Research on the wind environment and air quality of parallel courtyards in a university campus

Journal Pre-proof Research on the wind environment of parallel courtyard in campus Li Yang, Xiaodong Liu, Feng Qian, Shengnan Niu

PII:

S2210-6707(20)30006-8

DOI:

https://doi.org/10.1016/j.scs.2020.102019

Reference:

SCS 102019

To appear in:

Sustainable Cities and Society

Received Date:

23 June 2019

Revised Date:

11 December 2019

Accepted Date:

20 December 2019

Please cite this article as: Yang L, Liu X, Qian F, Niu S, Research on the wind environment of parallel courtyard in campus, Sustainable Cities and Society (2020), doi: https://doi.org/10.1016/j.scs.2020.102019

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Research on the wind environment of parallel courtyard in campus Research on the wind environment and air quality of parallel courtyards in a university campus Li Yang1, a, Xiaodong Liu2,b*, Feng Qian3,c, Shengnan Niu4,d 1,2,3,4

a

b

[email protected],

c

[email protected],

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[email protected], d [email protected]

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College of Architecture & Urban Planning, Tongji University, Key Laboratory of Ecology and Energy Saving Study of Dense Habitat (Tongji University), Ministry of Education, Siping Rd.1239, Shanghai, 200092, P. R. China

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Highlights This paper focus on the courtyard ventilation characteristics by wind driven. The best aspect ratio of parallel courtyard for air quality and ventilation is 1 to 2. 0° and 15° are the most recommended approaching wind angle for double combined courtyard wind environment. 75° to 90°are the worst inlet airflow direction for double combined courtyard flow field. For double combined courtyard, air quality could only be improved in the inlet courtyard.

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  

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Corresponding author: Xiaodong Liu [email protected]

Abstract: The purpose of this paper is to study the wind environment of parallel courtyard in campus which is affected by the yard aspect ratio and wind directions. Research done so far mainly focused on

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the thermal comfort of courtyards and wind environment at pedestrian level in urban. This article aims to study the wind environment of courtyards under aspect ratios via the high resolution RANS CFD simulation method. Two performance indicators are identified: 1)wind amplified velocity 2) age of air. The evaluation is based on the validation with field measurement. The results show that for single parallel courtyard, the best aspect ratio for air quality and ventilation locates in the range of 1 to 2. Furthermore, for double parallel combined courtyard, 0°and 15°is the most recommended approaching wind angle for ventilation. Contrarily, 75° to 90° are the worst inlet airflow direction for courtyard flow field because of insufficient abounding wind amplified area. Apart from this, only in the range of 0°to 30°, it is possible to enhance the inlet courtyard air quality in double combined yard shape compared to single courtyard.

Keywords: courtyard; wind environment; CFD; aspect ratio; pedestrian level;The abbreviation term in this paper is showed as followed table 1.

1.Introduction Courtyard is defined as an enclosed or semi-enclosed area opening to sky and surrounded by buildings or walls (Edwards, et al., 2006). Architects usually set atria and courtyards for natural ventilation and thermal adjusted demand and make them connect to the environment [Olsen et al., 2003] in order to accept natural ventilation and sunlight [Aldawoud et al., 2008, Yang et al., 2014].The characteristics of courtyard such as aspect ratio, orientation, configurations have an impact on the

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courtyard performance of thermal comfort (Reynolds,2002). More and more new and modern courtyards shape were created such as U, L etc considering the surrounding site restriction nowadays regardless of environment influence (Saeed, 2007).

Wind is another crucial influenced factor for thermal environment of courtyard, which has effect

on the human thermal comfort, temperature adaption, and building energy efficiency. In addition,

suitable airflow distribution also promotes the emission progress of air contaminant being significance

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for urban environment.

Previous researches has recognized courtyard as an efficient adaptive element for buildings in terms of thermal and ventilation aspects.( Zamani et al., 2015) An detailed overview of courtyard

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environment effect has been investigated by Zamani. Although yard has been known for a long time, it is a renewed research interest concept to meet friendly environmental requirement. Many literatures that have been made a detailed review illustrated that several studies have been performed to assess the following:

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courtyard environment effect basing on various digital methods. The conclusions mainly focused on the 

Thermal function:

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Thermal (Muhaisen&Gadi, 2006a) is an important issues which influences environmental

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comfort degree in hot and humid regions. Rajapaksha and Tablada have performed some studies on the thermal problem in courtyard (Rajapaksha, et al., 2003 and Tablada, et al., 2005). The studies showed that courtyard played an important role on indoor air ventilation

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when it acted as an air funnel rather than a suction zone recognized by conventional knowledge. Courtyard shape was another important influenced factor for thermal environment especially in reciprocal shading building groups which was often found in

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high density city.(Tereci et al., 2013) A increased modern courtyards adopted semi-enclosed shape nowadays with consideration of orientation in order to maximize its microclimatic performance. (Meir, 2000) It is clear that semi-enclosed courtyard has positive effect on the thermal environment of courtyard under some orientation and the wind velocity also could decrease by 2C° -3C° in air temperature. (Al-Hemiddi& Megren 2001) Besides, courtyard can also balance energy harvesting and thermal comfort in corresponded climates [Taleghani et al., 2014]. Therefore, it could conduct at least double functions for building such as the natural ventilation and light demand [Ahmad et al., 2000]. Thermal environment of courtyard were also influenced by shading impact of surround construction. (Yang et al., 2012) (Shashua-Bar et al., 2009) Muhaisen and Gadi

(2006b) found that due to the shading effect of courtyard, deep and long courtyard was more helpful than other courtyard in building energy efficient. Hence, the shallow courtyard forms is better appropriate for cold climate regions since it can enhance solar absorption and results in less heating requirements in winter time. Narrow and deep shapes led to a higher effective shaded function on courtyard as aspect ratio of width to height was the most remarkable influenced factor for shading. (Al-hafith et al., 2017) (Martinelli& Matzarakis, 2017)Natural ventilation 

The jet flows in courtyard were formed by wind pressure difference via natural ventilation. Courtyard width to height ratio had influences on the airflow pattern in yard. (Rojas et al., 2012) (Moonen et al., 2011) As the aspect ratio increased from 0 to 10, the flow stream line appears in the courtyard under the approaching wind direction was vertical to courtyard surrounded wall. Setting opening could improve air change efficiency for courtyard because holes on enclosure construction can decrease the whirlwind area at the centre of

