An Empirical Study on the Optimization of Building Layout in the Affected Space of Ventilation Corridors—Taking Shijiazhuang as an Example

Abstract

This article focuses on how to further explore the impact of building layout and form on the local wind environment in micro scale ventilation corridors connected to the urban scale. Taking Shijiazhuang as the research area, three typical blocks of complex building forms, including old and new ones, were selected near the built ventilation corridors. CFD numerical simulation and on-site observation experiments were conducted to analyze the impact of different building heights and layouts on the wind environment in each typical block qualitatively and quantitatively. The above can provide a reference and guidance for the construction of secondary and tertiary ventilation corridors and the spatial form design of functional buildings during urban renewal in the stock era. The results show the following: (1) average wind speed, Mean Wind Velocity ratio, and the proportion of the outdoor pedestrian comfort zone are negatively correlated with the building height, but there is a threshold for them to decrease with the increase in the building height. Observation experiments also indicate that in the background of the south wind, the internal and leeward wind environment of new high-rise residential areas is better than that of old low residential areas. (2) Regression analysis was conducted between the simulated average wind speed and the building height, indicating that regulating the average building height to be below 45 m can improve the wind environment as the building height decreases. (3) The enclosed building complex has the smallest impact distance on downstream wind speed compared to point, row, and staggered layouts, but its internal ventilation environment is relatively poor. To ensure the ventilation performance, the upper limit of the building height should be stricter, and it should be controlled within at least 40 m, especially below 30 m. (4) In the process of urban renewal in the future, it is recommended to conduct an overall ventilation efficiency evaluation for different blocks. Compared to others, increasing the height of buildings and leaving more space to increase the inter site ratio/building spacing is more beneficial for the overall ventilation environment.

1. Introduction

Ventilation corridors have been incorporated into national spatial planning as an effective planning tool for collaborative solutions to urban climate issues. It is a spatial form that includes multiple land use patterns, with a specific function of urbanization at the source and high hardness, in the chain of “urbanization brings climate problems, affects the functioning of ecosystems, and forms low environmental performance” [1]. Ventilation corridors guide urban space optimization to reduce wind loss, combined with the scientific protection of urban open spaces, and promote local air circulation. This is of great significance for improving the urban wind environment, alleviating the heat island effect, and reducing air pollution [1,2,3,4]. Before 2014, research and practice on ventilation corridors in China were mainly carried out in economically developed and densely populated cities. In 2019, nearly a hundred cities or regions across the country conducted this work. As the Ministry of Natural Resources pointed out in the notice on comprehensively carrying out national land spatial planning work, the pattern and control of ventilation corridors have become one of the key points for the review of the overall national land spatial planning at the municipal level for approval. Ultimately, the evaluation of ventilation corridors changed from a multiple-choice question to a mandatory question in planning and development.

When constructing ventilation corridors, it is necessary to consider the functional space, compensation space, and air guide channels [5,6]. The fresh cold air generated by compensatory spaces such as mountains, waters, forests, fields, lakes, and grasslands is transported through an “air guide channel” to the “functional space” where buildings, pedestrian and vehicular traffic are dense, anthropogenic heat emissions are heavy, and air circulation is obstructed. To maximize the benefits of ventilation corridors, on the one hand, it is necessary to plan and layout land reasonably at the urban scale, in order to achieve the goal of “attracting sources with less obstruction” and promote the introduction of fresh air from compensatory spaces into the city [7,8,9]. On the other hand, it is necessary to optimize the height, shape, layout, and other aspects of the spatial architecture at the local scale, so that fresh air can penetrate the urban area through multiple secondary corridors after reaching the edge of the city from the periphery. In order to achieve the effect of “transmitting wind and connecting wind”, promoting airflow to benefit more small and micro spaces [10,11,12].

At the urban scale, research on ventilation corridors has become relatively mature. It mainly relies on the cold source systems with ventilation and heat removal functions, such as Rivers and Lakes, green lands, forests and fields, and road networks, and conforms to the direction of the dominant wind, connecting the existing and future planned blue-green spaces with a good ventilation environment. Based on this, various planning strategies to enhance urban climate, such as overall urban layout and land optimization plans, are determined [13,14,15,16]. Japan formulated the “Wind Path” Research for eight major prefectures and cities within Tokyo Bay. It proposed three types of wind corridor forms for the construction and management of coastal cities, including bringing sea breezes though street and river course, guiding sea breezes though building height differences, or utilizing downdrafts on the leeward side of high-rise buildings [17]. Yin, J. et al. applied remote sensing and geographic information technologies to construct climatological and ecological models. Combining the temporal and spatial characteristics of mountain-valley breezes and sea-land breezes during the hot summer, they identified existing ventilation corridors to provide decision-making support for urban wind corridor planning [18].

At the local scale, research on ventilation mainly focused on comparing and analyzing the changes in the impact of different building layouts on outdoor wind environments through simulation, measurement, and wind tunnel experiments [19,20,21,22]. Li, L. et al. use the CEDVAL wind tunnel dataset and Urban Sub-domain Scale Model (USSM) to describe the impact of the building on the wind field near the ground [23]. Blocken, B. et al. present a general simulation and decision framework for the evaluation of pedestrian wind comfort and wind safety in urban areas with CFD [24]. However, they have not carried out quantitative analysis between the ventilation evaluation indicators and building parameters. In addition, plenty of studies have paid attention to the impact of new high-rise buildings, and ignored the old low-rise residential areas in the built-up city [25,26,27], and lack of mutual verification and summary between simulation and actual measurement about the ventilation effect [28,29].

