**3. Results**

**2.2. Simulation**

Average building height

**Table 4.** Related parameters of three areas

The main wind directions are shown in red.

(m)

Layout type Layout along the street

68 Sustainable Cities - Authenticity, Ambition and Dream

shopping district

**Materials Density [kg/m3**

**Table 5.** Details of ground surface material characteristics for simulation

Building Concrete 1600 1000 0.65 Road Asphalt 2120 920 0.74 Side walk Redbrick 1650 840 0.62 Inside surface Mortar 2000 800 1.3 Green Soil 1340 1700 0.7

A commercial CFD code, scSTREAM (Software Cradle Co., 2011), was used to simulate the urban ventilation properties. Detailed distributions of air current and pressure per direction can be visualized. The simulation models were built according to the realities shown in

The wind environment (Xi'an Weather Station, 2016) on a typical summer day (21st July, 2016) and a typical winter day (21st December, 2015) is selected for analysis. The wind direction and wind speed between 6:00 a.m. and 8:00 p.m. are shown in **Figure 4**. The average wind speed

**Xiao Zhai Shu Yuan men Tang West Market** 

shopping district

40.6 5.6 28.3

**Figure 4.** The wind condition in a typical summer day (21st July, 2016) and a typical winter day (21st December, 2015).

Traditional historic and culture block

**] Specific heat [J/(kg K)] Thermal conductivity [W/(m K)]**

**Group**

New

**Figure 3**, and the detailed input data are presented in **Tables 1**, **2** and **5**.

Construction time 2002 1991(1906) 2012 Building coverage (%) 32 46 33

Green coverage (%) 32.4 24.3 26.6

#### **3.1. Results of urban air quality and urban ventilation**

#### *3.1.1. Air pollution distribution in Xi'an city*

The field measurement results of the PM10 concentration distribution in the summer of 2016 are shown in **Figure 5**. The air pollution concentration distribution was not averagely distributed in the city. Comparison of **Figure 4** and the satellite photo in **Figure 2** showed the distribution of air pollution to be partially but directly related to urban density. This is because the urban typology affects urban ventilation and accelerates aggregation, and the air pollution in high-density urban districts is subsequently high.

In order to clarify the mechanism of the air pollution concentration in high-density areas, the low-rise and middle-rise areas that located in highly polluted areas are selected for simulation. This phenomenon will be discussed with the simulation results.

#### *3.1.2. Wind environment simulation*

On a summer day, five wind directions (N, ENN, EN, ENE, E) were simulated at a wind speed of 2 m/s. On a winter day, three wind directions (N, ENN, EN) were simulated at a wind speed of 1.25 m/s. **Figures 6** and **7** present the simulation results in the selected four urban areas.

In summer (**Figure 6**), with the effects from trees, the median wind speed in the low-rise area was slower than in the other areas, and the wind speed in the mix-rise area was fastest. With a wind direction of EN, the median wind speed in mix-rise area was 0.35 m/s, which was 0.15 m/s higher than in the low-rise and middle-rise areas, respectively. The median wind speed in high-rise area was slightly slower (0.03–0.06 m/s) than in the low-rise and middlerise areas, but there were small areas of higher wind speed in the high-rise area, and these reached 2.13 m/s (2–3 times of the max wind speed in the low-rise and middle-rise areas). This could explain the results of high PM10 concentration measured in low-rise and middle-rise areas shown in **Figure 7**.

In winter (**Figure 7**), without the effects from trees, the median wind speed in the mixed-rise area was higher than in the other areas. With a wind direction of ENN, the median wind speed in mixed-rise area was 0.24 m/s higher than in the low-rise area and 0.19 m/s higher than in the middle-rise area. The median wind speed in the high-rise area is 0.09 and 0.28 m/s lower than in the middle-rise and mixed-rise areas. This is to say, the low-rise area (high-built density) and the high-rise area (high urban roughness) are reducing the urban wind speed.