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courtyard. (Rojas et al., 2012) Gao et al. classified studies into four cases as "a street canyon, a semi-closure, a courtyard form and a relative open space" basing on a low-rise building complex. It has concluded that airflow stopped under several wind directions in courtyard

which produced by a gap in corner.(Gao et al., 2012) Multi-courtyard also had remarkable

effects on mitigating head loads for building via convection cooling mechanism. (Ernest et

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al., 2012) In this regard, openings also promoted the flow of hot air and radiation by cross

ventilation pattern which result in temperature rising. (Berkovic et al., 2012) However, in another study, as the opening allowed the cold air into courtyard and interior, enclosed (Chatzidimitriou& Yannas, 2016)

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courtyard was considered as the better way compared to courtyard with openings. Humidity:

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Another effective measure on heat relief focused on the humidity changing. All porous

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materials and construction such as vegetation, water ponds enhance air humidity and then amplify the cooling function upon surrounding air. (Di&Jiang, 2012; He&Hoyano, 2010; Wanphen& Nagano, 2009; Brutsaert 1975) Salata et al. investigated the outdoor thermal

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condition via comparing five cases that combining different vegetation and materials. It was shown that plants locating in courtyard could decrease the PMV value up to 1.5 in the hottest day through evapotranspiration and reflect mechanism which diminished the

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shortwave radiation and temperature. (Salata et al., 2015) Chatzidimitriou and Yannas studied the soil and water humidity influences upon a square and courtyard thermal environment using some simulated software such as envi-met. The results showed that the

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integrating of trees and wet soil could reduce the heat load more efficient than courtyard shading walls.(Chatzidimitriou& Yannas, 2016.) Apart from this, integrated with water ponds

and

vegetarian

courtyard

could

decrease

temperature

by 1.6

to

1.1

centigrade.(Taleghani et al., 2014)

In the past, wind environment thesis had been studied through many methods such as field

measurement (Niu et al., 2015), wind tunnel experiment (Cermak, 2003; Tsang et al., 2012; Du et al.,2017a; Tse et al., 2017), and CFD (computational fluid dynamics) modelling (Blocken et al., 2016; Du et al.,2017b; Liu et al., 2016; Tominaga&Stathopoulos, 2011). CFD has many apparent advantages comparing to wind-tunnel experiment such as there is no limitation on model scale. Currently, CFD research on wind simulation has been supported by several international studies focusing on

establishing general practice guidelines (e.g. Franke et. al., 2007; Tominaga et al., 2008; Franke et. al., 2004; Casey et al, 2000; Jakeman et al., 2006; Britter et al., 2007). Taking consideration of the complexity of environment behaviour issue, increased researches done so far about the courtyard wind environment using RANS simulation method. Table 2 summarized review results of courtyard literatures basing on simulated measurements. Turbulence model, validation, inlet atmospheric layer, and performance indicators are considered as the investigated object in this table. 

Most of these investigated literatures focused on the thermal environment and energy consumption studies, in which surrounded temperature was considered as the perform indicator. The literatures in relation to courtyard natural ventilation research is scarce, only taking up 12 of the whole 32 above surveyed papers.

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Berkovic, et al., (2012) have performed simulation about thermal comfort for diverse courtyards utilizing ENVI-met 3.1 BETA II. It is observed

that the courtyard thermal

condition have something to do with the position of openings and the lower opening

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position can provide less heat and radiation. The site measurement is also imperative for CFD validation (Bagneid, 2006). Sadafi, et al.,(2011) not only performed the experiments

and also used the numerical evidences to study the thermal comfort relationship between

indoor and outdoor. The result verified that internal courtyard was conducive to natural ventilation and thermal condition. In addition, it also found that the courtyard also had

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influence on the sunlight shading. 

Earlier relative CFD researches about courtyard have indicated that courtyard proportion, position, orientation and forms could influence inside airflow field apparently. However,

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past related studies was limited to investigate flow field condition of combined courtyard shapes and impact by different approaching wind direction which was usually found in school campus.

Most of above researches studied ventilation performance for courtyard using the wind

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speed as the indicator, and disregard the air change efficiency. Both of them should be considered as the ventilation indicator for courtyard in order to assess air flow field quantitatively.

High resolution coupled 3D steady RANS CFD simulation has been demonstrated as an

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efficient way to perform courtyard ventilation performance. Simulated method can be conducive to provide better understanding of the courtyard condition characteristics

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because there is no restriction of variable number under this method (Groat& Wang, 2002).Another reason of using computer simulation tool is that it can conduct different scenarios caused by different design variables so as to filter optimum building plan

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(Al-Masri& Abu-Hijleh, 2012; Berkovic, et al.,2012) which is helpful for architecture design.

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Beyond field measurement and wind tunnel experiment, most CFD simulation methods should be validated by measured data. The standard k- ε turbulence model were commonly used to conduct courtyard wind environment field simulation research whose credibility has been proved by Rajapaksha et al. Note that Large-Eddy Simulation (LES) model simulated more accurately than other models in terms of flow field, nevertheless, it also took more CPU time and computational resources.

Therefore, it was observed that researches done so far had studied courtyard thermal comfort and energy consumption. Numerical research on wind environment characteristics at pedestrian level of

courtyards was still scarce. In this paper, the parallel shaped (semi-enclosed) courtyard and its combined shape was selected for wind condition study by surveying two types of evaluation indicators: wind velocity and age of air. The purpose was to study the flow field in courtyard which was affected by the combined courtyard aspect ratio and approaching wind direction. High resolution coupled 3D steady simulation was performed for all cases implementing standard k-ε turbulence model and CFD validation was conducted by field measurement data in Tongji university campus. In addition, wind effects were divided into natural wind-driven and buoyancy-driven. This study only considered about natural wind effect and pedestrian wind environment. The results of this research could assist architects to array buildings’ positions more reasonable promoting dispersal of pollutants and design courtyard aspect ratio regarding airflow factor instead of only about aesthetics.