In the reality that built-up cities are not suitable for large-scale demolition and construction, architectural layout optimization and public space renovation is the key to giving full play for ventilation corridors at the local scale. Therefore, we based our research on the “6 + 13” multi-level ventilation corridor delineation and control planning studies which were accepted into the Revision of Shijiazhuang Master Plan (2017–2030) [30], furthering our work on building layout optimization and control indicators at the local scale. According to on-site investigation, three typical blocks covering complex building forms were selected for the following research: (1) using numerical models to simulate the wind environment before and after changing different building heights and arrangement forms, completing the calculation of ventilation evaluation indicators and quantitative analysis for the impact of building parameter changes on the ventilation environment, and proposing a reasonable range and control suggestions of building parameters that are conducive to ventilation; (2) based on the actual measurement, observation of the ventilation effects and simulation results on the typical blocks are compared to and verified by each other. However, the demolition and adjustment of existing buildings cannot be finished in one step, it needs to be gradually implemented. This study aims to provide references and guidance for the design of buildings’ spatial form in the “affected space” building. On the one hand, it can make a reference for the old residential areas that need to be demolished. On the other hand, areas that have been vacated and rebuilt in urban renewal, as well as newly built residential areas, can also be referred to.

2. Study Area

We took Shijiazhuang as the study area, which is located in the southwestern part of Hebei Province. The western part of the city is located in the middle section of the Taihang Mountains, and the east is on the alluvial plain of the Hutuo river. Therefore, it shows a downward slope from northwest to southeast, and the basic wind conditions are not conducive to air circulation and pollutant diffusion, with a static wind frequency of up to 1/5 throughout the year [30]. As the capital of Hebei Province, Shijiazhuang is an important central city of the economic integration in the Beijing–Tianjin–Hebei region. However, from 2013 to 2018, due to the influence of stable air circulation, Shijiazhuang suffered persistent large-scale haze weather frequently, the air quality remained severely polluted for several days, so that the city ranked first in air pollution all over the country and attracted high public attention.

Shijiazhuang metropolitan area is located in the central region encompassing eight districts, including Chang’an, Qiaoxi, Xinhua, Yuhua, Gaocheng, Luquan, Luancheng, and Zhengding (Figure 1a). The prevailing wind directions throughout the year are south–southeast (SSE) and north–northwest (NNW). Along these prevailing wind directions, there are few open spaces traversing the city with high ventilation potential. Ventilatable open spaces such as water bodies, green spaces, parks, and squares are scattered and lack connectivity, while predominantly oriented east–west [1]. Most existing buildings are long in the east–west direction, perpendicular or in large angles to the coming flow wind. This creates significant obstruction to the prevailing winds. In some areas, there is a lack of control when it comes to building heights or they are arranged in staggered patterns, causing negative effects on the wind environment of the internal zone and downstream leeward zones.

Based on the Shijiazhuang Ventilation Corridor Delineation and Management Planning, this study selected three typical blocks according to on-site investigation and research. They are adjacent to No. 3 Primary Ventilation Corridor, No. 5 and No. 11 Secondary Ventilation Corridor (Figure 1b). The three typical blocks are as follows:

Typical block ①: Traditional row layout consisting of 6-story buildings. Average building height is approximately 18 m, building distance is about 25 m, and frontages’ width is 65–100 m. It represents a typical old residential area with low-rise buildings and elongated linear rows. The geographical location and extent are shown in Figure 1c, top Jianling Residential Area.

Typical block ②: Dominated of high-rise buildings exceeding 20 stories. Average building height is over 50 m, building distance is about 70 m, and frontages width with 55–85 m. It is a typical example of newly built residential area mainly composed of high-rise buildings with shorter width. The geographical location and extent was shown in Figure 1c top Wanda Residential Area.

Typical block ③: Primarily consists of high-rise buildings, building height is 60–70 m, presenting a distinctly enclosed building form. The geographical location and extent are shown in Figure 1c bottom.

3. Data and Methods

3.1. Data

• The Map of Ventilation Corridor Planning and Climate Recommendations of Shijiazhuang Metropolitan Area is the achievement of the Ventilation Corridor Delineation and Management Planning Study project, which is adopted from Shijiazhuang Master Plan Revision (2017–2030) [30]. It is used to identify representative study areas, assess local climatic environments, and select typical blocks.
• Building data is acquired from the 1:2000 topographic map data of 2017 in Shijiazhuang metropolitan area, which composed information on the number of building floors and their location. It was used to estimate the building coverage ratios and building heights based on GIS spatial analysis techniques. This data is provided by the Shijiazhuang Municipal Institute of Territorial Spatial Planning and Design. It is used for constructing 3D models of buildings, analyzing building spatial morphology, and extracting building parameters, including average building height, floor area ratio, building layout, and other urban spatial form parameters.
• Meteorological Data (1981–2010) is from representative National meteorological observation stations in Shijiazhuang. It is used to set initial meteorological conditions for simulations. The average wind speed is 1.7 m·s⁻¹, and the local prevailing wind direction along the ventilation corridor is west–northwest (WNW). The other meteorological data is minute-by-minute observation data from DZB4-XVSB portable stations at the three typical blocks. It is used for the observational analysis of the impacts on wind environment.

