Integrated assessment of indoor and outdoor ventilation in street canyons with naturally-ventilated buildings by various ventilation indexes

Journal Pre-proof Integrated assessment of indoor and outdoor ventilation in street canyons with naturally-ventilated buildings by various ventilation indexes Xia Yang, Yong Zhang, Jian Hang, Yuanyuan Lin, Magnus Mattsson, Mats Sandberg, Ming Zhang, Kai Wang PII:

S0360-1323(19)30740-1

DOI:

https://doi.org/10.1016/j.buildenv.2019.106528

Reference:

BAE 106528

To appear in:

Building and Environment

Received Date: 17 September 2019 Revised Date:

4 November 2019

Accepted Date: 5 November 2019

Please cite this article as: Yang X, Zhang Y, Hang J, Lin Y, Mattsson M, Sandberg M, Zhang M, Wang K, Integrated assessment of indoor and outdoor ventilation in street canyons with naturallyventilated buildings by various ventilation indexes, Building and Environment, https://doi.org/10.1016/ j.buildenv.2019.106528. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

It provides an insight to assess the coupled indoor and outdoor ventilation by CFD. ACH by mean flows(ACHmean)/turbulence(ACHturb)/purging flow rate(ACHPFR) are adopted. Outdoor ACHPFR are from 18h-1 to 4h-1 as H/W=0.5 to 3 and only 0.8-0.9h-1 as H/W=5. Indoor ACHs is smaller than outdoor, and window sizes hardly affect outdoor ACHs. Both outdoor/indoor ACHPFR are greater than ACHmean but smaller than ACHmean+ACHturb.

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To be submitted to Building and Environment 2019

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Integrated assessment of indoor and outdoor ventilation in street canyons with

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naturally-ventilated buildings by various ventilation indexes

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Xia Yang1, Yong Zhang1, Jian Hang1*, Yuanyuan Lin1, Magnus Mattsson2,

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Mats Sandberg2, Ming Zhang3,Kai Wang4*

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1

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Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou, P.R. China

School of Atmospheric Sciences, Guangdong Province Key Laboratory for Climate

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2

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Gävle, SE-80176 Gävle, Sweden

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3

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and Pollution Control，State Power Environmental Protection Research Institute,

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Nanjing, 210031,China

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4

16

London, UK

Department of Building, Energy and Environmental Engineering, University of

State Environmental Protection Key Laboratory of Atmospheric Physical Modeling

Department of Civil, Environmental and Geomatic Engineering, University College

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*Corresponding author: Jian Hang, Kai Wang

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Tel: +86-13710248541

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E-mail address: [email protected]; [email protected]

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Abstract

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The integrated assessments of indoor and outdoor ventilation are still rare so far.

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As a novelty, this paper aims to quantify the influence of street aspect ratios(building

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height/street width, H/W=0.5-5) and window sizes(1m×1m, 1.5m×1.5m) on

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indoor-outdoor

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naturally-ventilated buildings. Numerical simulations with RNG k-ε model are

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validated against experimental data and the grid independence are tested as well. Air

29

change rates per hour(ACH, h-1) are adopted for assessing indoor-outdoor ventilation

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by mean flows(ACHmean) and turbulent fluctuations(ACHturb) respectively. Age of

31

air(τ), purging flow rate(PFR) and its corresponding ACHPFR are used to evaluate

32

overall ventilation capacities.

ventilation

in

two-dimensional

streets

with

single-sided

33

Shallower streets experience better indoor-outdoor ventilation. Outdoor ACHPFR

34

drop from 14.69-17.55h-1 to 3.96-3.97h-1 as H/W rises from 0.5 to 3. In extremely

35

deep canyon(H/W=5), two-counter-rotating vortices produce much smaller velocity at

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low-level regions(U/Uref~10-3-10-5), resulting in small ACHPFR for outdoor

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(~0.76-0.91h-1) and indoor in 1-13th floors(~0.03-0.61h-1). When H/W=0.5-1, leeward

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5-6th floors experience smaller ACHPFR(e.g.~1.13-1.40h-1 as H/W=1) than the other 2

39

floors(e.g. ~1.54-9.52h-1 as H/W=1). Particularly, as H/W=2-3, leeward-side indoor

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ACHPFR in the middle floors (except the first and top two floors) are nearly

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constants(~1.02-1.69h-1) and much smaller than windward-side ACHPFR(~1.41-4.35h-1)

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which increase toward upper floors. Besides, the smaller window size reduces indoor

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ACHPFR by 19.38%~88.28%, but hardly influences outdoor ventilation. Moreover,

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both outdoor and indoor ACHPFR are greater than ACHmean but smaller than

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ACHmean+ACHturb. Although further investigations are still required, this paper

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provides an insight and scientific foundation on integrated indoor-outdoor ventilation

47

evaluation with various effective ventilation indexes.

48 49

Key words: Computational fluid dynamics (CFD); Urban ventilation; Building

50

natural ventilation; Air change rate per hour (ACH); Age of air; Purging flow rate

51

(PFR)

52 53

1. Introduction

54

Under rapid urbanization, an increasing number of people are living in cities.

55

According to the statistics, urban population accounts for 55% of the world’s

56

population in 2018, and it is expected to reach 68% in 2050[1]. High-density urban

57

morphology can raise the efficiency of land use and resource utilization. However,

58

such congested urban conditions may also produce serious environmental problems 3

59

such as reduced urban wind speed, weakened pollutant dilution capacity and

60

strengthened urban heat island intensity etc.[2-4].

61

Moreover, on average, people spend approximately 90% of their time indoors.

62

Indoor air quality issues, which influenced by building ventilation, are drawing

63

increasingly more attention[5]. Since mechanical ventilation requires much more

64

energy to obtain satisfactory ventilation performance, natural ventilation strategy is

65

preferred for its more healthy, lower cost and energy saving alternative, especially in

66

resource-limited regions. As displayed in Fig.A1a in Appendix, urban and building

67

layouts are the essential factors determine the ventilation in both urban streets (i.e.