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2. Methodology 2.1 Governing equation

This paper performs some wind simulation using standard k-εmodel because this method is

appropriate for simulating courtyard ventilation performance which has been used in the past (Janssen et al., 2013; Ai et al., 2013; Liu et al., 2015;) and it can also provide sufficient accuracy under acceptable numerical cost. (Blocken et al., 2016)

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The form of the time-averaged governing equation which includes energy, momentum, and mass for the neutral and incompressible fluid is shown as following equation [1]: (Fluent, 2010; Yang, 2014) ∂t

+ ∆(𝑢̅ 𝜑) = ∇(Γ𝜑 ∇𝜑 + 𝑆𝜑 )

(1)

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∂(φ)

In the above equation, φ represents the scalars such as: the velocity under different vectors, u(m/s), v(m/s) and w(m/s); the turbulent kinetic energy k(m2/s2) and the turbulent dispassion rate

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ε(m2/s2). The mean velocity vector is represented by 𝑢̅ ; Γ𝜑 is the effective diffusion coefficient for every variable; 𝑆𝜑 is the source term in this equation which is decided by experimenter. Turbulent kinetic energy:

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Γ𝜑 = 𝛼𝑘 𝜇𝑒𝑓𝑓

(2)

Turbulent dispassion rate: Γ𝜑 = 𝛼𝜀 𝜇𝑒𝑓𝑓

(3)

where, 𝜇𝑒𝑓𝑓 is the effective turbulent viscosity; 𝛼𝑘 and 𝛼𝜀 express the inverse effective

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Prandtl numbers for k and ε, respectively. 2.2 Performance indicators

To evaluate the courtyard wind environment, this study uses the mean velocity and the concept of

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age of air as performance indicator (Sandberg, 1981). Firstly, this paper assesses wind environment using mean wind velocity considering the airflow

speed can effectively reflect the exceedance of wind increased and decreased. In order to quantitatively evaluate flow field, some concepts are defined as follows. The normalized wind effect amplified and diminished is abbreviated as the term of WVA and WVD. In this paper, the percentage area ratio of WVA and WVD which is abbreviated as RWV presents airflow velocity changed level. These items are defined and calculated as follows: 𝑊𝑉𝐴 =

(𝑈𝑝 − 𝑈𝑟𝑒𝑓 ) ⁄𝑈 (𝑈𝑝 > 𝑈𝑟𝑒𝑓 ) 𝑟𝑒𝑓

(4)

𝑊𝑉𝐷 =

(𝑈𝑝 − 𝑈𝑟𝑒𝑓 ) ⁄𝑈 (𝑈𝑝 < 𝑈𝑟𝑒𝑓 ) 𝑟𝑒𝑓

𝑅𝑊𝑉(%) =

(5)

𝑋(𝑡ℎ𝑒 𝑎𝑟𝑒𝑎 𝑢𝑛𝑑𝑒𝑟 𝑊𝑉𝐴 𝑜𝑟 𝑊𝑉𝐷)

(6)

𝑌(𝑡ℎ𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒 𝑐𝑜𝑢𝑟𝑡𝑦𝑟𝑎𝑑)

Where, the 𝑈𝑝 represents the local mean wind velocity in the courtyard, moreover 𝑈𝑟𝑒𝑓 is the reference mean wind velocity of the inlet wind at 10 m height from the local weather station. According to the Eq.4,5, WVA value is positive while WVD value is negative. The concept of age of air is a measurement for the freshness level of air which could be also considered as an indicator of indoor air quality and ventilation efficiency. (Sandberg, 1981; Etheridge, 2015) Definition of age of air referred to the time that has elapsed since the air flows into an area via an opening. In present research, local mean age of air is calculated by a user scalar transport equation and the diffusivity for the equation as follows： 𝑢𝑒𝑓𝑓

(7)

𝑆𝐶𝑡

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𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑣𝑖𝑡𝑦 = (2.88 × 10−5 )𝜌 +

Where 𝜌 is the air density; 𝑢𝑒𝑓𝑓 is the air effective viscosity; 𝑆𝐶𝑡 is turbulent Schmidt constant what is chosen as 0.7 in this study.

AOA is the abbreviation of area ratio of age of air. This means that the percentage of area ratio for

age of air during one time period which can be estimated using following Eq.8. For example, the

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followed Figure 1 represented the condition of local mean age of air performance for one courtyard. In this figure, X1 presents the whole area when the age of air is 28.12s, simultaneously, Y expresses the size

of the entire courtyard. Consequently, AOA for 28.12s is the specific value between X1 and Y. In

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addition, if X suggests the area range with air of age lower 31.25s, value of AOA means the percentage area ratio lower than 31.25s age of air which is estimated by the equation of (X1+X2)/Y. . 𝑋 (𝑡ℎ𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑖𝑟 𝑜𝑓 𝑎𝑔𝑒 𝑖𝑛 𝑎 𝑡𝑖𝑚𝑒)

𝑌(𝑡ℎ𝑒 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑎𝑖𝑟 𝑜𝑓 𝑎𝑔𝑒 𝑖𝑛 𝑤ℎ𝑜𝑙𝑒 𝑐𝑜𝑢𝑟𝑡𝑦𝑎𝑟𝑑)

(8)

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AOA(%)=

AOA can represent the ventilation efficiency and air change efficiency for a classroom. (Etheridge&Sandberg, 1996) The lower time AOA shows bigger, the ventilation condition indicates better. Contrarily, the longer time AOA presents bigger, the ventilation efficiency presents worse which

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could be also used to measure air quality. (MOHURD, 2014) It should be noted that this study mainly focused on the courtyard air quality instead of human thermal comfort, therefore, larger AOA and RWV under WVA are considered as the better environment

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performance indicator.

