Optimization Design of Underground Space Overburden ... - MDPI

sustainability Article

Optimization Design of Underground Space Overburden Thickness in a Residential Area Concerning Outdoor Thermal Environment Evaluation Xiaochao Su 1,2 , Zhilong Chen 1,2, *, Xudong Zhao 2, *, Xiaobin Yang 1,3 , Qilin Feng 2 and Haizhou Tang 1,2 1 2 3

*

Underground Space Research Center, Army Engineering University of PLA, Nanjing 210007, China; [email protected] (X.S.); [email protected] (X.Y.); [email protected] (H.T.) State Key Laboratory of Explosion & Impact and Disaster Prevention & Mitigation, Army Engineering University of PLA, Nanjing 210007, China; [email protected] Engineering Design and Research Institute of PLA Army Research Institute, Beijing 100000, China Correspondence: [email protected] (Z.C.); [email protected] (X.Z); Tel.: +86-25-8082-5101 (Z.C.); +86-25-8082-5399 (X.Z.)

Received: 16 June 2018; Accepted: 27 August 2018; Published: 7 September 2018

Abstract: Reasonable design of the overburden thickness of underground space (OTUS) can influence the outdoor thermal environment by affecting the ground plant communities. To optimize the design of the OTUS for improving the outdoor thermal environment, this study summarized the influence mechanism of the OTUS on the outdoor thermal environment and proposed a framework of the optimization design of underground space overburden thickness. A typical row layout residential area in Nanjing, China, was taken as the research object on which to perform a numerical study of the influence of plant communities formed by two types of plant collocations (a middle- and low-level plant collocation and a middle- and high-level plant collocation) on the outdoor thermal environment (airflow field, air temperature, relative humidity and thermal comfort) under three different ratios of trees to shrubs (2:3, 1:2, and 1:3), and to provide suggestions regarding the design of the OTUS according to the designer’s requirements. The conclusions were summarized as follows: (1) If a designer wants to enhance outdoor ventilation, the OTUS should be designed to satisfy the requirements for the middle- and low-level plant collocations and the overburden thickness of the 2/5 underground space development area should be set to 80~100 cm, the overburden thickness of the other 2/5 area should be set to 45~60 cm and the overburden thickness of the remaining 1/5 area should be set to 30~45 cm. (2) If a designer wants to reduce air temperature, increase relative humidity, and improve outdoor thermal comfort, the OTUS should be designed to satisfy the requirements for middle- and high-level plant collocations and the overburden thickness of the 1/4 underground space development area should be set to 80~100 cm, and the overburden thickness of the remaining 3/4 area should be set to 45~60 cm. Keywords: underground space overburden thickness; residential area; plant collocation; outdoor thermal environment; ENVI-met

1. Introduction In recent years, the urban thermal environment has been deteriorating due to China’s urbanization, and the urban heat island effect is the most pronounced in summer, especially in residential areas [1–3]. The heat island effect causes many problems, which can decrease outdoor thermal comfort, influence

Sustainability 2018, 10, 3205; doi:10.3390/su10093205

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Sustainability 2018, 10, 3205

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the outdoor activities of urban residents [4,5], increase the energy consumption of buildings [2], and even increase the risk of heat-related death due to heat waves [6–9]. It has been reported that the heat waves that swept across Europe in 2003 caused approximately 20,000 deaths in Britain, France, Italy, and Portugal [9,10]. To address these problems, various easing measures have been proposed [2], and landscape greening is accepted as the most effective way to ease the heat island effect [2,11], which can provide shade and contribute to reducing the surface temperature of buildings and the ground [1,4]. In this respect, Ooka used the multi-objective genetic algorithm and coupled simulation to optimize the tree design for a comfortable outdoor environment [12]. Bo Hong used numerical simulation to optimize the tree design for sunshine and ventilation [11]. Li Zhang used the ENVI-met model to investigate the effects of tree distribution and species on outdoor environments [13]. These studies had a positive effect on easing the urban heat island effect, however, which neglected the influence of underground space development on ground greening and the outdoor thermal environment. At present, residential areas have developed underground space on a large scale to free up more land for landscape greening, especially in China [14,15]. The growth environment of plants above the underground space is different from that under natural conditions. In areas with underground space development, the overburden thickness of underground space (OTUS) is a vital part of the landscape design above underground buildings [14]. The OTUS refers to the soil thickness used for plant growth between the underground building and the ground. If the OTUS is too thin to satisfy the requirements of growth for trees or shrubs, this will affect the formation of plant communities, resulting in a single landscape design, which will affect not only the landscape’s diversity but also the survival of plants. However, few studies have concerned with how to properly design the OTUS to pursue a comfortable outdoor environment. In our previous studies, we chose a residential area in Nanjing, China as the research subject and quantified the effects of three kinds of vegetation, lawn, large shrubs, and small trees, on the outdoor thermal environment and suggested, according to the simulation results, that the OTUS was best designed to satisfy the survival requirements of small trees would contribute to creating a comfortable outdoor environment [14]. The study provided preliminary data support for the design of the OTUS. However, the greening configurations considered in this study were relatively few and idealized. The effects of plant communities formed by different plant collocations under different ratios of trees to shrubs on the outdoor thermal environment were not taken into account. Landscape design above underground buildings, reasonable plant collocation, and an appropriate ratio of trees to shrubs can not only make full use of the space resources, form a layered landscape, and increase the visual beauty of the landscape, but can also form a multilayered plant community, improve biodiversity, and benefit the ecology [16,17]. Therefore, it is necessary and more meaningful to further quantify the effects of plant communities with different ratios of trees to shrubs on the outdoor thermal environment under different OTUS values. The purpose of this article was to investigate the optimization of the design of the OTUS to improve the quality of the outdoor thermal environment according to the designer’s different requirements. The influence mechanism of the OTUS on the outdoor thermal environment and a framework for the optimization design of the underground space overburden thickness were proposed in this study. We chose a residential area in Nanjing, China with a typical row layout as the research object. Considering different ratios of trees to shrubs, we used the computational fluid dynamics (CFD) simulation software ENVI-met to quantify the influence of plant communities formed by middle- and high-level plant collocations and middle- and low-level plant collocations on the outdoor thermal environment from four aspects: airflow field, air temperature, relative humidity and thermal comfort. In addition, we developed some reasonable suggestions for designing the OTUS according to the designer’s different requirements.