3.2. Research Methods

Based on Computational Fluid Dynamics (CFD) numerical simulation and the control variable method, we established a quantitative relationship model between building parameters and the ventilation evaluation indicators. The model derives reasonable ranges for building morphological parameters, and then we verified the rationality of this range through on-site observations of actual ventilation effects in typical blocks. Finally, in this way, we can propose design suggestions for building spatial forms that could promote good ventilation. The main processes include the following:

3.2.1. Simulation Analysis Method for Building Impact on Ventilation Environment

CFD numerical simulation is the primary tool for investigating the interaction and influence between building layout and the local microclimate at the community scale [31,32,33]. This study used 1:2000 topographic map data to obtain the 2D planar data of building distribution and the information of the three typical blocks. Then, by using SketchUp2016 software, the 2D data was adjusted to the actual scale through translation, scaling, and other operations, and the morphological boundaries of the buildings were drawn. Based on the actual height of each building, a push–pull tool was used to construct 3D building models of the typical blocks. In addition, the model was imported into WindPerfectDX1.0 fluid dynamics software and then all of the terrain and surface objects and their surroundings were gridded in order to finish the simulation parameter settings of the scale and azimuth. WindPerfectDX is three-dimensional heat flow analysis software designed specifically for architecture, landscape, and urban planning. It supports the joint design of BIM and GIS and can simulate environments from individual buildings to complex urban scales. It complies with the new evaluation standards for green buildings, and can meet the requirements of urban planning and architectural design under different building heights, building orientations, complex terrains, representative climates, and other conditions, especially for the outdoor wind environment, urban heat island simulation, natural ventilation simulation, wind pressure simulation, humidity simulation, and so on. The modeling and simulation domain is a 1000 × 1000 m grid that covers the entire typical block region, serving as the simulation scope of community scale. The horizontal grid resolution was set to 10 m intervals, while the vertical resolution was set to 1.5 m intervals near the ground [31,32]. The simulation parameter settings of WindPerfectDX mainly include meteorological conditions such as initial wind direction, wind speed, air temperature, and solar radiation, as well as turbulence models, calculation time, etc. According to Section 3.1, the initial meteorological conditions were set to be consistent with the local wind direction along the ventilation corridor area, i.e., northwest–west wind, and the average wind speed was set to be 1.7 m·s⁻¹. Based on the load code for the design of Building Structure (GB 50009-2012) [34], the roughness category of the height variation factor of wind pressure was determined. Overall, the inflow wind profile could be calculated. The Large Eddy Simulation (LES) method [35] was used as the turbulence model for simulation, with a set of 1500 s for the integral calculation time. The simulated result shows a converging curve which indicates that the calculation is believable. Then, we can view the distribution and set contour lines and vector representations for the results, as well as output wind environment analysis maps at heights of 1.5 m, 10 m, and 20 m, respectively.

The impact of buildings on the ventilation environment was studied by adjusting building height and layout [10,36]. Using the control variable method, we performed the following: (1) Height Variation: the internal arrangement morphology of the typical block remained constant, six experimental cases of building height were set, and they are as follows: lowering each building by 10 m or 5 m, and raising each building by 5 m, 10 m, 15 m, or 20 m. Wind field distributions at 1.5 m height above ground under these different cases were simulated, and compared with the wind field under the original building heights to identify rules of wind change caused by different building heights. (2) Layout Variation: building heights were kept constant while the building layout was adjusted, in order to identify the impact of different building layout forms on the internal and downstream wind environment of the typical block, and to determine which building layout form has less of an effect on the reduction in wind speed in the leeward zones, and is also conducive to wind speed recovery in downstream areas. After field survey and classification analysis, the architectural forms in Shijiazhuang are mainly divided into three categories: one is Residential areas with 3–6 floors, comb shaped layout that are about to exceed their age limit; another is Residential areas with a house age of over 20 years, 8–18 floors, mainly rectangular or barrel shaped; and the other is Newly built residential areas, mostly 15–30 story high-rise buildings shaped in tower structures with pointed roofs. Therefore, concentrating on the current situation in Shijiazhuang, four representative simplified layouts were proposed, and they are as follows: Point Style (A), Linear Row Style (B), Staggered Row Style (C), and Enclosed Style (D). The criterion for buildings to reduce influence on wind speed was the wind speed recovering to 80% or more compared to the original wind, and simulations of wind fields for these four layout forms were conducted [22,37], resulting in their different effects on the block’s wind field. The research results are only suggestions and references for basic evaluation, and further targeted evaluation is needed in the process of renewal for local land parcels.

3.2.2. Observational Analysis Method for Building Impact on Ventilation Environment

• Observation Sites:

In order to study the impact of different building heights on the wind environment, typical block ① and typical block ② were selected for comparative observation on wind influences, as they were adjacent and had different building heights. In the same weather conditions, on-site ventilation environment observations were conducted in each block, respectively, stations are shown in Figure 1c, top: Southeast corner station (Point A/Point D), which is the windward side in summer, and leeward side in winter; Interior station (Point B/Point E); Northwest corner station (Point C/Point F), which is the windward side in winter, and the leeward side in summer. For the impact of different building layout forms on the wind environment, typical block ③ was selected, whose building form presented an enclosed layout; on-site ventilation observations were conducted at three points within the block, and stations are shown in Figure 1c, bottom: Southeast corner station (Point J); Interior station (Point K); Northwest corner station (Point L). These typical blocks were selected based on a detailed investigation of the characteristics of architectural forms in Shijiazhuang. As the Shijiazhuang metropolitan area is a densely urbanized area, the underlying surface is very complex with various buildings and streets intersecting. In order to determine the location of the observation instruments, we needed to consider the space for placement, and it cannot be damaged by the constant stream of people. It is also necessary to stay away from kitchens and various exhaust outlets such as air conditioners in restaurants and shops. We also conducted field investigations and observation experiments repeatedly with researchers from the local meteorological bureau to ensure the conditions for robust simulation verification.