2.3 Model settings and grid analysis This study considers the ABC square of tongji university campus in China which locates beside

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the college architecture and urban planning as the modeling prototype . This model consists of two H height buildings and a courtyard shaped by these two constructions. According to the best practice guidelines by Franke et al. (Franke et al., 2007) and Tominaga et al. (Tominaga et al., 2008), the domain upstream and downstream sizes are 5H and 15H, respectively (Fig. 2). The resulting dimensions of the domain are WXDXH = 408X450X90 m3 (Fig. 2) and the maximum blockage ratio is 0.47% which is below the recommended maximum blockage ratio. (Franke et al., 2007; Tominaga et al., 2008) In addition, as shown in Fig.2, the two side domain lengths from the model side wall are also 5H and 15H. In terms of the computational grids, structured cells are produced in this paper via ICEM software to discretize whole calculation domain (Fig.3). A grid sensitivity analysis determines the final mesh resolution to reduce simulated error and computational time. An linear factor √2 is performed for

coarse grids with 2.12 million and fine grids with 4. 25 million based on 3.01 million meshes. In addition, the minimum mesh size corresponding to the above three types ranges from 0.037m, 0.027m and 0.014m. Wind velocity profiles of U/Uref values comparison along two test lines L1 and L2 located at 1.5m height level from ground are provided in Figure 4. The mean deviation between coarse and medium grid is 1.6% while it is 1.1% between medium and fine grid. Hence, medium grid is selected for further analysis. There are 3.01 million meshes in this simulated system so as to guarantee the accuracy of simulation and the minimum grid size is set as 0.02m. High grid quality in the nearby of the courtyard is imperative for the precise simulation of the natural ventilation under ABL (neutral atmospheric boundary layer) airflow profile. The minimum and maximum cell volumes in the domain are approximately 8X10 -8 m3 and 216 m3, respectively. The length from the center point of the wall adjacent cell to the wall, for the upstream, downstream, roof and ground are all about 0.020 m. In center courtyard between buildings, this distance ranges from 0.02 m to 0.03 m. This corresponds to y* values between 30 and 500 which

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guarantee that the center point of the wall-adjacent cell is placed in the logarithmic layer so as to fully utilize the standard wall function. 2.4 Boundary configuration

At the inlet of the domain, neutral atmospheric boundary layer (ABL) inflow profiles of mean computed as follow equations: (Richards and Hoxey, 1993)

𝜀(𝑧) =

𝜅

ln(

𝑧+𝑧0 𝑧0

)

(9)

∗2 𝑢𝐴𝐵𝐿

(10)

√𝐶𝑢 ∗3 𝑢𝐴𝐵𝐿

𝜅(𝑧+𝑧0 )

(11)

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𝑘(𝑧) =

∗ 𝑢𝐴𝐵𝐿

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𝑈(𝑧) =

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wind speed U (m/s), turbulent kinetic energy k (m2/s2) and turbulence dissipation rate ε (m2/s3) are

where z is the height coordinate, uABL is the ABL friction velocity, k the von Karman constant (=0.40–0.42) and Cu is a model constant of the turbulence model. (Richards and Hoxey, 1993) It is assumed that the building is situated on a large grass covered terrain with an aerodynamic roughness

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length z0=0.03 m (J. Wieringa, 1992). The reference wind speed at 10 m height is 3.4 m/s in accordance with weather station data It yields building Reynolds numbers of 4.9X106 based on the height of the courtyard building (H=18 m). For the ground surface, the standard wall functions by Launder and

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Spalding (Launder&Spalding, 1974) with roughness modification are used (Blocken, B., Stathopoulos, T., Carmeliet, J 2007). The physical sand-grain roughness height ks (m) and the roughness constant Cs, are determined by their consistency relationship with the aerodynamic roughness length z0. (Blocken, B.,

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Stathopoulos, T., Carmeliet, J 2007) For ANSYS/ Fluent 17, this relationship is: 𝑘(𝑠) =

9.793𝑧0 𝑐𝑠

(12)

Standard wall functions are also used at the building surfaces but with zero roughness height ks=0

(Cs=0.5). Zero gauge static pressure is applied at the outlet plane. Symmetry conditions are set at the top and lateral sides of the domain. Apart from above, standard k-ε model is performed for the turbulence model which has been demonstrated to be validation in courtyard airflow simulation by Rajapaksha I, et al. (2003). SIMPLE algorithm is chosen as the pressure-velocity coupling calculation pattern and the pressure interpolation method uses a staggered scheme named PRESTO. Every simulation takes more than 8000 iterations in order to reach the convergence. The calculated progress will be terminated when

the discretization error shrink and maintained at a reasonable level in which the residual values are 10-5 for k, ɛ, and every axis velocity; 10-4 for continuity term. Meanwhile, the variations, such as the air velocity, temperature and age of air at specific points, are steady or appear acceptable fluctuation amplitudes. All computations are performed by Fluent 17.0 on an 8-core workstation (Intel Xeon E5 2680 v3, 2.7 GHz) with 16 GB DDR of system memory.

3 List of cases In this study, the wind simulations are performed for a courtyard enclosed by two buildings. Courtyard shape pattern can be divided into two types as enclosed courtyard and semi-enclosed courtyard according to the enclosed level. (Fig. 5) Furthermore, semi-enclosed courtyard can be classified into three types in further such as three side enclosed courtyard, parallel courtyard and

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resemblance enclosed courtyard. (Fig. 5) As this paper focuses on the campus university which is comprised mainly by parallel courtyard pattern, therefore this study considers this courtyard type as the research object.

In order to investigate the impact factors for the courtyard wind environment, in this study, 27 cases are taken in which models are different in aspect ratio and inlet airflow direction. A) Effect on the parallel courtyard wind environment by different aspect ratios

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Firstly, six cases of courtyard are defined basing on aspect ratios of parallel courtyard. As shown in

Fig.6, the building has dimensions width × depth × height 90×15×18 m3. The courtyard aspect ratio （KA） is calculated by the following equation 13 and Fig. 6 indicates nomenclature for whole

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scenarios. In this study, all cases simulation are performed by commercial CFD software Fluent 17.0 to quantitatively assess the wind environment characteristics via normalized air speed and local age of air. 𝐷

(13)

𝐻

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𝐾𝐴 =

Where, D is the courtyard width and H represents the height of buildings. B) Effect on the double combined courtyard wind environment by different aspect ratios and inlet wind directions

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Secondly, as a result of the yards in campus usually uses the complex patterns combined with some small sub-courtyards, this study also conducts researches on the flow field of double combined parallel courtyard. In Fig. 7, relevant cases are classified into three forms in accordance with the

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courtyard centre aspect ratio KB value calculated by the Eq.14 as follows. 𝐾𝐵 =

𝐷1 𝐷2

(14)

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Where, D1 and D2 represent the centre width of the courtyard respectively which is shown in Fig.