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2. Methodology

2. Methodology 2.1. Influence Mechanism of the OTUS on Outdoor Thermal Environment 2.1. Mechanismthe of the OTUSthermal on Outdoor Thermal Environment TheInfluence OTUS influences outdoor environment by affecting the ground plant communities, as shownThe in Figure 1. First, the ground plant communities, formedbybyaffecting a varietythe of plants OTUS influences the outdoor thermal environment groundthrough plant different plant collocations, will depend on the design of the OTUS. In landscape design above communities, as shown in Figure 1. First, the ground plant communities, formed by a variety of plants underground buildings, requirements of the OTUS theOTUS. orderInoflandscape grasses, design shrubs, through different plantthe collocations, will depend on the ascend design ofinthe and above trees underground [14]. For example, when OTUS is of in the theOTUS rangeascend of 10~30 cm, land only buildings, the the requirements in the order of plants grasses,can shrubs, and trees [14]. For example, when the OTUS is in the range of 10~30 cm, land plants can only be be planted in the underground space development area, and only when the OTUS is in the range of 120 thetrees underground space development area, and when requirements the OTUS is inof the range oflocal 120 ~150planted cm canin big be planted. If the OTUS can satisfy theonly survival trees, the cm can bigcan trees planted. If the OTUS can satisfy theother survival requirements ofcan trees, thesatisfy local plant~150 communities bebe created with more diversity. On the hand, if the OTUS only plant communities can be created with morewill diversity. On thelow. other hand, if the OTUS can only the survival of shrubs or lawn, the biodiversity be relatively satisfy the survival of shrubs or lawn, the biodiversity will be relatively low. influence the outdoor Second, the ground plant community controlled by the OTUS will directly Second, the ground plant community controlled by the OTUS will directly influence the the radiative outdoor thermal environment. First, high trees and large shrubs can block solar radiation, reducing thermal First,ofhigh trees and large shrubs the canheat block solar from radiation, reducingtothe heating of theenvironment. external surfaces buildings, in turn reducing transfer the buildings the radiative heating of the external surfaces of buildings, in turn reducing the heat transfer from the surrounding environment [18]; Additionally, the plant canopy can reduce wind velocity [2,14]. Second, buildings to the surrounding environment [18]; Additionally, the plant canopy can reduce wind as the height of the shrubs is close to the height of pedestrians, the evapotranspiration of shrubs velocity [2,14]. Second, as the height of the shrubs is close to the height of pedestrians, the can consume radiant heat and affect the energy distribution at pedestrian height [14,19]. In addition, evapotranspiration of shrubs can consume radiant heat and affect the energy distribution at terrestrial plants, through photosynthesis and transpiration, can reduce the amount of solar radiation pedestrian height [14,19]. In addition, terrestrial plants, through photosynthesis and transpiration, absorbed by the ground and enhance soil heat dissipation, thus reducing the heat from the can reduce the amount of solar radiation absorbed by the ground and enhance soil transfer heat dissipation, landthus to the surrounding environment [14,20–22]. reducing the heat transfer from the land to the surrounding environment [14,20–22]. Ground level

Plant

Height (m)

0 15 Land plant

0.2—1

Small shrub

1—1.5

Big shrub

1.5—3

30

OTUS (cm)

45 60

Ground plant community

⚫ Air flow

determine

80 Small tree

6—10

100

Outdoor thermal environment

Ground plant community formed by different plant collocations

⚫ Air temperature

influence ⚫ Relative humidity ⚫ Mean radiation temperature

120 Big tree

20—30

150

Figure 1. The influence process of the OTUS outdoorthermal thermalenvironment environment (picture (picture source: Figure 1. The influence process of the OTUS onon outdoor source: author author self-drawing). self-drawing).

2.2. Optimization Design Framework 2.2. Optimization Design Framework Currently, in the field of urban microclimate research, simulation Currently, in the field of urban microclimate research,the theapplication application of of numerical numerical simulation methods has become increasingly widespread [2,11,12,23]. In thiswe study, we used numerical methods has become increasingly widespread [2,11,12,23]. In this study, used numerical simulation simulation methods optimize theOTUS design the OTUS improve the outdoor thermal methods to optimize theto design of the toofimprove thetoquality of the quality outdoorofthermal environment. environment. According to the above theoretical analysis, we summarized a framework the According to the above theoretical analysis, we summarized a framework of the optimization of design optimization design of thethermal OTUS for the outdoor thermal environment on the designer’s of the OTUS for the outdoor environment based on the designer’sbased different requirements, different requirements, as shown in Figure 2. The framework is composed of four parts. as shown in Figure 2. The framework is composed of four parts. (1) Setting of the problem. In this stage, optimal design objective was optimize the designof (1) Setting of the problem. In this stage, thethe optimal design objective was to to optimize the design of the OTUS to enhance the ventilation or improve outdoor thermal comfort, and the evaluation the OTUS to enhance the ventilation or improve outdoor thermal comfort, and the evaluation method method and standard value for choosing the optimal plans candidates were determined. and standard value for choosing the optimal plans candidates were determined. (2) Modeling. This part mainly served as the case design and was composed of four main (2) Modeling. This part mainly served as the case design and was composed of four main elements: the initial boundary conditions, grid size, ground greening configuration, and building elements: the initial boundary conditions, grid size, ground greening configuration, and building model. The initial boundary conditions mainly included the wind velocity, wind direction, initial model. The initial boundary conditions mainly included the wind velocity, wind direction, initial atmospheric temperature, outdoor atmospheric pressure, and relative humidity. The grid size


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atmospheric temperature, outdoor atmospheric pressure, and relative humidity. The grid size included Sustainability 2018, 10, x FOR PEER REVIEW 4 of 15 the grid number and grid step. The grid number determined the range of the simulation area, and the grid step determines the spatial Thenumber building model included theof building materials, included the grid number and grid grid resolution. step. The grid determined the range the simulation building height, building orientation,the etc.spatial The ground greening configuration has a direct influence area, and the grid step determines grid resolution. The building model included the materials, building height, building orientation, etc. TheOTUS. groundItgreening configuration onbuilding the outdoor thermal environment and is determined by the should be noted that has in the a direct influence on the outdoor thermal and determined by the OTUS. It should modeling process, the OTUS was the onlyenvironment variable used in is the optimization study of this paperbe and noted that in the modeling the modeling OTUS wasprocess, the onlythus, variable the optimization study it is impossible to be shown process, during the we used usedin different ground greening of this paper to and it is impossible to be shown configurations represent the different OTUS.during the modeling process, thus, we used different ground greening configurations to represent the different OTUS. (3) Simulation and analysis. Here, ENVI-met was adopted for the numerical study. This program (3) Simulation and analysis. Here, ENVI-met was adopted for the numerical program mainly consisted of an atmospheric model, a soil model, a vegetation model, study. and a This ground surface mainly consisted of an atmospheric model, a soil model, a vegetation model, and a ground surface model [24], and its applicability was validated by field measurements in our previous study [14]. model [24], and its applicability was validated by field measurements in our previous study [14]. We We obtained the indexes of wind velocity, air temperature, relative humidity, and mean radiation obtained the indexes of wind velocity, air temperature, relative humidity, and mean radiation temperature (MRT) through ENVI-met, and calculated the average and time-averaged values of these temperature (MRT) through ENVI-met, and calculated the average and time-averaged values of these indexes to analyze the changes in the outdoor thermal environment. indexes to analyze the changes in the outdoor thermal environment. (4) Evaluation. The effects of plant communities formed by middle- and high-level plant (4) Evaluation. The effects of plant communities formed by middle- and high-level plant collocations and middle- and low-level plant collocations with different ratios of trees to shrubs on the collocations and middle- and low-level plant collocations with different ratios of trees to shrubs on outdoor thermal environment were studied. The optimal greening configuration could be acquired the outdoor thermal environment were studied. The optimal greening configuration could be according to the designer’s different requirements. According to the corresponding relationship acquired according to the designer’s different requirements. According to the corresponding between the OTUS and in Section in 2.1, the OTUS to the to optimal relationship between theplants OTUSmentioned and plants mentioned Section 2.1, thecorresponding OTUS corresponding the greening configuration was the optimal one. So far, the optimization design of the OTUS for optimal greening configuration was the optimal one. So far, the optimization design of the OTUS forthe outdoor thermal environment has been carried out.out. the outdoor thermal environment has been carried (1) Setting of problem

