment and analysis that were obtained from the use of the software, the results were compared to the thermal field measurement data collected by the researcher. The simulation was conducted according to four directions of street orientation further to the variation in H/W ratio varied between high and low ratio in the two selected sites. The author’s results showed that in summer, the traditional configuration with a high H/W ratio and dense pattern performed more positively than the contemporary one with the respect of environmental comfort. Figure 2.18 shows the comparison of shading percentages between the two H/W ratio; a higher H/W of 3.0 provides more shading percentages than the lower one at 0.6 (Andreou, 2014). Furthermore, the researcher studied the effect of trees on the solar access and shading on outdoor space, and proved a significant effect of this urban variable on the urban microclimate. The researcher stated that the finding of this study is important as it provides a formulation guideline for urban designers in the same region and the same weather characteristics.
54 Figure 2.18: Shading percentages of the horizontal surface of streets on N-S, E-W, NE-SW and
NW-SE axis, with H/W ratio of 0.6 (left), and 3.0 (right) (Andreou, 2014)
Djukic, Vukmirovic and Stankovic (2015) analysed and compared two suggested proposals for developing the central area of Leskovac, Serbia. The researchers evaluated the two proposals against the existing design. They conducted a computer software simulation on four different times on one summer day. Simulating the existing case shows that there is a significant deference in microclimate parameters between the vegetated and non-vegetated areas, and the difference between these areas reached 3ᵒC in air temperature. Furthermore, the area covered with trees is less in air temperature by 0.5ᵒC -0.6ᵒC than the area covered with grass. Simulating the suggested scenarios shows that the difference in air temperature between these two scenarios was less than 0.5ᵒC. Therefore, the researchers suggested a
‘Solution scenario’ to develop and enhance the suggested proposals (Figure 2.19). The
‘Solution scenario’ kept the diagonal street of the existing case and more vegetation were suggested along the northwest side of the park, further to a suggestion of paving the main part of the square area with reflective material instead of asphalt, adding water features, covering the rest of the area with grass and increase the number of trees.
55
Figure 2.19: Air temperature simulation of existing case study (Djukic, Vukmirovic and Stankovic, 2015)
The study compare the air temperature collected from the five receptor points as field measurements with the simulation results of the solution scenario, and the results show that the average temperature reduction between the existing case and the solution scenario varied between 0.3-1.5ᵒC during the four simulation periods of the selected simulation day.
These findings highlight the significant effect of vegetation, paving or cladding material, and water features on urban microclimate. In another study, the H/W ratio of urban morphology was adopted by Schulte, Tan and Venkatram (2015) to develop a model for reducing the emissions caused by transportation, the researchers examined the effect of the relationship between building heights and street widths to develop their model using the comparison between two cities; Hanover, Germany and Los Angeles, USA to find how the dense pattern can reduce the emissions caused by transportation. On the other hand, the effect of the urban canyon geometry on the airflow and microclimate in Morocco was explored through a study carried out by Johansson (2006).
56 The researcher adopted the field measurements data over 1.5 years and selected both types of canyon as a case study area; the deep and the shallow canyon with an aspect ratio of 9.7 and 0.6, respectively. The researcher concluded that wind speed, as one of the microclimate parameters, is around 0.4 m/s in the deep canyon which is considered more stable and slower when compared with the wind in a shallow canyon of 0.7 m/s. The research found that the deep canyons in Morocco provide more preferable microclimates in the two main seasons, summer and winter. On the other hand, a large number of studies adopted and developed models to evaluate the effect of developments on the surrounding environment, and the surface albedo temperature is the most evaluated parameter in most of these studies. Yang and Li (2015) used a three-dimensional numerical models to study the urban surface temperature and the role of the horizontal surfaces in urban albedo averages, the researchers used the SVF as an alternative to the H/W ratio to represent the urban geometry configurations. The researchers found a linear relationship between the SVF of horizontal surfaces and urban albedo averages (Figure 2.20). Aida (1982) was one of the first researchers who studied the effect of the urban geometry on the surface solar absorption using the experimental method, the researcher adopted concrete blocks to simulate the urban canyon.
Figure 2.20: The relationship between SVF and the urban albedo(Yang and Li,2015)
On the other hand, Boutet (1987) explored the effect of five primary building shape air movement and illustrated the cubic shape effect on air flow within the complex author stated that buildings being
50 % - 60 % (Figure 2.21).
Figure 2.21: The effect of cubic shape orientation on air flow
Moreover, the researcher highlighted the
movement and claimed that arranging the building
the air flow around most of the buildings within the complex (Figure 2.22).
a) Linearly buildings arrangement b)
Figure 2.22: The air flow effect on tow of building configurations (Boutet On the other hand, Boutet (1987) explored the effect of five primary building shape air movement and illustrated the cubic shape effect on air flow within the complex
s being angled to the air direction decreases the air velocity by
The effect of cubic shape orientation on air flow (Boutet
Moreover, the researcher highlighted the effect of two building configuration
movement and claimed that arranging the buildings alternately will enhance the potential of the buildings within the complex (Figure 2.22).
a) Linearly buildings arrangement b) Alternating buildings arrangement Figure 2.22: The air flow effect on tow of building configurations (Boutet
57 On the other hand, Boutet (1987) explored the effect of five primary building shapes on urban air movement and illustrated the cubic shape effect on air flow within the complex. The
angled to the air direction decreases the air velocity by
(Boutet, 1987)
effect of two building configurations on air flow and alternately will enhance the potential of the buildings within the complex (Figure 2.22).