**Figure 5.** Distribution of PM10 concentration in the central of Xi'an.

In most of the cases, wind direction of EN provides the highest wind speed in allover the area. But, the results of high-rise area in summer are lower than the other directions. This is because of the effects from trees.

#### *3.1.3. Wind speed distribution*

**Figures 8** and **9** show the details of wind speed distribution inside these four areas in the summer and winter. In the low-rise area, because of the high building density and narrow corridors between buildings, the overall wind speed was low, and it also had a pronounced effect on downwind areas. In the high-rise area, some areas of high wind speed were observed in between the high-rise buildings, but the wind speed in the leeside of the big volume buildings was extremely low. The mix-rise area showed the best ventilation properties of the four areas. Therefore, the traditional urban typology with low density and high built coverage creates the low urban ventilation at the human level. High-rise districts with large open space inside the district also reduce the overall wind speed.

In **Figure 11**, it is shown the result of the wind speed stimulation of three shopping districts. In a previous paper, Steemers selected six different layouts of building combinations to simulate the wind speed of the regional environment and found when the building parallel to the direction of the wind, ventilation rate is the highest in the street space, but the ventilation rate of building space is poor in the combination of combination and courtyard [13]. As can be seen from the simulation diagram, the average wind speed in Tang West Market Group shopping district (0.77 m/s) is stronger than others (Xiao Zhai is 0.75 m/s and Shu Yuan men is 0.76 m/s). This is because of the lower land cover in Tang West Market Group shopping district which creates more open spaces and wider streets. Meanwhile, the street space in this area is more orderly and vertical. However, Shu Yuan men shopping district's wind speed is stronger than Xiao Zhai, although its site coverage is higher. Because it has less trees in the

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In **Figure 12**, it shows the air temperature of three areas. We can see Xiao Zhai's temperature value is lower than other two. Shu Yuan men's average air temperature is 29°C, and it is 1°C higher than Xiao Zhai's and Tang West Market Group. The proposed reason for this is that more plants are built in Xiao Zhai shopping district, and these trees can effectively reduce

area and its average building height is lower than Xiao Zhai.

**Figure 6.** Wind speed in four selected areas in a typical summer day (21st July, 2016).

**Figure 7.** Wind speed in four selected areas in a typical winter day (21st December, 2015).

In winter, without the effects from the trees, higher ventilation could be observed inside the four districts. Especially in the high-rise district, the wind property inside the community is promoted in the winter.

#### **3.2. Results of urban typology and urban environment in typical shopping areas**

ENVI-met was used to calculate the wind speed, the air temperature and the sky view factor (SVF) in all over the area at human height level (1.5 m height from the ground). The results are shown in **Figure 10**. They are all stimulated data at 1400 h.

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**Figure 6.** Wind speed in four selected areas in a typical summer day (21st July, 2016).

In most of the cases, wind direction of EN provides the highest wind speed in allover the area. But, the results of high-rise area in summer are lower than the other directions. This is because

**Figures 8** and **9** show the details of wind speed distribution inside these four areas in the summer and winter. In the low-rise area, because of the high building density and narrow corridors between buildings, the overall wind speed was low, and it also had a pronounced effect on downwind areas. In the high-rise area, some areas of high wind speed were observed in between the high-rise buildings, but the wind speed in the leeside of the big volume buildings was extremely low. The mix-rise area showed the best ventilation properties of the four areas. Therefore, the traditional urban typology with low density and high built coverage creates the low urban ventilation at the human level. High-rise districts with large open space

In winter, without the effects from the trees, higher ventilation could be observed inside the four districts. Especially in the high-rise district, the wind property inside the community is

ENVI-met was used to calculate the wind speed, the air temperature and the sky view factor (SVF) in all over the area at human height level (1.5 m height from the ground). The results

**3.2. Results of urban typology and urban environment in typical shopping areas**

of the effects from trees.

promoted in the winter.

*3.1.3. Wind speed distribution*

inside the district also reduce the overall wind speed.