⚫ Design object

(2) Modeling

⚫ Initial boundary conditions ⚫ Grid size

⚫ Evaluation method and standard value for optimal design

⚫ Ground greening configuration

⚫ Building model Designer

(3) Simulation and analysis

⚫ Numerical simulation of outdoor thermal using ENVI-met

⚫ Outdoor wind velocity, air temperature, relative humidity and MRT were obtained

(4) Evaluation ⚫ Outdoor thermal environment evaluation for every greening configuration ⚫ Acquire optimal greening configuration according to designer’s needs

Optimization design of the OTUS

Figure 2. 2.The design of of underground undergroundspace spaceoverburden overburden thickness Figure Theframework frameworkof ofthe the optimization optimization design thickness (picture source: author self-drawing). (picture source: author self-drawing).

3. Case Study 3. Case Study 3.1. Optimization Design Object 3.1. Optimization Design Object The purpose of this research was to optimize the design of the OTUS for the outdoor thermal The purpose of this research was to optimize the design of the OTUS for the outdoor thermal environment. In the summer, people prefer to enhance outdoor ventilation, reduce air temperature, environment. In the summer, people prefer to enhance outdoor ventilation, reduce air temperature, as as well as as improve outdoor relative humidity and outdoor well improve outdoor relative humidity and outdoorthermal thermalcomfort. comfort.The Theoptimal optimaldesign designofofthe OTUS was investigated according to designer’s different requirements. the OTUS was investigated according to designer’s different requirements. 3.2.3.2. Case Setup Case Setup For landscape buildings,designers designersusually usuallychoose choose a method that For landscapedesign designabove above underground underground buildings, a method that combines trees, shrubs, and grasses to build a rich plant community, therefore creating a beautiful combines trees, shrubs, and grasses to build a rich plant community, therefore creating a beautiful landscape with positive ecological landscape with positive ecologicaleffects. effects.InInthis thisstudy, study,the theground groundgreening greeningconfigurations configurations formed formed by middleand high-level plant collocations and middleand low-level plant collocations were mainly by middle- and high-level plant collocations and middle- and low-level plant collocations were considered, as shown in Figure 3. According to engineering experience, designers usually choose mainly considered, as shown in Figure 3. According to engineering experience, designers usually shallow-rooted small trees, and the best planting locations correspond the structural columns of the choose shallow-rooted small trees, and the best planting locations to correspond to the structural columns of the underground building. underground building.


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The most commonly used size of an underground building column grid in Nanjing is The used size of an underground buildingthe column in Nanjing is 8.1 m ×building 8.1 8.1 m × 8.1 mmost [25].commonly In this study, to facilitate the simulation, size grid of the underground m [25]. In this study, to facilitate the simulation, the size of the underground building column grid column grid was set to 8 m × 8 m, and a residential area with a row layout and underground parking in wasSustainability set to 8 m2018, m, andPEER a residential area with a row layout and underground parking in Nanjing, 10, x FOR REVIEW of 15 Nanjing, China was× 8used as the research object. The reason for choosing a residential area5 in Nanjing, China was used as the research object. The reason for choosing a residential area in Nanjing, China China as the research object is that the heat island effect in Nanjing has become increasingly serious in as the research object is thatused the heat effect in Nanjing has become increasingly serious in×recent The most commonly size island of an underground building column grid in Nanjing is 8.1 m 8.1 recentyears, years, andthe the outdoor air temperature can exceed 40 ◦ofCthe [22]; in addition, scale the scale of underground m [25]. this study, to the simulation, grid andIn outdoor airfacilitate temperature can exceedthe 40size °C [22]; inunderground addition, thebuilding ofcolumn underground spacespace development withwith the with development ofand the economy and is is anticipated wasdevelopment set to 8 mis× growing 8 m, and aalong residential area a row layout underground parking in Nanjing,totoreach is growing along the development of the economy and anticipated 2 2 by 52,000,000 m by 2020 to the urban offor Nanjing [15]. China was used as the research object. reason choosing a residential area in Nanjing, China reach 52,000,000 maccording 2020 according toThe theplan urban plan of Nanjing [15]. as the research object is that the heat island effect in Nanjing has become increasingly serious in recent years, and the outdoor air temperature can exceed 40 °C [22]; in addition, the scale of underground space development is growing along with the development of the economy and is anticipated to reach 52,000,000 m2 by 2020 according to the urban plan of Nanjing [15].

(a)

(b)

Figure 3. Two common plant collocationsabove aboveunderground underground buildings. (a)(a) Middleandand high-level Figure 3. Two common plant collocations buildings. Middlehigh-level plant collocations; (b) middleand low-level plant collocations. (Picture source: Author self-drawing). plant collocations; (b) middle- and low-level plant collocations. (Picture (a) (b) source: Author self-drawing). Figure 3. Two aimed collocations underground buildings. (a) Middle- andthe high-level This research toplant optimize the above design ofOTUS the OTUS by investigating relationship This research aimedcommon to optimize the design of the by investigating the relationship between (b) middleandcollocations, low-level plantand collocations. (Picture source: Author self-drawing). betweenplant thecollocations; OTUS, ground plant outdoor thermal environment; therefore, six the OTUS, ground plant collocations, and outdoor thermal environment; therefore, six configurations configurations were analyzed. In the modeling stage, the grid number (X × Y × Z) was set to 80 m × were analyzed. the modeling thethe grid number (XOTUS × Y ×byZ)investigating was set to 80 × 80 m × 30 m, This In research aimed tostage, optimize design of the the m relationship 80 m × 30 m, and the grid step (X × Y × Z) was set to 1 m × 1 m × 7.5 m. Each greening configuration between the OTUS, ground plant collocations, and outdoor thermal environment; therefore,formed six and the grid step (X × Y × Z) was set to 1 m × 1 m × 7.5 m. Each greening configuration formed by different plants corresponded to a kind of underground space overburden thickness that by configurations were analyzed. In the modeling stage, the grid number (X × Y × Z) was set to 80 m × different plants toofa growth kind offor underground overburden thickness that could could meet corresponded the requirements plants. We setspace six plant collocations (Figure 4). The plantmeet 80 m × 30 m, and the grid step (X × Y × Z) was set to 1 m × 1 m × 7.5 m. Each greening configuration the requirements growth forwere plants. We set sixlow-level plant collocations (Figure 4). The planttocollocations collocations ofofFigure 4a–c middleand plant collocations corresponding tree to formed by different plants corresponded to a kind of underground space overburden thickness that of Figure 4a–c were middleand low-level plant collocations corresponding to tree to shrub ratios of shrub ratios of 2:3, 1:2, and 1:3. The plant collocations of Figure 4d–f were middleand high-level could meet the requirements of growth for plants. We set six plant collocations (Figure 4). The plant plant collocations, corresponding tree toand shrub ratios of 2:3, 1:2, and 1:3. The relevant parameters 2:3, 1:2, and 1:3. The collocations of Figure 4d–f were middleand high-level plant collocations, collocations of plant Figure 4a–c were to middlelow-level plant collocations corresponding to tree to of the vegetation, model, and the boundary conditions shown in Table 1. It vegetation, should corresponding to tree to 1:2, shrub ratios ofplant 2:3,initial 1:2, and 1:3.of The relevant parameters ofhigh-level the shrub ratios of building 2:3, and 1:3. The collocations Figure 4d–f are were middleand be noted that each plant in each greening configuration occupied an area of 1 square meter. Thus, the that plant collocations, corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3. The relevant parameters building model, and the initial boundary conditions are shown in Table 1. It should be noted proportion of the number of different types ofboundary plants was equivalent to the proportion of the of the vegetation, building model, and the initial conditions are shown in Table 1. It should each plant in each greening configuration occupied an area of 1 square meter. Thus, the proportion of underground space area occupied by plants. be noted that each development plant in each greening configuration occupied an area of 1 square meter. Thus, the the number of different types of plants was equivalent to the proportion of the underground space proportion of the number of different types of plants was equivalent to the proportion of the development area occupied by plants.area occupied by plants. underground space development