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outdoor) and natural-ventilated buildings (i.e. indoor). It is of great importance to

69

evaluate their impacts on both indoor and outdoor natural ventilation when

70

developing sustainable urban-built designs for the healthy and low-carbon urban-built

71

environment.

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The urban canopy layers, consisting of buildings and street space with a

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macroscopic roof interface from its above layers, are usually natural-ventilated by

74

external wind from the surrounding rural area and the above atmosphere. Wind from

75

external regions may provide relative clean air into urban areas or street canyons to

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help pollutants dilution (i.e. urban ventilation)[6-12]. Natural ventilation in the

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three-dimensional (3D) urban districts includes three processes: pollutants being

78

mixed and redistributed within urban areas, pollutants being diluted horizontally by

79

wind flushing urban areas and removed across urban boundaries (i.e. horizontal 4

80

ventilation), pollutants being removed out or re-entering vertically through canopy

81

roofs (i.e. vertical ventilation)[6-12]. The most key urban parameters within these

82

processes usually include frontal area index (λf, i.e. the ratio of the frontal area of

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buildings to the total floor area) and plan area index (λp, i.e. the ratio between the

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top-view planar area of buildings and the total floor area) [e.g.9-11]. In addition,

85

building height variations and uneven urban layouts[11-14], street shape and overall

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urban form[15-17], ambient wind directions[16-19], lift-up building design[17-22]

87

etc., are also suggested to be the key influencing factors.

88

In particular, two-dimensional (2D) street canyon, which simplifies the urban

89

geometry and complex urban form to an infinitely long street surrounded by buildings

90

on both sides with a perpendicular approaching wind direction, is widely employed to

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analyze micro-scale climatology in urban areas [e.g.22-31]. The ventilation

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performance of the simplified 2D street canyons is usually worse than that of the 3D

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urban models, since lateral boundary effects are neglected and pollutants can only be

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vertically removed out across street roofs. Street aspect ratio (street height/street

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width, H/W) becomes the first key parameter for local turbulence and pollutant

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dispersion. Three flow regimes, i.e. the isolated-roughness flow regime (H/W<0.3),

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the wake-interference flow regime (0.3
98

one-main-vortex structure (0.67
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with considerable urban ventilation performance[23-26, 29]. The fourth regime,

100

named as the multi-vortex regime in which two or more vertically-aligned vortices 5

101

appear, usually induces much weaker pedestrian-level wind and worse urban street

102

ventilation [e.g.27-31]. However, there are different findings on the multi-vortex

103

regime in the literature [e.g.27-31]. The wind-tunnel-scale studies report that, with the

104

reference Reynolds number (Re) of 12000 (H=6cm), two contra-rotative vortexes are

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formed as H/W>1.67 and three to five vertically aligned vortexes are formed as

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H/W=3-5[27-28]. However, the other group finds only one main vortex exists as

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H/W=1-4 and two main vortexes appear as H/W=5-6 in full-scale street canyons

108

(W=24m, H=24-144m, Re~106–107)[30-31]. Recently, by conducting water channel

109

experiments with three aspect ratios(H/W=1, 1.5 and 2, Re~104-105), Chew et al.[32]

110

experimentally verifies that the widely adopted Re=11,000 is not applicable for Re

111

independence of 2D street canyons as H/W>1.5. Thus full-scale 2D deep street canyon

112

models(H/W=2-6)

113

wind-tunnel-scale models sometimes cannot [27-28].

usually

satisfy

Re

independence

requirement[30-31]

but

114

Furthermore, natural building ventilation is an important strategy in the

115

development of sustainable and healthy indoor environments, which represents the

116

process of exchanging indoor air with outdoor external air by natural wind. In the past

117

decades, many researches contributed to natural building ventilation assessment as

118

reviewed by Chen[5]. Similar Reynolds number independence phenomena is also

119

reported for the flow and pollutant dispersion coupling indoor and outdoor[33]. In

120

addition, the literature experimentally confirmed natural ventilation through the

121

windows or doors can reach large air change rate per hour of ACH=69h-1[34] which 6

122

can effectively reduce 24-hour air-borne infection probability predicted by Well-Riley

123

model. Different from mechanical ventilation in enclosed space such as hospital

124

isolation rooms[35], aircraft cabins[36] and high-speed trains[37] etc., building

125

natural ventilation significantly depends on the coupling impacts between indoor and

126

outdoor airflow[38-44]. Therefore, besides the locations and sizes of the ventilation

127

openings[38-40], ambient building arrangement and wind conditions [41-46], balcony

128

settings[47-48] etc. have been verified as key influencing factors. Furthermore, if

129

background wind speed is relatively small, thermal buoyancy force may significantly

130

interact with wind-driven dynamic force or dominate the flow for both urban

131

(outdoor)[49, 50] and building (indoor)[51, 52] ventilation. It represents natural

132

ventilation performance can be maximized or compromised by the proper or improper

133

external flow conditions which are determined by the surrounding urban morphology

134

and background atmospheric conditions. For example, greater building packing

135

densities usually reduce pressure difference across buildings, and subsequently

136

decrease natural building ventilation potential.

137

To conclude, it is meaningful to develop sustainable urban-built design for

138

improving both urban ventilation and natural building ventilation at the same time,

139

which are influenced by building packing densities, street aspect ratios, building

140

height variations and ambient wind directions etc. Previous investigations have been

141

performed for outdoor ventilation modelling in urban areas or indoor ventilation

142

assessment in naturally-ventilated buildings separately, however the integrated 7

143

evaluations of both indoor and outdoor ventilation with effective ventilation indexes

144

are still rare so far.