• Observation time:

We analyzed meteorological observation data from all stations in Shijiazhuang metropolitan area over the past 30 years, and identified representative weather backgrounds with frequent occurrences of each season. Meanwhile, we took into consideration the forecast of 3–5 days in advance from the local meteorological bureau to find observation days that match the representative weather background. Observation days were selected based on meteorological conditions: (i) wind direction should be consistent with the prevailing wind directions of Shijiazhuang; (ii) the wind speed should be close to the annual average wind speed; (iii) strong winds or precipitation should be absent on the observation days. Climate data analysis indicates that the prevailing wind directions in Shijiazhuang are southeast, northwest, and northeast. Therefore, 1–3 days were selected for each of these three prevailing wind directions, resulting in a total number of 10 observation days. Although the monitoring period is relatively short, it actually represents the most common weather conditions due to our careful analysis and selection.

• Observation Parameters:

Simultaneous observations of meteorological parameters were conducted, including wind speed, wind direction, air temperature, and humidity, in order to obtain minute-by-minute meteorological data for statistical and comparative analysis.

• Observation instruments:

We used DZB4-XVSB micro intelligent portable station for observation; it conducts measurements with an accuracy of minutes. The calibration method includes calibrating the compass by displaying a prompt screen. First, we rotated the instrument around the vertical direction in a horizontal plane as slowly and uniformly as possible for 2–3 turns. Then, we pressed the settings button to end the calibration. The LCD screen automatically prompts the calibration result. If it prompted “calibration failure”, we repeated the above step until the calibration was successful and automatically jumped back to the calibration compass main interface.

3.2.3. Wind Environment Evaluation Standards and Methods

• Average Wind Speed

The wind speed at a pedestrian-level height of 1.5 m above ground in urban space is considered, with the units of m·s⁻¹. This index serves as a positive indicator of ventilation effectiveness, meaning an increase in near-ground wind speed, and indicating that the ventilation capacity at ground level enhanced. According to the Beaufort wind scale, wind speeds of 0–0.2 m·s⁻¹ represent calm wind, 0.3–1.5 m·s⁻¹ represent light air, and 1.6–3.3 m·s⁻¹ represent light breeze.

For Simulation Analysis, the wind speed simulation result, which is the wind speed values at each grid cell of the typical blocks, were used to analyze the number and proportion of grid cells occupied by different wind speed ranges, respectively.

For Observational Analysis, the 2-min average wind speed observed at each station was calculated from 1-s time-step instantaneous values measured at the integer-second timestamps of a pedestrian-level height of 1.5 m above ground. The variance and proportion of each wind speed range were calculated for each observation station. These are used to reflect the discrete degree of wind speed in statistics.

The calculation formula for variance is

$${\mathrm{S}}^2={\displaystyle \frac{1}{n}}\left[{\left(X_1-\overline{X}\right)}^2+{\left(X_2-\overline{X}\right)}^2+\dots +{\left(X_n-\overline{X}\right)}^2\right]$$

(1)

In the formula, $x_n$ represents the individual sample, $\overline{x}$ represents the average value of samples, and $n$ represents the number of samples.

• Mean Wind Velocity Ratio

The impact of building form on the wind environment can be evaluated by the Mean Wind Velocity Ratio, which is the ratio of the initial wind speed of the incoming flow to the wind speed of the downstream flowing though the buildings. It serves as a ventilation evaluation indicator to assess the quality of the wind environment, reflecting the influence degree on wind speed caused by the existence of buildings [37].

The formula for Mean Wind Velocity Ratio is

$$R_i={\displaystyle \frac{V_i}{V_0}}$$

(2)

In the formula, $R_i$ is the ratio of i, $V_i$ is the wind speed at a pedestrian-level height of i, and $V_0$ is the wind speed of the incoming flow, always the initial wind speed.

Based on the principle that $R_i$ should not be lower than 0.5, we can conclude which building forms have a relatively small impact on the reduction in wind speed in the typical blocks, and proposed the reasonable control range and recommendations for building parameters to optimize air circulation capacity.

• Proportion of Outdoor Comfortable Wind Zones

Based on the extensive field measurements of existing buildings and statistical analysis of wind tunnel tests on established models, Simiu et al. had summarized the relationship between probability of wind speed and comfort at the pedestrian-level height [38]. This resulted in the threshold of comfortable wind speed, and wind speed between 1 m·s⁻¹ and 5 m·s⁻¹ was considered as the evaluation criterion for comfortable wind environments at the pedestrian-level height. The proportion of the above wind speed range is calculated as the Comfortable Wind Zone Value. The higher the value, the greater the coverage of comfortable outdoor wind zones and the wind environment quality [39]. The proportion of comfortable wind zones was statistically analyzed for wind speed simulations and observational results though three typical blocks to evaluate the performance of their wind environments, respectively.

3.2.4. Correlation Analysis Methods for Building Parameters and Wind Speed

A regression analysis was conducted between the wind environment simulation results of the typical blocks and the building parameters obtained from the fundamental geographic information data. We used linear regression and Pearson correlation coefficient methods to establish the relationship between building parameters and wind speed, where the average building height was used as the independent variable, and the average wind speed was used as the dependent variable. This enabled quantitative assessment of the effect that different building forms and layouts have on the wind environment at the community scale.