(a) (a)

(b) (b)

Figure 4. Cont.

(c) (c)


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(d)

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(e)

(f)

4. Models of plant collocations under different ratios ratios of to to shrubs. (a–c)(a–c) were were middleand Figure 4.Figure Models of plant collocations under different oftrees trees shrubs. middleand low-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d–f) were low-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d–f) were middlemiddle- and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3. and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3. Each greening Each greening configuration corresponded to a kind of underground space overburden thickness that configuration corresponded to a kind of underground space overburden thickness that could meet the could meet the requirements of growth for plants (picture source: ENVI-met). requirements of growth for plants (picture source: ENVI-met). Table 1. Vegetation and initial boundary conditions parameters.

Table 1. Vegetation and initial boundary conditions parameters. Parameter

Definition

Parameter

Definition

Vertical trees

Vertical trees

Transverse trees

Vegetation parameters Vegetation parameters

Transverse trees Large shrubs

Large shrubs Small shrubs

Small shrubs Lawn

Lawn

Building dimensions Building model

Building dimensions Building material

Building model

Building color Building material Wind velocity Building color(m/s) Wind direction (°) Wind velocity (m/s) Initial atmospheric temperature (K) Wind direction (◦ ) Outdoor atmospheric pressure (Pa) Initial atmospheric temperature (K) Relative humidity (%)

Initial boundary conditions (typical weather in summer)

Initial boundary conditions (typical weather in summer) 3.3. Evaluation Index

Outdoor atmospheric pressure (Pa) Relative humidity (%)

Values 5 m × 5Values m × 10 m (L × W × H) 5m×5m × 10 m 7 m(L× × 7m 6m W×× H) 7(L m××W7×mH)× 6 m 3 m(L× × 3m 2m W×× H) (L × W × H) 3m×3m×2m 1 m(L×1×mW× × 1m H) W1 ×mH) 1(Lm×× ×1m (L 0.2× mW (H)× H) m ×(H) 30 m × 0.2 15 m 18 m (L × W × H) 30 m × 15 m × 18 m Concrete (L × W × H) Gray Concrete 2.4 Gray 157.5 2.4 294.95 157.5 100,250 294.95 80

100,250 80

Usually, 3.3. Evaluation Indexthe indexes of wind velocity, air temperature, and relative humidity can directly reflect

the changes in the outdoor thermal environment. However, these indexes cannot accurately evaluate

outdoor thermal comfort. For the evaluation of outdoor thermal thehumidity index of mean Usually, the indexes of wind velocity, air temperature, andcomfort, relative can radiation directly reflect temperature (MRT) was used in this study. the changes in the outdoor thermal environment. However, these indexes cannot accurately evaluate The MRT refers to the surface temperature of an imaginary isothermal enclosed surface where outdoor thermal comfort. For the evaluation of outdoor thermal comfort, the index of mean radiation the radiant heat exchange capacity from the human body is equal to the actual amount of radiant temperature (MRT) was used in this study. heat exchange between the human body and the actual non-isothermal surface [26]. In addition, the TheMRT MRTisrefers the surface temperature an imaginary isothermal surface the a keytofactor in evaluating human of outdoor thermal comfort and enclosed it considers both where the radiant heat exchange capacity from the human body is equal to the actual amount of radiant heat shortwave and long-wave radiation flux that the human body absorbs. On a sunny day, regardless the comfort used, the MRT is considered as the key variable in evaluating thermal exchangeofbetween theindices human body and the actual non-isothermal surface [26]. Inoutdoor addition, the MRT is sensation [27]. a key factor in evaluating human outdoor thermal comfort and it considers both the shortwave and long-wave radiation flux that the human body absorbs. On a sunny day, regardless of the comfort indices used, the MRT is considered as the key variable in evaluating outdoor thermal sensation [27]. Studies have shown that human discomfort caused by strong sunlight is much greater than that caused by an increase in the average air temperature [28], and the change in comfort caused by an increase of 1 ◦ C in air temperature can be offset by a radiance decrease of approximately 70 W/m2 [29]. In the summer, the solar radiance in the outdoor environment of Nanjing is approximately l000 W/m2 , equivalent to an increase of 14 ◦ C in air temperature; therefore, when compared to air temperature,


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Studies have shown that human discomfort caused by strong sunlight is much greater than that caused by an increase in the average air temperature [28], and the change in comfort caused by an increase of 1 °C in air temperature can be offset by a radiance decrease of approximately 70 W/m2 Sustainability 2018, 10, 3205 7 of 15 [29]. In the summer, the solar radiance in the outdoor environment of Nanjing is approximately l000 W/m2, equivalent to an increase of 14 °C in air temperature; therefore, when compared to air temperature, which may exhibit the littleMRT variance, the MRT can thethermal actual human thermal which may exhibit little variance, can better reflect thebetter actualreflect human sensation in an sensation in an outdoor thermal environment. In addition, MRT was it has outdoor thermal environment. In addition, the index of MRT the wasindex used of because it hasused beenbecause widely used been widely used in evaluating outdoor thermal environments and could satisfy the requirements of in evaluating outdoor thermal environments and could satisfy the requirements of our research [14]. ourmore research [14]. on the MRT see References [30,31]. For details onFor themore MRTdetails see References [30,31]. 4.4.Results Resultsand andDiscussion Discussion 4.1. 4.1.Airflow AirflowField Field Figure Figure55shows showsthe thechanges changesin inwind windvelocity velocityfor forthe thetwo twoplant plantcollocations collocationsunder underdifferent differentratios ratios of trees to shrubs (1.5 m above ground, 15:00). In all configurations, the outdoor pedestrian wind of trees to shrubs (1.5 m above ground, 15:00). In all configurations, the outdoor pedestrian wind fields change in in wind wind velocity velocityfrom fromthe theaverage averagewind windvelocity velocitywas wasininthe the range fieldswere were similar. similar. The The change range of of 0.005~0.014 m/s. contrast, spatialdistribution distributionofofthe the outdoor outdoor pedestrian pedestrian wind wind field 0.005~0.014 m/s. In In contrast, thethespatial field was was significantly thethe building layout. Buildings blockblock the spread of airflow, and a wind significantlyaffected affectedbyby building layout. Buildings the spread of airflow, and shadow a wind forms at the back of buildings, weakening the airflow from the southeast. In addition, a narrow pipea shadow forms at the back of buildings, weakening the airflow from the southeast. In addition, effect waspipe created in the direction due to adjacent buildings; this effectbuildings; increasedthis the wind narrow effect wasnorth–south created in the north–south direction due to adjacent effect velocity, promoting air flow circulation. increased the wind velocity, promoting air flow circulation. ＜0.04