145

Therefore, this paper aims to adopt effective ventilation indexes to assess both

146

indoor and outdoor ventilation driven by mean flows and turbulent fluctuations. As a

147

start, we first consider urban ventilation and wind-driven single-sided building

148

ventilation in 2D street canyons with naturally-ventilated buildings under neutral

149

atmospheric conditions. The influences of aspect ratios (H/W=0.5, 0.67, 1, 2, 3, 5) and

150

window sizes (1.5m×1.5m or 1m×1m) are first emphasized. Because only ACH is not

151

sufficient to assess how external air is supplied into a room or street for indoor or

152

outdoor ventilation[44], three ventilation indexes— age of air, air change rate per

153

hour by mean flows and turbulence fluctuations (ACH), purging flow rate (PFR) and

154

the corresponding ACHPFR are used to quantify both building and urban ventilation

155

capacity. Overall, this paper is one of the first attempt to couple indoor and outdoor

156

ventilation assessment in street canyons with multi-storey naturally-ventilated

157

buildings. Previous CFD studies of the inter-unit dispersion [e.g. 37, 48] or natural

158

building ventilation [e.g. 41-44] by coupling indoor and outdoor airflow can provide

159

meaningful reference to present CFD simulations for integrated urban-built natural

160

ventilation evaluation.

161

This paper is structured as follows: Section 2 presents model descriptions of the

162

investigated and CFD methodologies by coupling indoor and outdoor airflow. Section

163

3 introduces ventilation indexes, i.e. air change rate per hour, age of air and purging 8

164

flow rate for indoor and outdoor ventilation assessments. Section 4 depicts results and

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discussions. The conclusions are drawn in Section 5.

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2. Model descriptions and CFD methodology

168

At present, comparing with wind tunnel experiments and field measurements,

169

computational fluid dynamics (CFD) simulations can be fully controlled and can offer

170

the data of every parameter at each point of CFD domain. Thus CFD is becoming the

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most widely used method for indoor and outdoor ventilation modeling, i.e. indoor

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ventilation [e.g. 35-36], outdoor ventilation[8-22, 27-31, 55-62] and that by coupling

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indoor and outdoor[33, 39-48,52-54] simulations. There are coupled and decoupled

174

approaches in indoor-outdoor flow simulations[41-44, 52-54]. In a coupled method,

175

both the outdoor and indoor environments are simulated in a single computational

176

domain. As recommended and reviewed in the literature[53-56], this paper will

177

employ the coupled approach which has been verified to be more accurate for flow

178

modelling in the proximity of and through the ventilation openings than the decoupled

179

approach. The accuracy and reliability of CFD simulations for indoor and outdoor

180

airflow will be carefully validated following the CFD guidelines[53-61].

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2.1 Turbulence models in CFD simulations Although large eddy simulation (LES) models are known to perform better in 9

184

predicting turbulent flows than the Reynolds-Averaged Navier-Stokes (RANS)

185

approach does[53-57, 62], RANS models have been adopted more widely for both

186

indoor and outdoor ventilation modelling[8-12, 14-19, 35-37, 39-50] because they

187

predict mean flows and the spatial average flow properties generally well with much

188

less computational time than LES. Among RANS approaches, the Renormalization

189

Group (RNG) k-ε model is recommended as one of the best choices and the most

190

widely-adopted for predicting the flow coupling indoor and outdoor[e.g. 44, 48, 53].

191

In this study, Ansys FLUENT[63] with the RNG k-ε model is used to solve the

192

steady-state flow field. The governing equations for the flow and turbulent quantities

193

of incompressible fluid are shown as below.

194

The mass conservation equation: =0

195

(1)

The momentum conservation equation: =−

1 +

196

−

''

(2)

''

The transport equations of turbulent kinetic energy (k) and its dissipation rate (ε): =

+

−

(3)

10

=

4

5+'

0

6

7 − '20

2

(4)

197

Where #" means time-averaged velocity components (Ux,Uy,Uz is the velocity in

198

stream-wise, span-wise and vertical directions). v represents the kinematic viscosity.

199

The Reynolds stress tensor: −

’’

’’

=

2 − kδij 3

+

8

(5)

200

$ij is the Kroneker delta (a piecewise function of variables i and j). And the kinematic

201

eddy viscosity:

8

202

= '(

2

(6)

The turbulence production term: =

8

×

;

;

+

;

(7)

203

The default constants in the RNG k-ε model are as follows[63]:

204

'( = 0.0845，

= 1.393，/0 = 1.393，'

0

= 1.42，'20 = 1.68

205 206

2.2 Validation of airflow for building ventilation coupling indoor and outdoor

207

In this study, a wind tunnel experiment at Cardiff University[64] involving

208

windward and leeward single-sided ventilation of a single cubic building was used to 11

209

validate present CFD methodologies including the turbulence models and grid

210

settings.

211

For accurate full-scale airflow simulation, Re similarity is significant in this study.

212

In Jiang’s experiment[64], the reference Re number is about 162,000, which is large

213

enough to ensure the Re independence. As shown in Fig.1b, a full-scale building

214

model with the height of H=2.5m was built in our CFD simulations. The scale ratio of

215

building model to wind tunnel model equals 10:1. The window with a size of

216

1.25m×0.84m is located in the middle of windward or leeward building wall. Fig.1a

217

displays the computational domain. The distances from the domain inlet, domain

218

outlet, domain top, domain lateral boundaries to the building model are 4H, 8H, 4H,

219

4H respectively. The dimensions are similar with those in the literature [44] and

220

match the wind tunnel experiment as well.

221 222

At the domain inlet, the vertical profiles of stream-wise velocity follow a logarithmic law, and the turbulent quantities were defined as follows[44]. < 6=7 = 6 ∗ /@7 AB6 =/=C 7 6=7 =

∗

2

(8a)

/D'(

6=7 = '( E/F

E/2

(8b) /6@=7

(8c)

223

where u* is the friction velocity, equals 1.068m/s, κ is Von Karman’s constant which

224

equals 0.41, =C equals 0.05m denoting the roughness height in the full-scale model,

225

velocity components in y and z directions are zero, '( is 0.09. These profiles at the 12

226

domain inlet refer to those measured in wind tunnel[64] and previous literature[44].