7. Apart from this, in order to study the approaching airflow direction effects, the flow field feature under several approaching wind angles should also be investigated. In addition, to evaluate results more conveniently, the whole courtyard is estimated by classifying some zones named as inlet zone (zone C), outlet zone (zone A), centre zone (zone B). (Fig. 7)

4 CFD Validation 4.1 Field measurement For CFD validation, ABC square in Tongji University campus in China is selected as the field

measurement location. Full-scale on-site measurements have been performed at 5 positions during three months in the courtyard campus using ultrasonic anemometers at different height levels. Fig. 8 indicates the positions of all measuring points and the field measurement was mainly performed on some windy days. The courtyard signed in the green area was enclosed by four buildings of ABCD. Four tested locations were distributed in the courtyard and point a and b located at the entrance of the courtyard where it could reflect the inlet wind direction and velocity. Points c and d placed in the centre of the courtyard. Besides, point e stood at the roof of building A with the height of 17m in order to measure reference wind velocity and direction (Uref). Simultaneously, so as not to disturb the pedestrian traffic, any ground measurement position except probe e is placed at 3m height above the ground level. During measurement period, each testing probe proceeded continuously for more than two hours to ensure the accuracy of the measurement data. (Blocken, et al., 2009) 4.2 CFD simulation

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Computational grid for ABC square model is established using the gambit tool under the proportion 1:1 comparing to the actual buildings. For the sake of fully developing of airflow, the upstream,

downstream, lateral, and height length of the computational domain are 5H, 15H, 5H and 5H, respectively. (Franke et al., 2007) All governing equation and boundary condition has been discussed in

the above section 2. Structured cells covers whole calculated domain with the minimum distance from

center wall-adjacent cell to the building wall guaranteeing standard wall function validity. All equations

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compute using the SIMPLE algorithm, and the second-order upwind scheme discretized for viscous and

convection terms in governing equation. The residuals extreme values in the simulation are all set as 10-5. 4.3 Comparison between simulation and measurement defined and by the following equation:

（15）

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Normalized wind velocity= 𝑈⁄𝑈 𝑟𝑒𝑓

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In order to assess the CFD validation more conveniently, the normalized wind velocity is

Fig. 9 presents the comparison of normalized wind speed and inlet wind directions between simulated and measured data at a, b, c and d locations under the same reference airflow direction. Fig.

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9a compares field measurement approaching wind directions with simulation. In general, this result shows good agreement of wind direction between simulation and measurement primary within 25° deviation especially for two probes at the entrance of courtyard. However, probe d indicates bad performance in line with tested result because of the complex airflow field condition in the choke

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position. For location a, b and c, the deviation with measurement data is 10°,5°and 15°, respectively. While, the maximum deviation for flow direction validation is archived at the location d. Fig. 9b shows the comparison between simulated and measured results in normalized wind speed at

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testing locations a,b,c,d. It can be clearly observed that the field measured data is well reproduced by simulation method and the mean deviation between simulation and measurement is smaller than 5% moreover the highest deviation value is about 5% in location d. Overall, the simulation results suggest good agreement with the measured results. Hence this type of mesh configuration retains for further research.

5 Results 5.1 The effect of KA on the wind environment of parallel courtyard 5.1.1 Flow field According to the different KA values, this simulation of one courtyard is divided into six cases. The general contours of normalized wind speed under different KA are shown in Fig. 10. Figure 10 shows that as the KA value increases, the local amplified wind velocity at the entrance of the courtyard is gradually reduced and reaches constant for higher value of KA at the horizontal plane. The highest normalized wind velocity emerges at the corner of the courtyard envelope building along airflow upstream

for

all

cases.

For

example,

the

highest

normalized

wind

speed

is

0.3,0.219,0.219,0.138,0.138,0.138 for case 0.5A, case 1A,case 1.5A, case 2A, case 2.5A and case 3A respectively

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Figure 11 presents RWV profiles for all cases as the function of KA at the pedestrian height

level. It can be obtained that by enlarging the courtyard aspect ratio, the wind amplified effect in parallel courtyard decreases until KA value reaches 2 and then remains approximately constant with

KA value larger than 2. For example, from case 0.5A to 2A, RWV with 0.056 WVA enhances by almost

42.4%, while it reduces only by 7.3% from 2 to 3 of KA. The RWV with WVA larger than 0.1375 5.1.2 Age of air and air change efficiency

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diminishes monotonically by almost 97.2% as a function of KA increasing.

Amplified wind velocity effect leads to lower values of age of air resulting in high air change

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efficiency and air quality. Percentage area ratio of age of air box plot under different KA is presented in Figure 12. Figure 12 shows that the highest mean age of air is achieved for case 1A and it is almost the same as the other two cases of 1.5A and 2A. Therefore, between KA value of 1 and 2, the best air

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quality is achieved and aspect ratio has no significantly influence on the air quality for single parallel courtyard. Note that although case 0.5A performs larger value in normalized wind speed, it leads to a lower air quality comparing to case 1A, 1.5A and 2A because of the larger area ratio with WVD -0.025. For example, for case 1A, 1.5A and 2A, the AOA value is 74.36%, 73.69% and 72.78% respectively,

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which are approximately identical with each other and much larger than other cases. Therefore, the best aspect ratio of parallel courtyard for air quality and ventilation locates in the range of 1 to 2. 5.2 The characteristic of wind environment on double combined parallel courtyard

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On campus, the courtyard tends to be combined with some parallel courtyards due to land restriction. This paper will investigate the basic flow characteristic next about the combined courtyard constituted by two parallel yards.