**Figure 5.** Distribution of PM10 concentration in the central of Xi'an.

70 Sustainable Cities - Authenticity, Ambition and Dream

are shown in **Figure 10**. They are all stimulated data at 1400 h.

**Figure 7.** Wind speed in four selected areas in a typical winter day (21st December, 2015).

In **Figure 11**, it is shown the result of the wind speed stimulation of three shopping districts. In a previous paper, Steemers selected six different layouts of building combinations to simulate the wind speed of the regional environment and found when the building parallel to the direction of the wind, ventilation rate is the highest in the street space, but the ventilation rate of building space is poor in the combination of combination and courtyard [13]. As can be seen from the simulation diagram, the average wind speed in Tang West Market Group shopping district (0.77 m/s) is stronger than others (Xiao Zhai is 0.75 m/s and Shu Yuan men is 0.76 m/s). This is because of the lower land cover in Tang West Market Group shopping district which creates more open spaces and wider streets. Meanwhile, the street space in this area is more orderly and vertical. However, Shu Yuan men shopping district's wind speed is stronger than Xiao Zhai, although its site coverage is higher. Because it has less trees in the area and its average building height is lower than Xiao Zhai.

In **Figure 12**, it shows the air temperature of three areas. We can see Xiao Zhai's temperature value is lower than other two. Shu Yuan men's average air temperature is 29°C, and it is 1°C higher than Xiao Zhai's and Tang West Market Group. The proposed reason for this is that more plants are built in Xiao Zhai shopping district, and these trees can effectively reduce

the air temperature in the area. Because the green plants mainly reduce the environment temperature by shadow and evapotranspiration. Through the role of the cover plants, two buildings 'walls and roof surface' temperature can be reduced by 11–25°C [14]. Moreover, the high-temperature area in Shu Yuan men is larger than others because it has less vegetation in the area. This is to say, tree planting in the urban area is providing contribution on wind

Urban Renovation and the Simulation Evaluation of Urban Climate Change in Residential…

In **Figures 10** and **13**, we can see the SVF images of three study areas. View factor is a geometric ratio, which is a part of radiation from the surface A blocked by object B [15]. Unger

related to the intensity of thermal environment in the microenvironment, which existed a good linear relationship [16]. As the SVF increases, heat intensity decreases [17]. The average of Xiao Zhai shopping district's SVF value is 0.36, but in Shu Yuan men is 0.52 and in Tang West Market Group is 0.45. Therefore, the SVF of Shu Yuan men is the highest of the three.

in Hungary Szeged and proved the SVF closely

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speed and air temperature reducing, and it can enhance thermal comfort.

studied on the 35 city areas for 0.25 km2

**Figure 10.** Sky view factor in the three shopping districts.

**Figure 11.** Wind speed distribution in the middle of a summer day (22nd July, 2016) at 1.5 m height.

**Figure 8.** Wind speed distribution in four selected areas at 1.5 m height from the ground in summer (wind speed: 2 m/s; wind direction: EN).

**Figure 9.** Wind speed distribution in four selected areas at 1.5 m height from the ground in winter (wind speed: 1.25 m/s; wind direction: ENN).

the air temperature in the area. Because the green plants mainly reduce the environment temperature by shadow and evapotranspiration. Through the role of the cover plants, two buildings 'walls and roof surface' temperature can be reduced by 11–25°C [14]. Moreover, the high-temperature area in Shu Yuan men is larger than others because it has less vegetation in the area. This is to say, tree planting in the urban area is providing contribution on wind speed and air temperature reducing, and it can enhance thermal comfort.

In **Figures 10** and **13**, we can see the SVF images of three study areas. View factor is a geometric ratio, which is a part of radiation from the surface A blocked by object B [15]. Unger studied on the 35 city areas for 0.25 km2 in Hungary Szeged and proved the SVF closely related to the intensity of thermal environment in the microenvironment, which existed a good linear relationship [16]. As the SVF increases, heat intensity decreases [17]. The average of Xiao Zhai shopping district's SVF value is 0.36, but in Shu Yuan men is 0.52 and in Tang West Market Group is 0.45. Therefore, the SVF of Shu Yuan men is the highest of the three.