227

Zero normal gradient boundary conditions are adopted at the domain outlet (i.e.

228

outflow), domain roof and lateral boundaries (i.e. symmetry).

229

RNG k-ε model was employed in this validation. Standard wall function was used

230

as near-wall treatment. The SIMPLE algorithm was adopted as pressure-velocity

231

coupling method. The second order upwind scheme was used for discretizing the

232

convection and diffusion-convection terms. The absolute residual for continuity

233

equation, velocity components, k, ε were all below 10-5. The calculation didn’t stop

234

until all residuals became constant.

235

Different grid arrangements with the minimum grid size of 0.05m (fine grid),

236

0.1m (medium grid), 0.2m (coarse grid) at wall surfaces were compared to test the

237

grid independence of numerical solutions. The grid expansion ratio is 1.25 from wall

238

surfaces toward the surroundings. Fig.2 depicts the vertical profiles of normalized

239

stream-wise velocity (Ux(z)/Uref) from both experimental data and numerical results at

240

the lines of x=-H/25, x=H/2 and/or x=3H/2 in the centre plane of the cube. The

241

numerical profiles of velocity below and near building height (z<1.2H) show a good

242

agreement with the wind tunnel data, which confirms that present CFD methods is

243

performing well in the prediction of single-sided ventilation airflow by coupling

244

indoor and outdoor. In terms of different grid arrangements, three computed statistics

245

named normalised mean square error (NMSE), fractional bias (FB) and correlation

246

coefficient (R) were computed [65]. Table. 1 compared the three grid arrangements 13

247

deviation, which shows high correlation between simulation results and wind tunnel

248

data (R>0.956) The FB values indicate that simulation results generally overestimate,

249

though the overestimation is small (e.g. -0.008~-0.261). In addition, the low values of

250

NMSE are found. From the table, fine grid arrangements and medium grid

251

arrangements have a higher R, besides, their NMSE and FB are smaller than coarse

252

grid arrangements. The medium grid and fine grid perform the same well and better

253

than the coarse grid in predicting velocity profiles(Table.1). Considering both the

254

simulation accuracy and computational time cost, the medium grid arrangement was

255

used in the indoor mesh generation and further CFD modelling.

256 257

2.3 CFD setups in case studies

258

An ideal two-dimensional urban street canyon is built for the CFD simulations in

259

this study. For example, Fig.3a shows the full-scale computational domain of H/W=2

260

and the detail description of target canyon with indoor rooms. The span-wise length(y

261

direction) of target street canyon(L) and building width(B) remain constant as

262

L=B=20m, the specific height of each floor is 3m including 0.3m floor slab. In this

263

study, only one window set in the windward or leeward wall in each storey of the

264

buildings alongside the street and single-sided natural ventilation is considered here.

265

Fig.3b provides the schematic view of mesh information on the vertical center plane.

266

The improved resolution cells is used for window area, considering its importance in

14

267

the indoor-outdoor air exchange. In this study, six aspect ratios (H/W) and two

268

window sizes, which consist twelve different cases are investigated(Table.2).

269

Different cases with various building configurations are named as Case [H/W,

270

w(window size)], where ‘H/W’ represents the various building heights and street

271

canyon width and ‘w(window size)’ is employed to discriminate the two window size.

272

For example, the case when building height and street canyon width are both 24m

273

thus the calculated H/W=1, and window size=1.5m×1.5m, is named as Case [1, w1.5].

274

The opening ratio which denotes the percentage of the area of window against the

275

area of the wall (i.e. 9.26% or 20.83% in this paper) has been used to characterize the

276

window size and window setting dimensions.

277

When dealing with the approaching flows from the domain inlet to the target

278

urban model, a certain number of buildings sets up in front of and behind the target

279

building to serve as roughness elements in order to develop an urban boundary layer[6,

280

28-31, 45,49]. In this study, both front and rear of the target street canyon has a

281

typical street canyon to reproduce roughness elements. At the domain inlet, the

282

vertical profiles of stream-wise velocity and turbulent quantities are defined in Eq.(9):
(9a)

= L6< 6=7 × M K 72

ε K 6=7 = 283

=−I J 7 =H

Q

(9b)

Q

S S OP R TU VW

(9c)

Where the reference velocity Uref=3m/s. H indicates the building height and the 15

284

reference height zref=24m. The power-law exponent of

285

underlying surface roughness above medium-dense urban area. The turbulent kinetic

286

energy

287

Eq.(9b), where turbulence intensity M K = 0.1 and a=1[60]. Von Karman constant

288

@ = 0.41 and '( = 0.09 are empirical constants.

K

= 0.22 indicates

is calculated from the mean wind speed and the turbulence intensity using

289

For the lateral and upper boundaries of the computational domain and the

290

downstream boundary, the normal velocity component and normal gradients of

291

tangential velocity components are set to zero (i.e. zero normal gradient).

292

To conduct the numerical simulations, the governing equations are discretized to

293

algebraic equations on a staggered grid system based on the finite volume method

294

(FVM). Standard wall function is used as near-wall treatment. The discretization

295

schemes used for the convection and diffusion terms are the second-order upwind

296

scheme. And the pressure and velocity couple with the SIMPLE algorithm.

297

Convergence is assumed to be obtained when all residuals [e.g. 53] stop decreasing

298

and reached a minimum value of 10-6 for x, y and z momentum, 10-5 for k and 10-4 for

299

ε and continuity.