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5.2.1 KB is 0.25

Fig. 13 shows the contours of the normalized wind speed in the horizontal plane above from

ground 1.5m level with different wind angles when KB is 0.25. Under inlet wind directions from 0°to 30°, the air impingement directly yields a stagnation area at the B and C zone. The flow subsequently accelerates and reaches the maximum normalized wind velocity of 0.125 at these two places. Then it decelerates in the courtyard AB and BC with complex wind directions. For case 0.25B45 and 0.25B60, the flow is bent at entrance of courtyard and accelerates within the zone BC. A recirculation area emerges at the zone A where the normalized wind velocity reduced significantly because of the changed airflow circuit. As the approaching wind direction increases from 75°to 90°, airflow couldn’t be amplified in the whole area inside the courtyard. For example, for case 0.25B70 and

0.25B90, the maximum normalized wind velocity can’t reach more than 0. Figure 14 presents the profiles of RWV condition for case 0.25B under different approaching wind directions. It can be observed that by enlarging the inlet airflow angle, the RWV for WVA reduce monotonically and that for WVD increases significantly. After 75°of wind direction, the RWV value of WVD in entire courtyard reaches the maximum 100%. In this case, wind can’t be amplified by the courtyard. For example, as the inlet wind direction enhances from 0°to 90°, the RWV value of WVA considerably decreases from 40% to 0%. In order to assess quantitatively the wind environment intensity, this paper accounts the AOA value under different wind directions as shown in Fig. 15. It is obviously obtained from the Fig. 15 that the highest AOA is achieved for 0°which is also similar with cases 0.25B15 and 0.25B30. As the age of air represents the air quality, so 0°, 15°and 30°are the three best inlet wind direction for courtyard ventilation in which the three cases have analogical air flow field with each other. Contrarily, 75°and 90°are not the recommended wind direction for courtyard ventilation. It should be noted that

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for case 0.25B60, airflow field can only be enlarged in the windward yard which can be explained by the deviation of the approaching wind direction. 5.2.2 KB is 0.5

Fig. 16 shows the wind simulation results of different inlet wind directions for KB 0.5. The result shows that the area with airflow amplified effect only emerges in the upwind ward courtyard of

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zone BC for all flow angles. Wind tunnel location of B could only magnify the flow velocity between 0° to 45°while it can be amplified at the wind inlet location C from 0°to 60°. For zone AB, for example, the highest normalized wind flow is achieved for case 0.5B0 with -0.437 which is

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considerable larger than other corresponded cases.

Profiles of RWV as a function of inlet wind direction for case 0.5B are provided in Figure 17. By enlarging the inlet airflow direction, the RWV value of WVA diminishes significantly from 0°to

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90°. In addition, from 75°to 90°, WVA area disappears in the courtyard. The above two phenomenon show the same flow characteristics as the case 0.25B under corresponded wind direction. The decrease of the RWV for WVA is more pronounced for wind angles from 0°to 15°under case 0.5B than case 0.25B. For example, the RWV of WVA for case 0.5B diminishes approximately by 10%,

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moreover, it decreases only by smaller than 1% for case 0.25B in the range of 0°to 15°wind direction. This is a crucial observation that the case 0.25B can fit more inlet airflow direction ranges than case 0.5B.

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The overall air quality is evaluated through the AOA statistics as illustrated in Fig. 18. By enlarging the wind direction, the air change efficiency and air quality for courtyard decreases considerably especially in the range of 30°to 90°. Meanwhile, enhancing the wind angle has

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negligible impact on the courtyard air quality from 0°to 15°. For example, the AOA only reduces from 98% to 97% under 0°to 15°, moreover, it diminishes from 70% to 40% for 30°to 90°. Hence, in combination with RWV consideration, 0°is the optimal wind direction for case 0.5B ventilation and the 15°is the secondary selection. In addition, case 0.5B75 and 0.5B90 remain the same as the case 0.25B75 and 0.25B90 basic flow characteristics which is not recommended for courtyard ventilation. 5.2.3 KB is 0.75 Fig. 19 presents the contour graph of double combined parallel courtyard under 0°-90°in the horizontal level at the height of 1.5m above ground level when KB is 0.75. Zone AB mean normalized wind speed deteriorates monotonically as increasing the approaching wind angle. The shadow area emerges at the corner of zone B increases significantly as the airflow direction rises. Beyond above,

other flow characteristics also remain the same as the two cases 0.25B and 0.5B. Therefore, double combined courtyard under different aspect ratios has some common ventilative features with each other. Figure 20 indicates the profiles of RWV performance for case 0.75B. It can be clearly obtained that RWV of WVA under 0°reduces apparently comparing to case 0.5B and 0.25B while the distribution of their flow almost maintain similar with each other. This is mainly because of the shadow area at the corner of zone B in which this area enhances across enlarging the ratio of courtyard under 0° inlet wind orientation. For 0°, for example, the value of RWV of WVA decreases from 25% in case 0.5B to 11% in case 0.75B. Profiles for area ratio of age of air as a function of wind direction about case 0.75B in the same horizontal plane are provided in Figure 21. The overall basic flow characteristic for this case is alike with above two cases that the air change efficiency in courtyard exacerbates by enhancing the inlet flow direction. Two wind angles of 0°and 15°are the optimal airflow direction for case 0.75B

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ventilation since the resemble RWV condition and AOA performance which performs better in flow

field than other directions. For instance, in terms of AOA, case 0.75B0 and 0.75B15 is approximately 90% indicating significantly larger than that in other angles. 5.2.4 Impact of different KB values

Fig. 22 shows the comparison of wind environment evaluated by AOA and normalized wind

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velocity condition impacted by different courtyard aspect ratios. The following observations can be drawn: 

For double combined courtyard aspect ratio, the best ventilation condition is achieved for

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the KB value of 0.25. In this case, AOA and RWV situation performs better for KB with 0.25 than other ratios such as 0.5 and 0.75. 

In combination with above RWV distribution of WVA and AOA condition under different

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KB, 0°and 15°is the most recommended approaching wind angle for double combined courtyard wind environment. Contrarily, 75°to 90°are the worst inlet airflow direction for courtyard flow field because of insufficient abounding wind amplified area. 

The courtyard air quality decreases significantly with the approaching wind angle

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enlargement regardless of the aspect ratio KB. Note that, the difference between cases on air change efficiency increases by enlarging the wind angle. For example, under 0° three cases are similar with each other on AOA condition, furthermore, the deviation of them

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reaches to almost 15% between case 0.25B and 0.75B under 60°. 5.3 Impact of double combined courtyard pattern Inlet and outlet courtyard as shown in Table 3 which is as large as the case A yard are selected to

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compare the wind environment between combined courtyard case B and single courtyard case A under same circumstance. The aim of this part is to study the amplified or diminished effect on single courtyard airflow condition by combined yard pattern. Table 2 presents the normalized AOA efficiency in the courtyard so as to assess the ventilative impact on the single courtyard by the combined double courtyard. Normalized AOA efficiency are defined as follows: 𝜀𝐴𝑂𝐴 = double combined courtyard AOA(inlet or outlet)/ single courtyard AOA Following observation can be found: 

Only in the range of 0°to 30°, it is possible to enhance the inlet courtyard air quality in double combined yard shape compared to single courtyard. For example, for case 0.25B and

0.5B, the AOA efficiency values are all larger than 1.0 which indicates that the air quality is improved. 