4. Results and Analysis

The relationship between buildings and wind speed is related to their height, shape, distance between buildings, building width, adjacent building shapes, and even underlying surface properties and materials. In this study, we only talk about building height and building layout, and provide the results of the impact. A detailed and targeted evaluation is needed during the detailed control planning process.

4.1. Impact of Building Height on Wind Environment

4.1.1. Simulation Analysis of the Impact of Different Building Heights

• Characteristics of wind field distribution

Figure 2, Figure 3 and Figure 4 present the simulation results of wind fields for typical block ①, ②, and ③ at 1.5 m height above ground under different building height cases. In general, reducing the height of each building in the blocks leads to an increase in the average wind speed. Specifically, for block ①, the average wind speeds are 0.96 m·s⁻¹, 0.89 m·s⁻¹, and 0.89 m·s⁻¹ when buildings are reduced by 10 m, 5 m, and at the original height, respectively. For block ②, the results are 0.99 m·s⁻¹, 0.94 m·s⁻¹, and 0.85 m·s⁻¹. For block ③, the results are 0.50 m·s⁻¹, 0.49 m·s⁻¹, and 0.49 m·s⁻¹.

It is evident that the greater reductions in building heights, the more increases in wind speed, particularly in areas such as buildings located in the upwind, the wide road and open space region, where the wind speed increase becomes more significant because of the reduction in building heights.

In contrast, increasing building height results in a slight decrease in the average wind speed in the blocks. For block ①, the average wind speeds are 0.86 m·s⁻¹, 0.85 m·s⁻¹, 0.86 m·s⁻¹, and 0.84 m·s⁻¹ when buildings are increased by 5 m, 10 m, 15 m, and 20 m, respectively. For block ②, the results are 0.80 m·s⁻¹, 0.80 m·s⁻¹, 0.78 m·s⁻¹, and 0.79 m·s⁻¹. For block ③, the results are 0.49 m·s⁻¹, 0.48 m·s⁻¹, 0.48 m·s⁻¹, and 0.47 m·s⁻¹.

• Statistics at proportion of different Wind Speed Ranges

The proportion of different wind speed ranges were calculated, respectively, including calm wind, light air, and light breeze. The results are shown in Figure 5a. It can be observed that building height decreases may lead to a reduction in the proportion of calm wind, while the proportion of light breeze increases, and the greater reductions in building height, the more significant the increase of the light breeze. For example, under the case of all buildings lowered by 10 m, the proportion of light breeze increases by approximately 2–6% compared to the original building heights. Conversely, as building height increases, the proportion of calm wind increases by 2–6%, and the proportion of light breeze decreases by 2–3%. This indicates a positive correlation between building height and the proportion of calm wind, while there is a negative correlation between building height and the proportion of light breeze.

• Analysis of Wind Environment Indicators

The results of wind environment indicators for each typical block under different building height cases were calculated separately, as shown in Figure 5b. Both the Mean Wind Velocity ratio and the proportion of outdoor comfortable wind zones have negative correlations with building height. As for the Mean Wind Velocity ratio, when building heights are reduced, all of the three typical blocks have shown an increasing trend. It can be indicated from the principle that Ri should not be lower than 0.5, that the recommended building height control for block ① should be less than or equal to 40 m, while for block ② it should be 45 m, and for block ③ it should be 30 m. This indicates that enclosed building layouts require a stricter upper control limit on building heights. It can be observed that by reducing building heights, all of the three typical blocks have shown an increasing trend in the proportion of outdoor comfortable wind zones. In contrast, by increasing building heights, only block ③ causes a significant decrease in comfortable wind zones, which is not the case for the other two blocks.

• Quantitative Relationship between Average Wind Speed and Building Height

We took the simulation results of 21 schemes (with an average building height ranging from 8 to 70 m) for adjusting the building height changes of 3 typical plots as data samples, and conducted a regression analysis with the building height. This established a quantitative relationship, as shown in Figure 6. It has indicated that there is a significantly negative correlation between average wind speed and building height, and the correlation coefficient is −0.732. As building height increases, average wind speed decreases proportionally. However, there is a certain threshold of this attenuation, that is when building heights exceed 45 m, the average wind speed will no longer decrease. In summary, limiting mean building height to no more than 45 m may effectively mitigate ventilation efficiency reduction.

4.1.2. Observation Analysis of the Impact of Different Building Heights

The observational and statistical analysis results of the wind speed at a height of 1.5 m above the ground for block ① are shown in

Table 1. Statistical results of ventilation environment observation experiment in typical block ①.

Perennial Dominant Wind Direction	Observing Days	Observing Points	Average Wind Speed	Variance	Proportion of Calm Wind (%)	Proportion of Light Air (%)	Proportion of Light Breeze (%)	Proportion of Outdoor Comfort Zone (%)
southeast	25 August 2023	windward side	1.14	0.21	0.0	80.2	19.8	54.2
		The interior	0.65	0.08	6.2	93.3	0.4	9.3
		leeward side	0.50	0.04	7.1	92.9	0.0	0.6
	13 September 2023	windward side	1.18	0.25	0.0	78.2	21.8	55.5
		The interior	0.75	0.09	3.3	95.7	1.0	15.1
		leeward side	0.54	0.05	6.7	93.3	0.0	2.1
	12 October 2023	windward side	0.78	0.13	0.0	96.8	3.2	21.5
		The interior	0.45	0.06	25.5	74.5	0.0	2.1
		leeward side	0.35	0.03	33.4	66.6	0.0	0.3
northeast	6 November 2023	windward side	0.90	0.26	2.6	88.7	8.8	35.5
		The interior	0.72	0.11	5.8	92.9	1.3	15.7
		leeward side	1.20	0.52	1.0	70.4	27.4	50.1
northwest	19 December 2023	windward side	0.84	0.23	3.1	88.1	8.9	27.1
		The interior	0.80	0.13	2.4	94.0	3.6	24.9
		leeward side	1.06	0.52	0.4	83.1	14.9	33.1