300 301

3. Ventilation assessment indexes for indoor and outdoor

302

In recent years, some indoor ventilation indices have been used to assess and

303

quantify urban ventilation and pollutant dilution capacity such as air change rate per 16

304

hour (ACH) [7-9, 17, 40-44], purging flow rate (PFR) and net escape velocity (NEV)

305

[16, 66], age of air (τ) and air exchange efficiency[8-11], etc. These studies treat

306

urban canopy layers (UCL) as outdoor space similar with indoor environments and

307

propose the assumption that the ventilation processes outdoors and indoors are similar:

308

supplying external air and distributing them within a space, pollutant dilution and

309

removal by mean flows, recirculation of contaminants by turbulent mixing. The major

310

difference is that there is a large-area open boundary (i.e. urban roof) at the urban top,

311

whose fraction of area is much larger than that of supplies and openings (windows etc)

312

to the total surface area of rooms or buildings. Therefore, pollutant removal induced

313

by vertical turbulent diffusion across street roofs are much more significant for urban

314

ventilation than that through windows/openings of rooms for indoor ventilation.

315

For considerations of ventilation effectiveness, Fig.4 briefly illustrates various

316

ventilation indices for outdoor (Fig. 4a) and indoor (Fig.4b) respectively. First, the age

317

of air (τ, unit: s) is employed in evaluating how external wind supplies relatively

318

clean air into a room or street canyon. Then air change rate per hour (ACH, unit: h-1)

319

due to mean flows (ACHmean) and turbulent diffusion (ACHturb) are separately defined

320

to assess the strength of air exchange across street roofs or window openings. Finally,

321

to assess the net natural ventilation performance induced by mean flows and turbulent

322

diffusion, purging flow rate (PFR) are adopted for both street canyon (outdoor) and

323

rooms (indoor).

324

It is worth mentioning that, in the following tracer gas simulations, carbon 17

325

monoxide (CO) is employed as the tracer gas in rooms or the target street canyon for

326

indoor and outdoor ventilation respectively. And the CO source with the pollutant

327

emission rate of 10-5kg·m-3s-1 is located in the entire target street(Fig. 4a) or rooms

328

(Fig. 4b) for outdoor and indoor age of air simulation and PFR calculation.

329

The governing equation of time-averaged concentration is: O

−

O

]^

= _`

(10)

330

where

331

pollutants ]^ =

332

by the literature [8-12, 66], the pollutant was set in the entire street or the whole

333

rooms separately after the steady airflow obtained at the emission rate

334

_`=10-5kg∙m-3∙s-1.

is the time-averaged velocity components, the turbulent eddy diffusivity of 8 /a^8 ,

a^8 = 0.7 is the turbulent Schmidt number as recommended

335

For boundary conditions of Eq.(10), the inflow concentration is defined as zero

336

at the domain inlet, zero normal flux condition is set at wall surfaces and zero normal

337

gradient condition is applied at the domain roof and outlet.

338 339 340

3.1 Age of air (τ) Local mean age of air (τ) was originally defined to represent how long the

341

external air can reach an arbitrary point after it enters a room[5, 65], by supposing that

342

external air of a room is relatively clean and its age is zero. Later, it was extended for

18

343

urban ventilation assessment[8-11] by assuming that the external air is cleaner than

344

that in the street canyon and external wind can bring fresh air into a street to help

345

pollutant dilution. Thus the poor-ventilated zones in a room or street require a longer

346

time for external air to arrive and experience larger age of air for indoor[5, 44, 67] or

347

outdoor[8-11]. Here we also note that the age of air for street canyon is actually the

348

effective age of air[8].

349

This paper adopts the homogeneous emission method[67] to calculate age of air

350

in target street canyon[8] and each room of multi-storey buildings[44]. If a

351

homogeneous pollutant release rate (_`, kg∙m-3∙s-1) is fixed in the entire target street or

352

ventilated room, age of air (τ, unit: s) is proportional to the concentration attained at

353

the same point as defined in Eq.(11): f = 'g /_`

(11)

354

where _` is the emission rate in target street canyon or each room which equals 10-5

355

kg∙m-3∙s-1 in this study. τ and Cp are the age of air and tracer gas concentration at a

356

point respectively.

357 358

3.2 Air change rate per hour (ACHmean and ACHturb)

359

Air change rate per hour (ACH) has been widely adopted to evaluate ventilation

360

in rooms [5, 40-44] which represents the rate at which the total indoor volume is

361

replaced with external fresh air. Later, it was adopted to quantify ventilation capacity 19

362

in 2D street canyons or 3D urban models[6-7, 11, 17]. ACH (unit:hj 7 induced by mean flows and turbulence fluctuation across the

363 364

urban canopy boundaries and room openings are defined as below [6-7, 11, 17]: k'Il

mK

=

k'I8pHq =

3600nl oA

mK

3600n8pHq oA

Qmean = ∫ V • ndA

(12a)

Qturb = ± ∫ 0.5σ u dA

(12b)

365

nl

366

where ACHmean and ACHturb are air change rate per hour induced by the volumetric flow

367 368

rates (m3·s-1) through boundaries of a room or street by mean flows (Qmean) and turbulent ur r fluctuations (Qturb). V is velocity vector, n is the normal direction of room openings or

369

street roofs, A is surface area; σ u = w ' w '= u ' u ' = 2 k / 3 is the fluctuation velocity

370

on room opening or street roofs based on the approximation of isotropic turbulence (k is

371

the turbulent kinetic energy)[6-7, 11, 17].

mK

and n8pHq is the volumetric flow rate (m3·s-1) of a room or canyon.

372

In this paper, ACHmean only indicates the indoor and outdoor air exchange under

373

the influence of the mean velocity at the top of street canyon or window in the rooms.

374

ACHturb further describes the air exchange caused by turbulent fluctuations across the

375

street canopy or windows, but they cannot describe the net ventilation capacity. Thus

376

purging flow rate will be defined later.

377 378 379

3.3 Purging flow rate (PFR) and the corresponding ACHPFR The purging flow rate (PFR, unit:m3·s-1) was first defined to assess the net 20

380

airflow rate of flushing a room induced by the mean flows (i.e. convection) and

381

turbulent diffusion[67]. Later, PFR was extended for evaluating the net capacity of

382

removing pollutant in urban domain[16, 66, 68].