For double combined courtyard, air quality could only be improved in the inlet courtyard under some approaching wind directions comparing to single courtyard. This can be explained that the reduced vertical cross-section in the area B prevent the air flows into outlet part. Consequently, the air quality may be possible enhanced in the inlet region instead of corresponding outlet area. For three cases B, for example, the 𝜀𝐴𝑂𝐴 value larger than 1.0 such as 1.12 for case 0.25B, 1.11 for case 0.5B all occurs in the inlet courtyard.



Outlet courtyard wind environment is much worse than that in inlet regardless of aspect ratios for yard. This is mainly because of the blocking effect at the area B in which the flow tube section diminishes significantly. In case of 0.25B, the highest gap with 68% between

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inlet and outlet is achieved under 90°approaching wind direction.

6. Discussion

The above studied results show that the aspect ratio of single courtyard has significant influence on the courtyard airflow environment. In addition, some double combined courtyard pattern could also be achieved under some specific wind directions.

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improve the courtyard air quality. For various aspect ratio yard cases, the best ventilation condition can In this study, the double combined courtyard is investigated to study the basic flow

characteristics. However, in fact, some campus courtyards indeed separate with each other instead of

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connecting directly shape in this paper at the area B. Hence, further researches need to be performed to investigate the basic flow characteristics for combined courtyard without connecting each other directly.

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High wind velocity contributes to promoting air pollutant disperse so that improve the air quality. Nevertheless, it should be noted that large airflow speed may has negative impact on human comfort at the height of pedestrian level. Therefore, in the future, further investigations are needed to focus on the impact on the people comfort degree with different ventilation conditions depending on

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the air quality indicators in this study.

In present research, the aspect ratio is identified as the specific value of width to height for courtyard. Moreover, it may also affect the flow field situation by the ratio along courtyard length

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direction. Consequently, the aspect ratio of width to length for yard could be also defined as a variant impacting the whole flow field in further research. The result will be conducive to the determination of

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the yard size by building designers.

7. Conclusion This paper presents the basic flow characteristics of single courtyard and double combined

courtyards impacted by different aspect ratios under multiple inlet wind directions. High-resolution quality grid is performed to simulate the wind flow with different airflow angles so as to reduce the discretization error. Two performance indicators AOA and RWV are used to assess the wind flow field and courtyard air quality. The evaluation is based on the field measurement validation of wind velocity using wind anemometer. Through the simulation and analysis, it can be concluded as follows:

1) Impact of aspect ratio on wind environment for single courtyard

 The wind amplified effect in single parallel courtyard decreases until aspect ratio value reaches 2 and then remains approximately constant with that value larger than 2. In addition, the best aspect ratio of parallel courtyard for air quality and ventilation locates in the range of 1 to 2. 2) Impact of aspect ratio on wind environment for double combined courtyards

 For case 0.25B, 0°to 30°are the best inlet wind direction range for courtyard air quality in which the three cases have analogical air flow field with each other. Contrarily, 75°and 90° are not the recommended wind direction for courtyard ventilation.

 Case 0.25B can fit more inlet airflow direction ranges than case 0.5B because of the decrease of the RWV for WVA under case 0.5B is more pronounced for wind angles from 0° to 15°than other angels which is apparently different with case 0.25B. Apart from this, 0° is the optimal wind direction for case 0.5B ventilation and the 15°is the secondary

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selection.

 For case 0.75B, the best courtyard ventilation environment can be achieved in the range of 0° to 15°.

 For double combined courtyard aspect ratio, the best ventilation condition is achieved for the KB value of 0.25. In this case, AOA and RWV situation performs better for KB with

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0.25 than other ratios such as 0.5 and 0.75.

 In combination with above RWV distribution of WVA and AOA condition under different KB, 0°and 15°is the most recommended approaching wind angle for double combined

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courtyard wind environment. Contrarily, 75°to 90°are the worst inlet airflow direction for courtyard flow field because of insufficient abounding wind amplified area.

 The courtyard air quality decreases significantly with the approaching wind angle

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enlargement regardless of the aspect ratio KB. Note that, the difference between cases on air change efficiency increases by enlarging the wind angle. 3) Impact of double combined courtyards on single courtyard airflow field characteristics

 Only in the range of 0°to 30°, it is possible to enhance the inlet courtyard air quality in double

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combined yard shape compared to single courtyard. For double combined courtyard, air quality could only be improved in the inlet courtyard under some approaching wind directions comparing to single courtyard. This can be explained that the reduced vertical cross-section in the area B prevent the air flows into outlet part.

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 Because of the blocking effect at the area B in which the flow tube cross-section diminishes significantly, outlet courtyard wind environment is much worse than that in inlet regardless of

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aspect ratios for yard.

The results presented in this paper provide an insightful understanding of wind condition for

parallel courtyard in campus. These findings can help the architects to design better courtyard environment so as to shape more comfort wind condition on pedestrian level of campus.

Acknowledgment This work is financially sponsored by the National Natural Science Foundation Committee of China (Subject Numbers:51378365)

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Conflicts of interest: none

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Figure 1 Instruction of the AOA calculation

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Figure 2 Computational domain

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Figure 3 Computational grid condition a) perspective of this computational domain; b)perspective of

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the model grid distribution; c) plan of the whole domain mesh; d) detail grid configuration of model

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and domain

Figure 4 computational grid sensitivity analysis in two lines a) comparison for three type grids for test line L1 b) comparison for three type grids for test line L2

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Figure 5 Courtyard patterns

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Figure 6 Research of the effect on the courtyard wind environment by different aspect ratios

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Figure 7 Research of the effect on the double combined courtyard wind environment by different aspect

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ratios and inlet wind directions

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Figure 8 positions of field measurement points

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Figure 9 CFD validation analysis

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Figure 10 Contour graph of parallel courtyard under different KA

Figure 11 RWV profiles under different KA values

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Figure 12 AOA under different KA values

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Figure 13 wind simulation environment under KB is 0.25

Figure 14 RWV under different wind directions when KB is 0.25

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Figure 15 AOA under different wind directions when KB is 0.25