**Figure 10.** Sky view factor in the three shopping districts.

**Figure 8.** Wind speed distribution in four selected areas at 1.5 m height from the ground in summer (wind speed: 2 m/s;

**Figure 9.** Wind speed distribution in four selected areas at 1.5 m height from the ground in winter (wind speed: 1.25 m/s;

wind direction: EN).

72 Sustainable Cities - Authenticity, Ambition and Dream

wind direction: ENN).

**Figure 11.** Wind speed distribution in the middle of a summer day (22nd July, 2016) at 1.5 m height.

This is because the high building density creates deeper urban canopy, and less vegetation in this area model makes more open space. Therefore in Tang West Market Group, we also can see a high numerical concentration in the central area, which is an open square. However, in Xiao Zhai, although there are many open spaces in the area, there are a lot of plants that cover

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In **Figure 14**, these are results of mean radiant temperature in three areas. In these images, we can see that lower values mainly concentrate in the green plants area in the shopping district,

Urban ventilation plays an important role in the urban environment. In summer, urban ventilation contributes to urban heat dissipation and urban heat island mitigation. In winter, high wind speed accelerates the aggregation of air pollution. Results demonstrated that the urban typology affects urban ventilation and urban air quality. However, it is usually difficult to

Most of China's cities have undergone fast economic growth, urban expansion, and urban redevelopment. This unique situation is the reason of the rapid environmental degradation in Chinese cities, which could also be useful to the other countries. The fast change in form in China's urban areas has provided an opportunity to optimize urban environments in short periods. This requires an urgent establishment of related policies to regularize environmental

This work demonstrated that the wind environment in the low-rise area and the high-rise area are characterized by high building density and the pronounced urban roughness. Wind speed was 0.04–0.09 m/s lower in the high-rise area than in the middle-rise area and 0.04–0.14 m/s lower in the low-rise area than in the middle-rise area. Wind speed is 0.19–0.27 m/s lower in the high-rise area than in the mixed-rise area and 0.21–0.28 m/s lower in the low-rise area than in the mixed-rise area. Overall, the balance between building height and building ratio should be considered in future urban development projects. The information from this work provides information useful to the cultivation of environ-

Overall, high-density urban residential and commercial development is providing a big impact on the urban wind environment and urban thermal environment. While this gives hints for UHI mitigation during the day, and it creates physical obstacles for heat release during the nights. A lower SVF reduces the urban radiation absorption from the Sun, but also reduces the outgoing longwave radiation from the urban surfaces. The spread of air pollution is affected by the wind turbulence around high buildings. Future studies should consider more detailes of the layout and volume of high-rise buildings in urban development projects

change the urban form over a short period in areas that have already been developed.

them. Therfore, its SVF value is the lowest of three districts.

and higher values is in the other spaces without vegetation.

urban development and redevelopment.

**4. Discussion**

**5. Conclusion**

mental urban policy.

**Figure 12.** Air temperature distribution in the middle of a summer day (22nd July, 2016) at 1.5 m height.

**Figure 13.** Sky view factors in the middle of a summer day (22nd July, 2016) at 1.5 m height.

**Figure 14.** Mean radiant temperature in the middle of a summer day (22nd July, 2016) at 1.5 m height.

This is because the high building density creates deeper urban canopy, and less vegetation in this area model makes more open space. Therefore in Tang West Market Group, we also can see a high numerical concentration in the central area, which is an open square. However, in Xiao Zhai, although there are many open spaces in the area, there are a lot of plants that cover them. Therfore, its SVF value is the lowest of three districts.

In **Figure 14**, these are results of mean radiant temperature in three areas. In these images, we can see that lower values mainly concentrate in the green plants area in the shopping district, and higher values is in the other spaces without vegetation.