383

Thus, this paper adopts PFR as the net airflow rate of flushing the whole room

384

(indoor) or street canyon (outdoor) induced by both convection and turbulent

385

diffusion. As defined in Eq.(13a), if a homogeneous tracer gas release rate (_` ,

386

kg∙m-3∙s-1) is fixed in the entire target street or ventilated room(Fig.4), PFR can be

387

calculated as the ratio of tracer gas release rate to the spatial mean concentration in

388

the volume. Moreover, to have a better comparison between indoor and outdoor

389

ventilation, ACHPFR which is the ACH calculated by PFR is also defined in Eq.(13b).

390

PFR and ACHPFR are defined as below: rst =

_` × oA < 'g >

k'Iwxy =

(13a)

3600rst oA

(13b)

391

Here _` =10-5 kg∙m-3 ∙s-1 is the volumetric emission rate, <'g > is spatial mean

392

concentration (kg∙m-3). In addition, this paper defined PFR as the source of a whole

393

room or a whole canyon rather than a source point to reflect dilution properties.

394 395

4. Results and discussions

396

4.1 Influence of aspect ratio in shallow canyon (H/W=0.5, 0.67, 1 as H=24m) 21

397

This subsection investigates the influence of aspect ratio on the indoor and

398

outdoor ventilation in three kinds of shallow street canyons(H/W=0.5, 0.67, 1) with

399

the constant building height(H=24m) but different street widths.

400

4.1.1 Outdoor ventilation in shallow street canyons (H/W=0.5-1)

401

Fig.5-6 first display the simulated normalized velocity magnitude(U/Uref),

402

normalized turbulent kinetic energy(k/Uref2) and the outdoor age of air (τ) as H/W=0.5,

403

0.67, 1 respectively. As depicted in Fig.5, only one main clockwise vortex appears in

404

all these three streets, and the vortex center with H/W=0.5 locates near the windward

405

wall, while those with H/W=0.67, 1 are near the street centre. In addition, U/Uref near

406

windward-side wall is found to be always greater than that near leeward-side wall.

407

Furthermore, the normalized turbulent kinetic energy is considerably larger when near

408

street roof and windward walls than that near street ground and leeward walls. The

409

maximum k/Uref2 (>0.08) appears at the upper corner of street roof and windward

410

building. Meanwhile, in such single-sided ventilated buildings, the indoor velocity is

411

always much smaller than outdoor. More importantly, when street width decreases

412

from 48m to 24m, k/Uref2 becomes smaller in the whole canyon. As a result of flow

413

field in Fig. 5, wind first transports external clean air into the windward side, thus the

414

windward side age of air is smaller than the leeward side and around vortex centre

415

(Fig.6). Moreover, narrower street canyon tends to experience greater age of air and

416

worse ventilation in the entire street. Especially for H/W=1, air near the corner of the

417

leeward building and street center is particularly old (τ>500s). 22

418

To quantify the outdoor ventilation capacity, Table.3 summarizes ACHPFR and air

419

change rates per hour across street roofs due to mean flows(ACHmean) and turbulent

420

diffusion(ACHturb) in these three street canyons with H/W=0.5, 0.67 and 1(H=24m). It

421

is noticed that as H/W rises from 0.5 and 0.67 to 1, ACHmean decreases from 6.07 and

422

7.98h-1 to 5.93h-1, meanwhile ACHturb is reduced considerably from 34.59 and

423

22.94h-1 to 15.15h-1. For the net ventilation capacity of mean flows and turbulence

424

fluctuations, ACHPFR goes down a little from 14.69 and 13.95h-1 to 10.78h-1. In all

425

cases, outdoor ACHPFR is larger than ACHmean but smaller than the sum of ACHmean

426

and ACHturb. For example, in H/W=0.5, the sum of ACHmean and ACHturb is 40.66h-1

427

and ACHPFR is 14.69h-1. Pertaining to the specific ventilation efficiency of ACHPFR

428

associating with ACHmean and ACHturb still require further researches. In summary,

429

results show that outdoor ventilation for narrower street of H/W=1 is poorer than

430

shallower canyons(H/W=0.5, 0.67).

431 432

4.1.2 Indoor single-sided ventilation of near-road buildings(H/W=0.5-1)

433

As an example, Fig.7a-b displays velocity vector and normalized stream-wise

434

velocity(Ux/ Uref) within and near all rooms of near-road buildings in Case[0.5, w1.5]

435

and indoor age of air in all rooms in Case[0.5, w1.5], Case[0.67, w1.5], Case[1, w1.5].

436

Generally, indoor space is naturally-ventilated by air exchange through openings (e.g.

437

flow or stream-wise velocity perpendicular to windows here). Fig.7a shows that

23

438

normalized stream-wise velocity in the windward-side rooms is larger than that in the

439

leeward-side rooms, and the flow fields are similar in the middle floors (2th to 7th

440

floor). Subsequently, the windward-side rooms possess better ventilation capacity

441

than leeward-side rooms and the upper floors obviously experience smaller age of air

442

than other floors because of the larger local velocity. Besides, in leeward-side rooms,

443

the age of air become larger as H/W increases from 0.5 to 1. On the contrary, in the

444

first or second floor of the windward-side rooms, age of air as H/W=0.5 and 0.67 is

445

greater than that as H/W =1(Fig.7b).

446

To evaluate the influence of aspect ratios on indoor ventilation, Fig.7c-e

447

compares ACHmean, ACHturb, ACHPFR for all leeward and windward rooms in Case[0.5,

448

w1.5], Case[0.67, w1.5], Case[1, w1.5]. Because of the uniform motion at the window

449

of the middle levels where the outdoor velocity vector is parallel to the

450

window(Fig.7a as an example), the smallest ACHmean as H/W=0.5-1 appears in 4th and

451

5th floor (~0.26h-1-1.04h-1) for both windward-side and leeward-side rooms(Fig.7c).