. It can be seen that under the southeast prevailing wind direction, the average wind speed observed on the windward side is the highest, followed by that in the interior, and the smallest is on the leeward side. The wind speed variance on the windward side is the largest, indicating the most intense fluctuations on the observation day, while the wind speed fluctuations in the interior and leeward side are small, and the overall wind speed values are also low. Especially for the leeward station, its variance is less than half of that on the windward side, suggesting that the daily variation in wind speed will weaken because of the obstruction by buildings. No calm wind zone with a wind speed below 0.2 m·s⁻¹ was observed on the windward side; however, the proportion of calm wind zones observed on the leeward side was larger than that in the interior. The windward side had the highest proportion of outdoor comfortable wind zones, which ranges 22–55%. After the air blows through the building, the comfortable wind zone in the interior decreases to 2–15%, and the calm wind ranges 3–26%. The reduction in comfortable wind zones on the leeward side is more obvious, with the proportion generally below 2% and the calm wind in the 7–33% range. Under the northeast dominant wind direction, the smallest average wind speed was observed in the interior, with the largest proportion of calm wind zones and the lowest frequency of light breeze, resulting in the proportion of comfortable wind zones being the smallest. Under the northwest dominant wind direction, the smallest average wind speed was also observed in the interior, as well as the lowest frequency of light breeze and the smallest proportion of comfortable wind zones. In summary, the “wake effect” of buildings makes it easy for calm wind zones to appear in the interior and leeward sides of the blocks, significantly reducing the proportion of comfortable wind zones.

The observation and statistical analysis results of wind speed at a height of 1.5 m for block ② are shown in

Table 2. Statistical results of ventilation environment observation experiment in typical block ②.

Perennial Dominant Wind Direction	Observing Days	Observing Points	Average Wind Speed	Variance	Proportion of Calm Wind (%)	Proportion of Light Air (%)	Proportion of Light Breeze (%)	Proportion of Outdoor Comfort Zone (%)
southeast	8 August 2023	windward side	1.36	0.24	0.0	69.2	30.7	72.6
		interior	0.98	0.16	0.0	90.2	9.8	42.7
		leeward side	0.89	0.12	0.0	95.8	4.2	30.1
	15 September 2023	windward side	0.52	0.25	2.0	81.1	16.0	45.7
		interior	0.43	0.16	8.9	86.2	3.6	18.7
		leeward side	0.77	0.08	0.6	97.1	1.7	16.0
	11 October 2023	windward side	1.10	0.23	0.4	83.8	15.8	51.9
		interior	0.88	0.16	0.7	91.5	7.8	29.9
		leeward side	0.72	0.09	2.8	97.1	0.1	12.8
northeast	9 November 2023	windward side	1.18	0.47	0.9	72.1	26.5	52.4
		interior	0.82	0.15	0.4	92.9	6.7	21.4
		leeward side	2.16	0.63	0.0	22.9	70.1	92.0
northwest	6 December 2023	windward side	1.07	0.40	2.8	73.3	23.9	45.1
		interior	0.80	0.20	6.7	85.1	8.2	25.2
		leeward side	1.49	0.46	0.6	53.5	45.4	70.8

. It can be seen that under the southeast dominant wind direction, the average wind speed observed on the windward side is the largest, the proportion of comfortable wind zones is the highest, followed by the interior, and the leeward side is the smallest. The wind speed variance of the windward side is the largest, and the leeward side is the smallest, which is consistent with the observation conclusion of block ①. The proportion of outdoor comfortable wind zones observed on the windward side is the highest, ranging 46–73%, the proportion of interior comfortable wind zone is reduced to 19–43%, and the calm wind frequency is 1–9%. The comfortable wind zone on the leeward side is reduced to 13–30%, and the calm wind frequency is 1–3%. The reduction in comfortable wind zones in the interior and leeward side is smaller compared with block ①. Under the northeast and northwest dominant wind directions, although the observed frequency of calm wind in the interior is relatively low, the average wind speed also remains the smallest, and the proportion of comfortable wind zones is the lowest, which is consistent with the observation conclusion of block ①. The proportion of comfortable wind zones in the interior is reduced more significantly compared with block ①.

To sum up, under the southerly wind direction, the average wind speed on the leeward side is generally the smallest, and the proportion of comfortable wind zones is the lowest. Among them, the frequency of calm wind in the interior and the leeward side of block ② is significantly lower than that of block ①, and the proportion of comfortable wind zones is also larger than that of block ①. However, under the northerly wind direction, the frequency of calm wind in both blocks is generally low, the average wind speed in the interior is the smallest, and the proportion of comfortable wind zones is significantly reduced. Compared with block ①, the proportion of comfortable wind zones in the interior of block ② decreases more significantly, that is, the reduction in the proportion of comfortable wind zones in the interior of the new high-rise community is more obvious. This shows that the increase in building height does not certainly lead to the decrease in ventilation capacity, and the attenuation of wind speed with the increase in building height has a particular threshold, which is consistent with the conclusion of the simulation results.