0.38

0.72

1.06

1.40

1.74

2.08

2.42

2.76

＞3.11

(m/s)

(a)

(b)

(c)

Average value: 0.986

0.985

0.977

(d)

(e)

(f)


0.976

0.972

Figure in in wind velocity for two plantplant collocations underunder different ratios of treesof to trees shrubs Figure5.5.The Thechanges changes wind velocity for two collocations different ratios to (1.5 m above 15:00); (a–c) were middlelow-level plantand collocations corresponding to tree shrubs (1.5 ground, m above ground, 15:00); (a–c)and were middlelow-level plant collocations to shrub ratios of 2:3, 1:2, and 1:3; (d–f) were middle- and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: ENVI-met).

The time-averaged values of the average wind velocity for each configuration were obtained by averaging the wind velocity values at nine time points from 8:00 to 16:00 (see Figure 6). When the ratio

corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d–f) were middle- and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: ENVI-met).

The time-averaged Sustainability 2018, 10, 3205values of the average wind velocity for each configuration were obtained by 8 of 15 averaging the wind velocity values at nine time points from 8:00 to 16:00 (see Figure 6). When the ratio of trees to shrubs was 1:3, the pedestrian wind velocity was the lowest among the six of trees to shrubs was 1:3, theaspedestrian wind velocityincreased, was the lowest among the six configurations, configurations, indicating that the number of shrubs the space environment became indicating that as the number of shrubs increased, the space environment became whichofwas crowded, which was not conducive to introducing air flow to the pedestrian level orcrowded, to the spread conducive to the introducing aireffects flow toofthe pedestrian level or to the of air flow. airflow Moreover, air not flow. Moreover, weakening large shrubs at a height of 2spread m on pedestrian the weakening effects of large shrubs at a height of 2 m on pedestrian airflow may be more pronounced may be more pronounced than those of smaller shrubs. In addition, the time-averaged value of the than those smaller for shrubs. In addition, the time-averaged value of the lower average wind velocities average wind of velocities middleand high-level plant collocations were than those for for middleand high-level plant collocations were lower than those for middleand low-level plant middle- and low-level plant collocations under the same ratio of trees to shrubs. The results showed collocations under the same ratio of trees to shrubs. The results showed that if a designer wanted that if a designer wanted to enhance outdoor ventilation, the OTUS should be designed to satisfy the to enhance outdoor ventilation, the OTUSplant should be designed to satisfy the requirements middlerequirements for middleand low-level collocations, and the improvement effectsfor were mostand low-level plant collocations, and the improvement effects were most obvious when the ratio trees obvious when the ratio of trees to shrubs was 2:3. Thus, the overburden thickness of theof2/5 to shrubs was 2:3. Thus, the overburden thickness of the 2/5 underground space development underground space development area should be set to 80~100 cm, the overburden thickness of thearea should be set to 80~100 cm,tothe overburden of the other 2/5 area should be set to1/5 45~60 other 2/5 area should be set 45~60 cm, andthickness the overburden thickness of the remaining areacm, and the overburden thickness of the remaining 1/5 area should be set to 30~45 cm. should be set to 30~45 cm.

Figure 6. Time-averaged value of the average wind velocity for each configuration. (a–c) were middleFigure Time-averaged value of the corresponding average wind velocity forshrub each configuration. (a–c) were and6.low-level plant collocations to tree to ratios of 2:3, 1:2, and 1:3;middle(d–f) were andmiddlelow-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d–f) and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2,were and 1:3 middleand high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: Author self-drawing). (picture source: Author self-drawing).

4.2. Air Temperature 4.2. Air Temperature Figure 7 shows the changes in air temperature for the two plant collocations under different ratios ofFigure trees to7shrubs above ground, 15:00). In thefor underground space development area, the values shows(1.5 the m changes in air temperature the two plant collocations under different of air temperature in (1.5 Figure 7a–c were clearly higher than those in Figure 7d–f. The change in air ratios of trees to shrubs m above ground, 15:00). In the underground space development area, ◦ C and 0.023~0.029 ◦ C for the average air temperature in the rangethan of 0.005~0.014 thetemperature values of airfrom temperature in Figure 7a–c werewas clearly higher those in Figure 7d–f. The change and low-level collocations and middle-was andin high-level plant in the air middletemperature from theplant average air temperature the range of collocations, 0.005~0.014 respectively. °C and With the °C increase in middlethe ratio and of trees to shrubs, thecollocations air temperature at the pedestrian level tended 0.023~0.029 for the low-level plant and middleand high-level plant to decrease, which means that increased of shrubs is conducive reducing the air temperature, collocations, respectively. Withanthe increasenumber in the ratio of trees to shrubs,tothe air temperature at the thus mitigating the heat island effect. pedestrian level tended to decrease, which means that an increased number of shrubs is conducive to reducing the air temperature, thus mitigating the heat island effect.


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＜26.04

26.16

26.29

26.42

9 of 15

26.54

26.67

26.79

26.92

27.05

＞27.17

(°C)

(a)

(b)

(c)


26.671

26.657

(d)

(e)

(f)


26.581

26.558

Figure 7. 7.The in air airtemperature temperature plant collocations different Figure Thechanges changes in forfor thethe twotwo plant collocations underunder different ratios ofratios trees of trees shrubs aboveground, ground,15:00). 15:00).(a–c) (a–c) weremiddlemiddle-and andlow-level low-levelplant plantcollocations collocations to to shrubs (1.5(1.5 m mabove were correspondingtototree treetotoshrub shrubratios ratiosof of 2:3, 2:3, 1:2, 1:2, and 1:3; (d–f) plant corresponding (d–f) were weremiddlemiddle-and andhigh-level high-level plant collocations correspondingtototree treetotoshrub shrubratios ratios of of 2:3, 2:3, 1:2, and collocations corresponding and 1:3 1:3 (picture (picturesource: source:ENVI-met). ENVI-met).