452

ACHturb in the upper floors are larger than that in the lower floors in windward

453

building(Fig.7d), for example 33.22h-1 in the top floor and 8.53h-1 in the first floor at

454

windward building as H/W=0.5, which is consistent with the distribution of k. While

455

ACHturb in leeward-side rooms (~3.20h-1-9.06h-1) is much smaller than the

456

windward-side (~4.95h-1-33.22h-1) in all three cases. In addition, as H/W increases

457

from 0.5 and 0.67 to 1, ACHturb decreases significantly. Subsequently, as shown in

458

Fig.7e, the air purification ability(i.e.indoor ACHPFR) improves in the wider canyon, 24

459

and the distribution of ACHPFR in different floors demonstrates great discrepancy with

460

that of ACHmean. Windward-side ACHPFR above the 3th floor basically rise toward

461

upper floors, while leeward-side ACHPFR decreases with the increasing floor number

462

except the roof-level two floors. For example, as H/W=1, leeward 5-6th floor

463

experience the smaller ACHPFR from 1.13 to1.40h-1 comparing with the other

464

floors(~1.54-9.52h-1).

465

Overall, when the building height remains the same, the wider the canyon, the

466

better both indoor and outdoor ventilation performance. And windward-side rooms

467

basically enjoy the better ventilation than leeward-side rooms. Indoor ACHPFR for

468

most rooms(~1-5h-1) are smaller than those of outdoor ACHPFR(10.78-14.69h-1).

469 470

4.2 Influence of aspect ratios in typical deep canyon(H/W=2, 3 as W=24m)

471

Street aspect ratio is the key parameter affecting the airflow characteristic. This

472

subsection investigates the influence of raising aspect ratio on both indoor and

473

outdoor ventilation in typical deep street canyons(H/W=2, 3) with the constant street

474

width(W=24m) but different building heights(H=48, 72m with 16, 24 floors).

475

Fig.8a-f represent the configuration of the normalized velocity(U/Uref) field and

476

outdoor age of air for Case [1,w1.5], [2,w1.5] and [3,w1.5](H/W=1-3). Obviously,

477

there is only one main vortex in all three street canyons. This one-main-vortex

478

structure in deep street with H/W=2 and 3 can be verified by the scale-model outdoor 25

479

measurement displayed in the Appendix (see Fig. A1) and our previous study[22].

480

Moreover, as H/W rises from 1 to 3, the wind speed reduces and urban age of air

481

increases dramatically in the whole street canyon. In particular, Fig.8g-h illustrate

482

horizontal profiles of pedestrian-level normalized velocity and age of air at z=1.5m in

483

target street canyon. In contrast to street with H/W=1, the pedestrian-level normalized

484

velocity with H/W=3 is nearly seven times smaller and its age of air is almost four

485

times larger (i.e. much older air stays calm along the lower part of the canyon).

486

As indicated in Table.3, compared to shallow street(10.78-14.69h-1 as H/W=0.5-1),

487

outdoor ACHPFR drop evidently in typical deep canyon(3.96-6.08h-1 as H/W=2-3).

488

Similarly, ACHmean decreases dramatically from 5.93-7.98h-1 to 1.02-1.51h-1, however

489

ACHturb is reduced less times(15.15-34.59h-1 to 11.52-17.11h-1). All these results

490

confirm the much worse outdoor ventilation in typical deep street(H/W=2-3) than the

491

shallow ones(H/W=0.5-1).

492

As shown in Fig.8a, c, e, there is a uniform vertical motion on the facade of

493

windward side and leeward side. Such flow pattern causes the poor ability to expel the

494

fresh air into the rooms in the middle floors as H/W=2-3. As a result, Fig.9a shows the

495

age of air in the middle-floor rooms of deep street(H/W=2-3) have comparatively

496

similar values(~3500s) which is much greater than that(~1000s) in shallow street

497

(H/W=0.5-1). Particularly, as H/W=3, the age of air in the first floor of windward and

498

leeward side reach up to 7000s and 5000s respectively. The top floors generally

499

experience much younger air than the other floors. Moreover, Fig.9a also depicted the 26

500

differences in indoor ventilation capacity(i.e. age of air) between windward-side and

501

leeward-side for the typical deep canyons(H/W=2, 3) is more evident than that for the

502

shallow canyons(H/W=0.5, 0.67, 1).

503

Then Fig.9b-d depict various indoor ACHs of all rooms as H/W=1-3. It

504

demonstrates that ACHPFR is also greater than ACHmean but much less than

505

ACHmean+ACHturb. Indoor ACHmean and ACHPFR in the middle floors are relatively

506

small (e.g. ACHPFR~1.36h-1, 1.08h-1 and 1.10h-1 in leeward 5th, 9th, 11th floor as

507

H/W=1, 2, 3 respectively), and those in the upper and lower floors are much larger. In

508

deep streets with H/W=2 and 3, the constant ACHmean and ACHPFR in the middle floors

509

(except the top and bottom two floors) are found much smaller(e.g. leeward-side

510

ACHPFR~1.02-1.69h-1 and windward-side ACHPFR~1.41-4.35h-1) than shallow streets

511

as H/W=0.5-1(leeward-side ACHPFR~1.13-2.72h-1 and windward-side ACHPFR

512

~2.54-7.14h-1 ).

513

Additionally, for indoor ventilation in leeward-side rooms as H/W=2 and

514

3(Fig.9c-d), we can find the small but relatively comparable ACHmean(0.20-0.82h-1

515

and 0.07-0.52h-1), ACHturb(3.41-5.45h-1 and 1.78-4.60h-1) and ACHPFR(1.07-2.29h-1

516

and 0.72-1.72h-1). In windward-side rooms, indoor ACHturb and ACHPFR are found

517

rising significantly with the increase of z/H. And leeward-side rooms always have a

518

poorer ventilation performance than windward-side, for example, leeward-side

519

ACHPFR (~1.93h-1) at the top-floor room is at least four times smaller than

520

windward-side ACHPFR (~8.59h-1) as H/W=2. 27

521

Overall, in contrast to shallow streets(outdoor ACHPFR~10.78-14.69h-1,

522

H/W=0.5-1), typical deep canyon as H/W=2 and 3 experience worse outdoor

523

ventilation(ACHPFR~3.96-6.09h-1) due to the smaller velocity, moreover the

524

leeward-side middle-floor rooms(except the first and second floors) attain smaller

525

ACHPFR(~1.02-1.69h-1) and windward-side ACHPFR rise toward the upper floors

526

(~1.41-4.35h-1).