4.2. Impact of Building Layouts on Wind Environment

4.2.1. Simulation Analysis of the Impact of Different Building Layouts

The simulation results of wind speed and flow field at a height of 2 m for four layout forms for the building height of 30 m are shown in Figure 7. The simulations have used the same boundary conditions to highlight the impact of different building layouts on airflow. It can be seen that the Point Style (A) has the largest average wind speed within the block, especially in the street area in the downwind direction, where a significantly larger wind speed area appears. This is because the ventilation blockage of the point-style layout is small, the wind environment between building rows is good, and the wind speed in almost the entire channel meets the outdoor comfort requirements, so the average wind speed is relatively large. It has shown that Point Style layout form has obvious advantages in ventilation [40]. When the initial wind speed is 1.7 m·s⁻¹ and the wake wind speed recovers to 80%, the influence distance of Point Style (A) on the downstream wind speed is about 300 m, almost 10 times the height of the buildings. Due to the blockage and friction of the front-row buildings, wind speed on the rear-row buildings in the Linear Row Style (B) gradually decreases, and low-wind-speed areas are mostly concentrated near the buildings in the downwind area [32]. Its influence distance on the downstream wind speed is about 280 m, and the influence range is about 9.3 times the height of the buildings. Due to the staggered arrangement of the building layout in the Staggered Row Style (C), a large area of low-wind-speed areas is formed on the leeward side of the buildings, which has a greater impact on reducing the wind speed in the leeward area of the building group [41]. The influence distance for Staggered Row Style is about 280 m, and 9.3 times the height of the buildings. The Enclosed Style D has a large wind speed in the inlet and the open space parallel to the wind direction, but it has a stronger wind-blocking effect on the downstream wake area inside the building area [41]. The influence distance is about 220 m, and 7.3 times the height of the buildings. According to the influence distance of different building layout forms, the order from small to large is as follows: Enclosed Style (D) < Staggered Row Style (C) = Linear Row Style (B) < Point Style (A).

4.2.2. Observation Analysis of the Impact of Different Building Layouts

Table 3. Statistical results of the ventilation environment observation experiment in typical block ③.

Perennial Dominant Wind Direction	Observing Days	Observing Points	Average Wind Speed	Variance	Proportion of Calm Wind (%)	Proportion of Light Air (%)	Proportion of Light Breeze (%)	Proportion of Outdoor Comfort Zone (%)
southeast	15 August 2023	windward side	1.5	0.48	1.6	54.5	43.8	68.8
		interior	0.5	0.08	11.4	87.9	0.7	5.7
		leeward side	0.7	0.13	6.2	93.3	0.4	22.4
	25 September 2023	windward side	1.2	0.20	0.0	79.0	21.0	57.5
		interior	0.4	0.05	23.8	76.2	0.0	0.6
		leeward side	0.6	0.05	4.5	95.5	0.0	2.0
northeast	18 October 2023	windward side	1.05	0.59	3.1	75.5	19.5	41.1
		interior	0.29	0.05	50.9	49.1	0.0	0.4
		leeward side	0.58	0.13	9.6	88.4	2.0	11.6
	7 November 2023	windward side	1.06	0.32	2.9	76.7	20.4	47.5
		interior	0.40	0.05	31.3	68.8	0.0	0.7
		leeward side	0.85	0.13	1.5	94.4	4.1	29.9
northwest	25 January 2024	windward side	1.09	0.41	3.2	75.7	20.4	42.1
		interior	0.46	0.07	26.2	73.9	0.0	1.4
		leeward side	0.76	0.18	1.4	94.0	4.6	22.1

shows the observation and statistical analysis results of the impact of building forms on the wind environment in typical block ③. It can be seen that under the southeast wind direction, the average wind speed observed on the windward side is the largest, the average wind speed in the interior is the smallest, the proportion of calm wind in the interior is the largest, and the proportion of comfortable wind zones is only about 1–6%. Therefore, the ventilation environment in the interior is relatively the worst. The proportion of small wind areas on the leeward side is smaller than that in the interior, while the proportion of comfortable wind zones is bigger. Under the northeast dominant wind direction, the average wind speed observed on the windward side is the largest, while in the interior it is the smallest, and the proportion of calm wind in the interior is the largest. The proportion of comfortable wind zones is extremely low, which is less than 1%. Although the comfortable wind zone on the leeward side is reduced by about 30–70% compared with that on the windward side, it is still larger than the proportion of comfortable wind zones in the interior. Under the northwest dominant wind direction, the average wind speed observed on the windward side is the largest, and also in the interior is the smallest, and the proportion of calm wind zones in the interior is the largest. The proportion of the comfortable wind zone in the interior is only about 1%, while the proportion of comfortable wind zones on the leeward side is about 22%.

It can be seen that regardless of the background wind conditions, the proportion of calm wind observed in the interior of the block is relatively large, while the comfortable wind zones are extremely small. It was indicated that the ventilation environment in the interior area of the enclosed buildings is relatively the worst. This layout form has a greater impact on the wind environment in the interior of the block than on the downstream, which is consistent with the conclusion of the simulation results in Figure 7.