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Figure 16 wind simulation environment under KB is 0.5

Figure 17 RWV under different wind directions when KB is 0.5

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Figure 18 AOA under different wind directions when KB is 0.5

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Figure 19 wind simulation environment under KB is 0.75

Figure 20 RWV under different wind directions when KB is 0.75

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Figure 21 AOA under different wind directions when KB is 0.75

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Figure 22 AOA and RWV between different KB values

Table 1 Abbreviation Abbreviation term

Wind velocity amplified

WVA

Wind velocity diminished

WVD

Area ratio of wind velocity

RWV

Area ratio of air of age

AOA

Aspect ratio of case A

KA

Aspect ratio of case B

KB

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Original term

Table 2 Literature review results feature Author

Method &Model

Toe DHC,&Kubota T. （2015）

Object

Field Measurement

of

Performance

Validati

paper

indicator

on

Indoor

Operative

NN

thermal

temperature;

comfort

Air

Natural

temperature

ventilation Ahmed S. et al

(2005)

Virtual Environment

Energy

Heating

(IES)

Consumpti

load

on

Cooling

Program;

ModelIT,

SunCast and Apache. ENVI-met

load Thermal

PMV;

Y

ro of

Ghaffarianhoseini A, et al (2015)

N

environme

Physiologic

nt

ally

Equivalent

Temperature (PET)

ur

Muhaisen AS. (2006)

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Du X, et al (2014)

Consumpti

Consumptio

on

n

Thermal

Heating

environme

demand;

nt

Operative

-p

Energy

Thermal

Temperature

Field measurement

environme

;

nt; Energy

Energy

consumpti

consumptio

on

n

Shaded

Shading

effect

area

CourtSun

N

N

temperature

Energy plus;

na

Cantón MA, et al (2014)

Design Builder

Energy

re

Taleghani M, et al (2014)

DOE2.1E

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Aldawoud A. (2007)

NN

N

percentage Design builder;&k-e

Thermal

Temperature

standard model

environme

; Humidity;

Field measurement

nt;

Air velocity

NN

Airflow environme nt

Yaşa E,& Ok V. (2014)

Fluent 6.3;

Energy

Solar

consumpti

radiation

N

on Ernest R,& Ford B. (2012)

Field Measurement

Thermal

Cooling

NN

environme

energy; Air

nt;

velocity

Air flow Yang X, et al. (2012)

discrete

transfer

method (DTM)

Temperatu

Air

re

surface

and

N

Temperature Chatzidimitriou A, &Yannas S. (2016)

Envi-met

Thermal

Physiologic

Fluent;

environme

ally

Radtherm

nt

Equivalent

N

Temperature (PET) Heliodat

Thermal

PMV(predic

environme

ted

nt Almhafdy A, et al. (2013)

IES;

Field

measurement

N

mean

ro of

Mertens E.(1999)

vote)

Thermal

Air speed;

environme

Relative

nt;

humidity;

Airflow

Y

Wind speed

-p

environme nt

Jamaludin AA, et al. (2014)

Field Measurement

Natural

Temperature

NN

Thermal

Solar

Y

environme

radiation;

nt

Radiation

re

ventilation

Al-Saud K a M, &Al-Hemiddi N a

Field Measurement

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ur

M.(2003)

ENVI-met

na

Taleghani M, et al. (2015)

ENVI-met

lP

Taleghani M, et al. (2014)

Moonen P, et al. (2012)

Gao Y,

et al. (2012)

temperature Thermal

PET;

Y

environme

Temperature

nt

;

Wind

speed Natural

Temperature

NN

N

ventilation; Cooling effectivene ss

CFD

Natural

Air change

simulation&Large

ventilation

rate

eddy simulation

unsteady

Field measurement

Natural

Wind speed

NN

Natural

non-dimensi

NN

ventilation

onal

ventilation Sharples S, &Bensalem R. (2001)

Wind tunnel

flow

coefficient

Moonen P, et al. (2011)

CFD

Natural

Normalized

simulation&Large

ventilation

exchange

eddy simulation

Tablada A, et al. (2009)

N

flux

Field measurement

Thermal

PMV;

NN

environme

Air speed

nt fluidyn-PANACHE&

Natural

Normalized

standard k-e model

ventilation

concentratio

Y

ro of

Kumar P,& Feiz A-A. (2016)

n

Rajapaksha I, et al. (2003)

Field measurement;

Thermal

Temperature

CFD

environme

;

nt;

speed

simulation&

standard k-e model

Y

Wind

Natural Almhafdy A, et al. (2015)

IES

-p

ventilation Thermal

Temperature

N

Wind speed

NN N

environme

re

nt;

Natural ventilation

Freefem++;

Thermal

Temperature

Field measurement

environme

;

nt; Natural

speed

na

Rojas JM, et al. (2012)

Wind tunnel

lP

Ok V, et al. (2008）

López-Cabeza VP, et al. (2018)

ENVI-met;

ventilation Field

ur

measurement

Dehghani Mohamadabadi H, et al.

Jo

(2018)

Zhang A et al. (2017)

Wind

Thermal

Temperature

Y

Wind speed

NN

Thermal

PET;

Y

environme

temperature

environme nt;

Wind tunnel

Natural ventilation

ENVI-met;

Design

builder

Air

nt; Kumar P, &Feiz A-A.(2016) (Y-Yes; N-No; NN-No Need)

CFD

Natural

simulation&SST k-w

ventilation

Wind speed

Y

Table 3 𝜀𝐴𝑂𝐴 value of single and combined courtyard for different positions under various inlet approaching wind direction 𝜺𝑨𝑶𝑨

Position

0°

15°

30°

45°

60°

75°

90°

courtyard

1

1

1

1

1

1

1

Inlet

1.12

1.12

1.06

0.89

0.88

0.90

0.99

Outlet

0.91

0.92

0.98

0.73

0.57

0.52

0.31

Inlet

1.11

1.10

1.04

0.88

0.88

0.89

0.67

Outlet

0.82

0.84

0.80

Inlet

1.11

0.80

1.03

Outlet

0.82

0.47

0.77

Case A

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Case 0.75B

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0.62

0.38

0.42

0.61

0.88

0.89

0.89

0.87

0.55

0.38

0.43

0.50

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Case 0.5B

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Case 0.25B