In In addition, airtemperature temperature middlehigh-level collocations addition,the thepedestrian pedestrian air forfor thethe middleand and high-level plant plant collocations was lower than that forfor thethe middleandand low-level plant collocations under the same ratio ratio of trees to was lower than that middlelow-level plant collocations under the same of trees difference may be that the the middleand and high-level plant plant collocations can to shrubs. shrubs. The Thereason reasonfor forthis this difference may be that middlehigh-level collocations block solar radiation and is is conducive to to reducing thethe canprovide providemore moreshade, shade,which whichcan caneffectively effectively block solar radiation and conducive reducing pedestrian-level air temperature. pedestrian-level air temperature. The time-averagedaverage averageair airtemperatures temperatures for for each byby averaging The time-averaged each configuration configurationwere wereobtained obtained averaging the average air temperature values at nine time points from 08:00 to 16:00 (see Figure 8). thethe the average air temperature values at nine time points from 08:00 to 16:00 (see Figure 8).For For middleand low-level plant collocations, the value in Figure 8c was the lowest, and was 0.025 °C and ◦ middle- and low-level plant collocations, the value in Figure 8c was the lowest, and was 0.025 C and 0.008 °C lower than those in Figure 8a,b, respectively. For the middle- and high-level plant 0.008 ◦ C lower than those in Figure 8a,b, respectively. For the middle- and high-level plant collocations, collocations, the time-averaged average air temperature in Figure 8f was the lowest, and was 0.016 the time-averaged average air temperature in Figure 8f was the lowest, and was 0.016 ◦ C and 0.013 ◦ C °C and 0.013 °C lower than those in Figure 8d,e, respectively. lower than those in Figure 8d,e, respectively. In addition, the values from Figure 8d–f were all lower than those from Figure 8a–c. This result In addition, the values from Figure 8d–f were all lower than those from Figure 8a–c. This result indicates that if a designer wants to reduce air temperature, the OTUS should be designed to satisfy indicates that if a designer wants to reduce air temperature, the OTUS should be designed towere satisfy the requirements for the middleand high-level plant collocations, as the improvement effects themost requirements for the middleand high-level plant collocations, as the improvement effects were obvious when the ratio of trees to shrubs was 1:3. Thus, the overburden thickness of the 1/4 most obvious when ratio of trees toshould shrubsbewas Thus,cm, theand overburden thickness of theof 1/4 underground spacethe development area set 1:3. to 80~100 the overburden thickness underground space development area should be set to 80~100 cm, and the overburden thickness of the the remaining 3/4 area should be set to 45~60 cm. remaining 3/4 area should be set to 45~60 cm.

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Figure 8. 8. Time-averaged Time-averaged value value of of the the average average air temperature for each configuration. (a–c) (a–c) were were Figure middle- and and low-level low-level plant collocations corresponding to tree to shrub ratios ofof2:3, 1:2, and 1:3; (d–f) middleplant collocations corresponding to tree to shrub ratios 2:3, 1:2, and 1:3; (d– Figure 8. Time-averaged value of the average air temperature for each configuration. (a–c) were were middleand high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2,1:2, andand 1:3 f) were middleand high-level plant collocations corresponding to tree to shrub ratios of 2:3, middle- and low-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d– (picture source: Author self-drawing). 1:3 (picture self-drawing). f) weresource: middle-Author and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: Author self-drawing). 4.3. Relative Humidity 4.3. Relative Humidity 4.3. Relative Humidity Figure 9 shows the changes in relative humidity for the two plant collocations under different Figure 9 shows the changes in relative humidity for the two plant collocations under different ratios of trees to shrubs (1.5 m above ground, 15:00). for In the space development Figure 9 shows the in relative the underground two plant collocations under differentarea, ratios of trees to shrubs (1.5changes m above ground,humidity 15:00). In the underground space development area, the values were clearly lower than thosespace in Figure 9d–f. The change ratiosof ofrelative trees to humidity shrubs (1.5inmFigure above 9a–c ground, 15:00). In the underground development area, the values of relative humidity in Figure 9a–c were clearly lower than those in Figure 9d–f. The the values of relative humidity in Figure were clearly than those in Figure 9d–f. in relative humidity from the average relative9a–c humidity was inlower the range of 0.428~0.504% for The the two change in relative humidity from thethe average relative humidity was in the range of 0.428~0.504% for in relative humidity from humidity was in the range of 0.428~0.504% for plant change collocations under the same ratio average of trees relative to shrubs. the two plant collocations under thethe same trees to shrubs. shrubs. theaddition, two plant collocations under sameratio ratioof ofat trees In the relative humidity was always theto highest level for the ratio of trees to shrubs of In addition, the relative humidity was at highestlevel levelfor for the ratio of trees to shrubs In addition, relative wasalways always at the the highest the ratio of trees to shrubs 1:3, indicating that anthe increase inhumidity the number of shrubs increased plant transpiration, thus increasing of 1:3,of indicating thatthat an an increase increasedplant plant transpiration, 1:3, indicating increaseininthe thenumber number of of shrubs shrubs increased transpiration, thusthus the relative humidity level. increasing the relative humidity level. increasing the relative humidity level. The time-averaged values of the average relative humidity for each configuration were obtained The time-averaged values of the averagerelative relativehumidity humidity for were obtained by by The time-averaged values of the average foreach eachconfiguration configuration were obtained by averaging the average relative humidity values at nine time points fromto 08:00 to(see 16:00 (see10). Figure 10). averaging the average relative humidity values at nine time points from 08:00 16:00 Figure For averaging the average relative humidity values at nine time points from 08:00 to 16:00 (see Figure 10). For For the andlow-level low-level plant collocations, the value in Figure 10c highest, was theathighest, at least themiddlemiddle- and plant collocations, the value in Figure least 0.144% the middleand low-level plant collocations, the value in Figure10c 10cwas wasthe the highest, at least 0.144% 0.144% and 0.071% higher than those in Figure 10a,b, respectively. middlehigh-level and 0.071% higher than those in Figure 10a,b, respectively. For For the the middleand and high-level plantplant and 0.071% higher than those in Figure 10a,b, respectively. For the middle- and high-level plant collocations, the value in Figure wasthe thehighest, highest, with with values 0.055% andand 0.075% higher collocations, the value in Figure 10f10f was valuesthat thatwere were 0.055% 0.075% higher collocations, theinvalue in 10d,e, Figurerespectively. 10f was theInhighest, with values that were 0.055% and 0.075% higher than those Figure addition, the values from Figure 10d–f were all higher than than those in Figure 10d,e, respectively. In addition, the values from Figure 10d–f were all higher than those in Figure 10d,e,10a–c. respectively. addition, values from Figure 10d–f were for all higher from Figure Thisindicates resultInindicates thatthe if the OTUS satisfies the requirements those than fromthose Figure 10a–c. This result that if the OTUS satisfies the requirements for thethe design than those from Figure 10a–c. This result indicates that if the OTUS satisfies the requirements for the design of the middleand high-level plant collocations, the pedestrian relative humidity could be of the middle- and high-level plant collocations, the pedestrian relative humidity could be increased designincreased of the middleand high-level plant collocations, the pedestrian relative humidity could effectively, and the improvement effects were most obvious when the ratio of trees to be effectively, and the improvement effects were most obvious when the ratio of trees to shrubs was 1:3. shrubs was 1:3. Thus, thickness of were the 1/4most underground increased effectively, and the the overburden improvement effects obvious space when development the ratio of area trees to Thus, the overburden thickness of the 1/4 underground space development area should be set to to 80~100 and the overburden of the remaining 3/4 area should be set to area shrubsshould was be 1:3.setThus, the cm, overburden thickness thickness of the 1/4 underground space development 80~100 cm,cm. and the overburden thickness of the remaining 3/4 area should be set to 45~60 cm. 45~60 should be set to 80~100 cm, and the overburden thickness of the remaining 3/4 area should be set to 45~60 cm. ＜49.59

＜49.59

49.89

49.74

49.74

(a)

49.89

(a)


50.04

50.04

50.19

50.19

(b)

50.35

50.35

(b)

50.502

50.50

50.50

50.65

50.80

50.65

(c)

50.80

(c)

50.589

Figure 9. Cont.