Alternating buildings arrangement Figure 2.22: The air flow effect on tow of building configurations (Boutet, 1987)
58 Block density and building design have a significant effect on urban design sustainability, and it is proved that the dense form has a significant effect on energy conservation. The challenge of the urban designer is to find the most optimised configuration to achieve the desired energy conservation without neglecting other design factors, such as viewshed and urban ventilation. The viewshed is one of the urban design factors that should be taken into consideration when planning according to climatic and other urban design factors.
The viewshed represents the geographical area that can be viewed from specific locations excluding the points that are located behind any obstruction, such as buildings. Sander and Manson (2007) studied the viewshed calculation using the 'Geographic Information System (GIS)' and 'Digital Elevation Model (DEM)' data to evaluate the viewshed of a specific region in Minnesota, USA, and they found that generalisation of a uniform height will serve better for some land use to achieve the desired viewshed.
On the other hand, Lehmann (2010) studied the effect of the compact form on energy conservation and found that the most compact form is the most sustainable form, which could be represented by the vertical form of mid-rise and high-rise buildings. Moreover, the researcher presented the most promising model in terms of solar gain and heat conservation, the researcher took into consideration the optimum land use required for the same number of units, and proved that the six floors mid-rise buildings are the most preferred form for energy conservation, but the author reached this result in specific weather conditions and generalising this result needs more research and investigation according to the other locations of studied areas (Figure 2.23).
The relationship between the main
speed, and relative humidity has been addressed in many studies. The relation
the outdoor air temperature and the relative humidity has been explored by Giannopoulou et al. (2014). The researchers conducted a survey and data analysis to collect the data of the two parameters from 26 stations in Athena, Greece, and they highlighted the inverse relation between the air temperature and the relative humidity.
Figure 2.23: The most promised and sustainable composition(Lehmann
Furthermore, the researchers found that the highest air temperature western station, while the lowest air temperature
2.24).
between the main microclimate parameters, such as air temperature, wind speed, and relative humidity has been addressed in many studies. The relation
the outdoor air temperature and the relative humidity has been explored by Giannopoulou et archers conducted a survey and data analysis to collect the data of the two
in Athena, Greece, and they highlighted the inverse relation between the air temperature and the relative humidity.
The most promised and sustainable composition(Lehmann
Furthermore, the researchers found that the highest air temperature was
western station, while the lowest air temperature was recorded in the northern station (Figure
59 such as air temperature, wind speed, and relative humidity has been addressed in many studies. The relationship between the outdoor air temperature and the relative humidity has been explored by Giannopoulou et archers conducted a survey and data analysis to collect the data of the two in Athena, Greece, and they highlighted the inverse relationship
The most promised and sustainable composition(Lehmann, 2010)
was recorded in the recorded in the northern station (Figure
Figure 2.24: The relation between the outdoor air temperature and the relative a) Centre station
b) Northern station
c) Western station
Figure 2.24: The relation between the outdoor air temperature and the relative humidity(Giannopoulou et al., 2014)
60 Figure 2.24: The relation between the outdoor air temperature and the relative
Furthermore, a field measurement study conducted by Irulegi, Serra and Hernande
find the relation between the air temperature and relative humidity in Mediterranean climatic zone, San Sebastian, Spain. Figure 2.25
two microclimate parameters.
Figure 2.25: The outdoor air
Mediterranean zone, Spain (Irulegi, Serra and Hernández
However, the relationship between the wind speed and the relative humidity was explored in an experimental study conducted by Lee and
effect of the wind flow and relative humidity on heat transfer
relationship between wind velocity and relative humidity. This relation
transfer coefficient positively by enhancing heat transfer. Table 2.1 shows the empirical data collected from the laboratory experiment.
measurement study conducted by Irulegi, Serra and Hernande
find the relation between the air temperature and relative humidity in Mediterranean climatic Figure 2.25 below shows the inverse relation
Figure 2.25: The outdoor air temperature and relative humidity file measurement in Mediterranean zone, Spain (Irulegi, Serra and Hernández, 2017)
between the wind speed and the relative humidity was explored in an experimental study conducted by Lee and Lau (2016). The researchers highlighted effect of the wind flow and relative humidity on heat transfer, and they presented the inverse
between wind velocity and relative humidity. This relationship
positively by enhancing heat transfer. Table 2.1 shows the empirical data experiment.
61 measurement study conducted by Irulegi, Serra and Hernandez (2017) to find the relation between the air temperature and relative humidity in Mediterranean climatic below shows the inverse relationship between the
temperature and relative humidity file measurement in the 2017)
between the wind speed and the relative humidity was explored in Lau (2016). The researchers highlighted the and they presented the inverse ship affects the heat positively by enhancing heat transfer. Table 2.1 shows the empirical data
Table 2.2: The experimental data for the relation velocity (Lee and Lau, 2016).
2.5 Building Physics and Outdoor