5. Conclusions and Discussion

This paper presents research on how to further carry out the optimization of a building layout at a local scale connected with ventilation corridor research of the urban scale based on the planning study of multi-level ventilation corridors in the revision of the Shijiazhuang Master Plan (2017–2030). In fact, in 2018, we edited the national standard for urban planning climate assessment (GB/T 37529-2019) and the industry standard for ventilation corridors (QX/T 437-2018), making ventilation corridors a necessary part of municipal land spatial planning. The original intention of this research is to influence planners and decision-makers to consider the overall airflow of the community when making detailed control plans. We want them to understand that on the premise of ensuring plot ratio and natural lighting, optimizing building height and layout can balance economic benefits and local airflow circulation, and this was overlooked in the past. As we keep up with our efforts, suggestions might be truly incorporated into the legal process and standards step by step. During that time, the impact on the overall airflow of the local area should be considered in advance for every new or renovated residential area. This study used numerical models to simulate the wind environment before and after changing the different building heights and arrangement forms, and to complete the quantitative analysis for the impact of building parameter changes on the ventilation environment and the calculation of ventilation evaluation indicators, in order to put forward a reasonable range and control suggestions for building parameters that are conducive to ventilation, and obtain the actual ventilation effects of the corresponding real blocks through on-site observation to verify the simulation results.

The main conclusions are as follows:

(1) The simulation analysis of the impact of different building heights on wind environment has shown that building height is positively correlated with the proportion of calm wind and negatively correlated with the proportion of light breeze. The average wind speed of the block increases with the reduction in building height. Reducing the height of each building by 10 m can increase the proportion of light breeze by about 2–6% and increase the average wind speed by 0.01–0.14 m·s⁻¹ compared with the original building height. Meanwhile, both the Mean Wind Velocity ratio and the proportion of outdoor comfortable wind zones are negatively correlated with the building height. The Mean Wind Velocity ratio and the proportion of comfortable wind zones increase with the reduction in building height. However, when the building height increases, the change in the proportion of comfortable wind zones is small, especially in the newly built high-rise communities. The observation results have additionally proved that after the air blows through the buildings, under the southerly wind direction, the frequencies of calm wind in the interior and leeward side of Block ② are 1–9% and 1–3%, respectively, which are significantly lower than the 3–26% and 7–33% of Block ①, and the proportion of comfortable wind zones is 19–43% and 13–30%, which are also larger than the 2–15% and 0–2% of Block ①. Under the northerly wind direction, the frequency of the calm wind is generally low, the average wind speed in the interior is the smallest, and the proportion of comfortable wind zones is significantly reduced. Specifically, Block ② has a greater reduction in the proportion of comfortable wind zones in the interior compared with Block ①.

It is also found that there is an inflection point in the relationship curve between the average wind speed and the building height. That is, when the building height is greater than 45 m, the average wind speed will no longer reduce with the increase in the building height. Instead, affected by the Narrow tube effect, the wind speed at the corridor of the channel increases significantly, resulting in a general slight increase in the average wind speed of the blocks, and even Narrow tube effect would cause local wind disaster hazards. Therefore, the building height should be controlled within no more than 45 m.

(2) The simulation analysis of the impact of different building layouts has shown that the building groups with an enclosed form have more strict requirements for the upper limit of building height control. Increasing the building height, the reduction in the comfortable wind zone in Block ③ is significantly larger than that in Block ① and Block ②. In order to ensure the ventilation performance, the upper limit of building height control corresponding to Block ① is 40 m, Block ② is 45 m, and Block ③ is 30 m. The observation conclusions also confirm that the ventilation environment inside the enclosed building group is relatively the worst, which has a greater impact on the wind environment. However, it has the smallest impact distance on the downstream wind speed. Indeed, it is a building layout mode conducive to the wind speed recovery in the downstream area.

In addition, the high Floor Area Ratio and low-density mode is conducive to long-wave radiation heat dissipation and ventilation convection heat dissipation, due to its large building interval. By increasing the building height, increasing the building interval and open space, and reducing the building density appropriately, that could be beneficial to the overall ventilation environment. In the future, at the local scale, for the practical design of actual blocks in the “affected space”, and the implementation of urban renewal, the above research can be referred to.

This paper discussed the strategies of building height and layout forms that need to be considered in architectural design, in order to ensure the comfort of the microclimate and improve ventilation efficiency. The results have put forward some control suggestions on building forms and layouts to optimize the air circulation capacity, which is similar to the research conclusions of Li, Q. et al. [42] and Ma, J. et al. [43], and reached consistent conclusions with Razak, A.A. et al. [44] on the impact of different building density, aspect ratio and other building forms to the wind environment at a pedestrian-level height. However, due to the complexity of the actual outdoor wind environment of buildings, the on-site synchronous observation of the impact of buildings on ventilation only selected 3 stations in each typical block, which was a lack in the number of instruments. In the follow-up research, we have to use more stations covering east, west, north, and south directions, and the sampling time should be increased as well. The experiments conducted in this article to increase or decrease the height of buildings are only a rough estimate, with the aim of providing planners and policy makers with an understanding of some concepts from scratch. For example, the increase in building height is based on the existing old residential areas that have around 5–6 or even 3 floors, by appropriately increasing the height, and reducing the number of buildings, which could improve ventilation, and leaving space for greenery in the future. The reduction in building height is indeed necessary for contiguous high-rise residential areas, which should be constructed in a layered and undulating manner when urban renewal or demolition is carried out in the future, in order to create more thermal mechanical differences and generate local circulation to promote airflow exchange. In practical operation, it is necessary to compare and select different planning schemes in a targeted manner. Furthermore, more typical blocks should be selected in the congestion areas around ventilation corridors where the wind environment is in urgent need of improvement, and the cross-verification of simulation and observation on the impact of building density, Floor Area Ratio, and wind environment should be performed as well. Thus, we can give more spatial form design guidelines for the buildings of the affected space and the construction of the secondary and tertiary corridors in Shijiazhuang, which can also provide references for other cities.