50.502

＞50.95 (%)

50.589

＞50.95 (%)

Sustainability2018, 2018,10, 10,x3205 Sustainability FOR PEER REVIEW

15 1111ofof 15


(d)

(d)



11 of 15

(e)

(f)

(e)

(f)

50.984

51.093

50.984

51.093

Figure 9. The changes in in relative for two collocations under different of trees to of Figure 9. The changes relativehumidity humidity for plant two plant collocations under ratios different ratios Figureshrubs 9. The changes in relative humidity for(a–c) two were plant collocations under different ratios of trees to (1.5(1.5 m m above ground, 15:00). middleandand low-level plant collocations trees to shrubs above ground, 15:00). (a–c) were middlelow-level plant collocations shrubscorresponding (1.5 m above ground, 15:00). (a–c) were middleand middlelow-level plant collocations to tree to shrub ratiosofof2:3, 2:3,1:2, 1:2, and 1:3; andand high-level plantplant corresponding to tree to shrub ratios and 1:3; (d–f) (d–f)were were middlehigh-level corresponding tocorresponding tree to shrubtoratios 2:3, ratios 1:2, and 1:3; were middleand high-level plant collocations tree toof shrub of 2:3, 1:2,(d–f) and 1:3 (picture source: ENVI-met). collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: ENVI-met). collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3 (picture source: ENVI-met).

Figure 10. Time-averaged value of the average relative humidity for each configuration. (a–c) were middle- and low-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and 1:3; (d– f) were middle- and high-level plant collocations corresponding to tree to shrub ratios of 2:3, 1:2, and Figure10. 10.Time-averaged Time-averagedvalue valueofofthe theaverage averagerelative relativehumidity humidityfor foreach eachconfiguration. configuration.(a–c) (a–c)were were Figure 1:3 (picture source: Author self-drawing).

middle-and andlow-level low-levelplant plantcollocations collocations corresponding corresponding to tree to shrub ratios of 2:3, middle2:3, 1:2, 1:2, and and 1:3; 1:3; (d–f) (d– middleand collocations corresponding to to tree to to shrub ratios of of 2:3,2:3, 1:2,1:2, and 1:3 f)were were middleandhigh-level high-level plant collocations corresponding tree shrub ratios and 4.4. Outdoor Thermal Comfortplant (picture source: Author self-drawing). 1:3 (picture source: Author self-drawing). Figure 11 shows the changes in the MRT for the two plant collocations under different ratios of trees to shrubs (1.5 m above ground, 15:00 p.m.). The MRT was significantly reduced where trees and 4.4. Outdoor Thermal Comfort 4.4. Outdoor Thermal Comfort shrubs were grown due to the cooling effect of greening. However, due to the lack of shade, the Figure 11 shows the changes in the MRT for the two plant collocations under different ratios of improvement effect shrubs on was than that collocations of trees. With under an increase in the ratios ratio Figure 11 shows theofchanges inthe theMRT MRT forweaker the two plant different of trees of to trees shrubs (1.5 m above ground, 15:00 p.m.). The MRT was significantly reduced where trees to shrubs, the average MRT tended to decrease, which means that an increased number of trees to shrubs (1.5 m above ground, 15:00 p.m.). The MRT was significantly reduced where trees and and shrubs iswere grown due to theoutdoor coolingMRT. effect greening. However, due to the lack of shade, conducive In of addition, the design of theto middleand of high-level shrubsshrubs were grown duetotoimproving the cooling effect of greening. However, due the lack shade, the plant collocations was conducive to lowering the outdoor MRTof than thatWith of theanmiddleandin the the improvement effect of more shrubs on the MRT was weaker than that trees. increase improvement effect of shrubs on the MRT was weaker than that of trees. With an increase in the ratio low-level plant collocations. ratio of trees to shrubs, the average MRT tended to decrease, which means that an increased number of

of trees to shrubs, the average MRT tended to decrease, which means that an increased number of shrubs is conducive to improving outdoor MRT. In addition, the design of the middle- and high-level shrubs is conducive to improving outdoor MRT. In addition, the design of the middle- and high-level plant collocations was more conducive to lowering the outdoor MRT than that of the middle- and plant collocations was more conducive to lowering the outdoor MRT than that of the middle- and low-level plant collocations. low-level plant collocations.


12 of 15


＜45.50

49.50

53.50

57.50

12 of 15

61.50

65.50

69.50

73.50

77.50

(a)

(b)

(c)


63.244

62.804

(d)

(e)

(f)


61.480

61.026

＞81.50 (°C)

Figure for two twoplant plantcollocations collocationsunder underdifferent differentratios ratios trees shrubs Figure11. 11.The The changes changes in MRT for ofof trees to to shrubs (1.5 (1.5 m above ground, 15:00. (a–c) were middleand low-level plant collocationscorresponding correspondingtototree treeto m above ground, 15:00. (a–c) were middleand low-level plant collocations toshrub shrubratios ratiosofof2:3, 2:3,1:2, 1:2,and and1:3; 1:3;(d–f) (d–f)were weremiddlemiddle-and andhigh-level high-levelplant plantcollocations collocationscorresponding correspondingto totree treetotoshrub shrubratios ratiosofof2:3, 2:3,1:2, 1:2,and and1:3 1:3(picture (picturesource: source:ENVI-met). ENVI-met).

The values of the MRT for eachfor configuration were obtained averaging Thetime-averaged time-averaged values of average the average MRT each configuration werebyobtained by the average MRT values at nine time points from 08:00 to 16:00 (see Figure 12). The time-averaged averaging the average MRT values at nine time points from 08:00 to 16:00 (see Figure 12). The timevalues of the average fromMRT Figure 12d–f were clearly than those from Figure averaged values of theMRT average from Figure 12d–f werelower clearly lower than those from 12a–c. Figure When the ratio of trees to shrubs was 2:3, the difference in the time-averaged values of the average 12a–c. When the ratio of trees to shrubs was 2:3, the difference in the time-averaged values of the ◦ MRT for the two of plant was 1.501 and°C, theand corresponding differences were average MRT fortypes the two types collocations of plant collocations wasC,1.501 the corresponding differences ◦ ◦ 1.583 and 1.923 when°C thewhen ratios the wereratios 1:2 and 1:3,1:2 respectively. For the sameFor plant were C1.583 °C andC 1.923 were and 1:3, respectively. thecollocations, same plant the time-averaged values of the average MRT tended to decrease as the ratio of trees to shrubs collocations, the time-averaged values of the average MRT tended to decrease as the ratioincreased. of trees to For increased. the middle- and low-level plant collocations, the time-averaged value of the average MRT in shrubs ◦ C and 0.391 ◦ C lower than those in Figure 12a,b, respectively. FigureFor 12cthe wasmiddlethe lowest, least 0.431 andat low-level plant collocations, the time-averaged value of the average MRT For middleand high-level collocations, Figure wasthose the lowest, a total of in the Figure 12c was the lowest,plant at least 0.431 °C the andvalue 0.391in°C lower12fthan in Figure 12a,b, ◦ ◦ 0.462 C and 0.340 lower than those in Figure 12d,e, respectively. Therefore, OTUS respectively. For theCmiddleand high-level plant collocations, the value in Figure the 12f was theshould lowest, bea designed to satisfy the requirements for the middleand high-level plant collocations, which will total of 0.462 °C and 0.340 °C lower than those in Figure 12d,e, respectively. Therefore, the OTUS help to effectively improve the pedestrian-level thermal comfort, and the improvement effects were should be designed to satisfy the requirements for the middle- and high-level plant collocations, most obvious when the ratio ofimprove trees to shrubs was 1:3. Thus, the overburden thickness of the 1/4 which will help to effectively the pedestrian-level thermal comfort, and the improvement underground space development area should be set to 80~100 cm, and the overburden thickness of the effects were most obvious when the ratio of trees to shrubs was 1:3. Thus, the overburden thickness remaining 3/4 area should be set to 45~60 cm. of the 1/4 underground space development area should be set to 80~100 cm, and the overburden

thickness of the remaining 3/4 area should be set to 45~60 cm.



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Figure 12. Time-averaged value of average the average for each configuration. middleFigure 12. Time-averaged value of the MRTMRT for each configuration. (a–c)(a–c) werewere middleand and low-level plant collocations correspondingtototree treeto to shrub ratios and 1:3;1:3; (d–f) werewere middlelow-level plant collocations corresponding ratiosofof2:3, 2:3,1:2, 1:2, and (d–f) and high-level plant collocations corresponding to tree totoshrub ratios of 2:3, 1:2,of and 1:31:2, (picture source: middleand high-level plant collocations corresponding tree to shrub ratios 2:3, and 1:3 Author self-drawing). (picture source: Author self-drawing).

5. Conclusions 5. Conclusions In this study, we investigated the optimization of the design of the OTUS for improving the quality In this study, we investigated the optimization of the design of the OTUS for improving the of the outdoor thermal environment according to the designer’s different requirements. The influence quality of the outdoor thermal environment according to the designer’s different requirements. The mechanism of the OTUS on the outdoor thermal environment and a framework of the optimization influence mechanism of the OTUS on the outdoor thermal environment and a framework of the design of the underground space overburden thickness were proposed. optimization design of the underground space overburden thickness were proposed. We chose a residential area with a row layout in Nanjing, China, as the research object and used We chose a residential area with a row layout in Nanjing, China, as the research object and used the CFD software ENVI-met to quantitatively analyze the influence of plant communities formed the CFD software ENVI-met to quantitatively analyze the influence of plant communities formed by two types of plant collocations (a middle- and low-level plant collocation and a middle- and twoby types of plant collocations (a middle- and low-level plant collocation and a middle- and highhigh-level plant collocation) on the outdoor thermal environment (airflow field, air temperature, level plant collocation) on the outdoor thermal environment (airflow field, air temperature, relative relative humidity and thermal comfort) under three different ratios of trees to shrubs (2:3, 1:2, and 1:3) humidity and thermal comfort) under three different ratios of trees to shrubs (2:3, 1:2, and 1:3) and to and to provide suggestions regarding the design of the OTUS. The results of this study led to the provide suggestions regarding the design of the OTUS. The results of this study led to the following following conclusions. conclusions. building layout exerted a greater influence that of collocations the plant collocations on the TheThe building layout exerted a greater influence than thatthan of the plant on the outdoor outdoor field. Under thetrees same ratio of the trees to shrubs, the middlelow-level plant airflow field.airflow Under the same ratio of to shrubs, middleand low-level plantand collocation was collocation was more conducive to the spread of outdoor airflow than the middleand high-level plant more conducive to the spread of outdoor airflow than the middle- and high-level plant collocation. collocation. However, it was not conducive to reducing air temperature, increasing relative humidity, However, it was not conducive to reducing air temperature, increasing relative humidity, and and improving For the same plant collocation, an in increase in the ratio of improving outdoor outdoor thermal thermal comfort.comfort. For the same plant collocation, an increase the ratio of trees trees to shrubs was not conducive to the spread of outdoor airflow, however, it was conducive to shrubs was not conducive to the spread of outdoor airflow, however, it was conducive to reducing to reducing air temperature, increasing relative humidity, andthe improving the outdoor thermal comfort. air temperature, increasing relative humidity, and improving outdoor thermal comfort. If a designer wants to enhance outdoor ventilation, the OTUS should be designed to satisfy If a designer wants to enhance outdoor ventilation, the OTUS should be designed to satisfy the the requirements for the middleand low-level plant collocations, and the improvement effects are most requirements for the middle- and low-level plant collocations, and the improvement effects are most obvious when theratio ratio of of trees shrubs is 2:3. the overburden thickness of the 2/5ofunderground obvious when the treestoto shrubs is Thus, 2:3. Thus, the overburden thickness the 2/5 space development area should be set to 80~100 cm, the overburden thickness the otherof2/5 underground space development area should be set to 80~100 cm, the overburdenofthickness thearea should be set to 45~60 cm,toand the cm, overburden of the remaining 1/5 area should set to other 2/5 area should be set 45~60 and the thickness overburden thickness of the remaining 1/5 be area 30~45 cm. should be set to 30~45 cm. a designer wants to reduce air temperature, increase relative humidity, improve outdoor If a If designer wants to reduce air temperature, increase relative humidity, andand improve outdoor thermal comfort, OTUS should be designed to satisfy the requirements for middlehigh-level thermal comfort, the the OTUS should be designed to satisfy the requirements for middleandand high-level plant collocations, and the effects are most obvious when the ratio of trees to shrubs is 1:3. Thus, plant collocations, and the effects are most obvious when the ratio of trees to shrubs is 1:3. Thus, the the overburden thickness of the 1/4 underground space development area should be set to 80~100 overburden thickness of the 1/4 underground space development area should be set to 80~100 cm, cm, overburden thickness of the remaining should beto set45~60 to 45~60 andand the the overburden thickness of the remaining 3/4 3/4 areaarea should be set cm. cm. Author Contributions: X.S., Z.C. and X.Z. conceived and designed the study. X.S., X.Y. and H.T. performed the Author Contributions: X.S.,and Z.C.result and X.Z. conceived and designed the Z.C., study.X.Z. X.S.,and X.Y. andreviewed H.T. performed the the numerical simulations analyses. X.S. wrote the paper. Q.F. and edited numerical simulations and result analyses. X.S. the wrote the paper. Z.C., X.Z. and Q.F. reviewed and edited the manuscript. All authors read and approved manuscript. manuscript. All authors read and approved the manuscript.

Funding: This study was supported by the National Natural Science Foundation of China (Grant No. 51478463).


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Funding: This study was supported by the National Natural Science Foundation of China (Grant No. 51478463). Conflicts of Interest: The authors declare no conflict of interest.

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3. 4. 5. 6. 7. 8.

9. 10.

11. 12.

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