527 528

4.3 Indoor and outdoor ventilation in extremely deep street canyon(H/W=5)

529

This indoor and outdoor ventilation are further investigated for the extreme deep

530

canyon(H/W=5) with W=24m and H=120m(40 floors). Fig.10 represents the flow

531

field and indoor-outdoor ventilation capacity in extremely deep canyon when H/W=5,

532

with window size of 1.5m×1.5m. Contrary to shallow and typical deep street canyon,

533

two counter-rotating vortices are observed as H/W=5, including one main, stronger

534

and clockwise vortex in the upper part and another weaker counter-clockwise vortex

535

in the lower part(Fig.10a). In general, the main vortex in the upper part is similar to

536

H/W=3, while the velocity in the lower part of canyon is very small (Fig.8g, U/Uref

537

~10-3-10-5).

538

As indicated in Table.3, outdoor ACHs become relatively small as H/W=5, i.e.

539

0.76 h-1, 0.61h-1 and 6.91h-1 for ACHPFR, ACHmean, ACHturb respectively. In particular,

540

ACHPFR as H/W=5(0.76 h-1) is only 19.2% of that as H/W=3(3.96 h-1). Fig.10b 28

541

displays that k/Uref declines exponentially toward low levels of the extremely deep

542

street canyon(H/W=5), which is 10-2 near windward side around the roof height and

543

10-6-10-7 at the pedestrian level. These variations result in seriously adverse effect to

544

the airflow at the lower part of canyon and age of air reaches as large as 3×104s at the

545

pedestrian level(Fig.10c). Age of air ranges from 5×103s to 3×104s in the lower part

546

(below 10th floor) of the canyon and age of air at pedestrian level(Fig.8h) increases

547

from leeward side to windward side owing to the weak and counterclockwise vortex.

548

In addition, the junction of two vortices is at the level of 10th to 16th

549

floor(Fig.10a), which leads the airflow entering into the rooms through different ways

550

below and above it. Fig.10d shows the detailed description of various indoor ACH

551

indices. Indoor ACHmean in leeward-side rooms is small(~10-3-10-1h-1). For the lower

552

floors, less ACHs(e.g. ACHPFR~0.04-0.61h-1) appears in 1st-13th floors and a great

553

increase can be found in the windward 13th-16th floors which is consistent with the

554

junction location of two vortices. Leeward-side rooms experience smaller ACHs than

555

windward-side rooms. Windward-side rooms at the 39th and 40th floor enjoy a much

556

better ventilation performance than other floors, and ACHPFR increases suddenly from

557

4.31h-1 of the 38th floor to 8.11h-1 of the 40th floor.

558

To sum up, when the canyon is extremely deep(H/W=5), the lower

559

counter-clockwise weak vortex will be produced below the upper clockwise vortex, as

560

a result, leeward-side pedestrian regions attain slightly younger air than windward

561

side. The wind in the lower-vortex region is 1-2 order weaker, thus street ventilation 29

562

in this region is extremely poor and rooms in the corresponding lower floors(1-13th

563

floors) experience extremely small indoor ACHPFR(~0.04-0.61h-1) than the

564

upper-vortex regions(16-40 floors, ~0.98-8.11h-1). Under such weak indoor and

565

outdoor airflow, future research will consider the influence of buoyancy force induced

566

by solar shading and air-wall temperature difference whose effects cannot be

567

neglected.

568 569

4.4 Effects of window size on indoor-outdoor ventilation

570

The indoor-outdoor air exchange is significant associated with the window size

571

which plays the critical role on the natural ventilation. As an example, Fig.11 shows

572

the velocity and age of air in Case[1,w1.5] and Case[1,w1] with window sizes of

573

1.5m×1.5m(Fig.11a,c) and 1m×1m(Fig.11b,d) with opening ratio of 20.83% and 9.6%

574

respectively. Fig.11a-b confirms that the vortex location and velocity are almost the

575

same for two window sizes. Thus, there is little difference in the air age

576

distribution(Fig.11c-d). Similarly, Table.4 lists ACHmean, ACHturb and ACHPFR under

577

different aspect ratios(H/W~0.5-5) with these two window sizes, showing that the

578

discrepancies of ACHs between two window sizes are very small, suggesting the

579

impact of window sizes on outdoor ventilation is negligible.

580

However, the indoor ventilation capacity may be seriously deteriorated with the

581

decrease of window size. Fig.12a-b display the indoor age of air in windward-side and 30

582

leeward-side rooms with two window sizes as H/W=1-3. Obviously, smaller window

583

size(1m×1m) experience about double age of air than larger one (1.5m×1.5m). For

584

example, as H/W=3, age of air in first floor of the leeward and windward buildings

585

reach from 5000s and 7000s to 10000s and 20000s respectively. Fig.12c-e depict the

586

percentage of ACH: ACH(1m×1m) /ACH(1.5m×1.5m) on each floor as H/W=1-3. Window

587

size of 1m×1m attains much smaller indoor ACHmean than window size of 1.5m×1.5m,

588

such percentage ranges from 25%-60% for most middle floors and those in the upper

589

and lower floors usually exceed the middle floors. ACHturb is also found to be

590

decreased to around 42% in each floor when reducing the window area. In addition,

591

the overall ventilation performance (ACHPFR) can be reduced to 11.72%-80.62%.

592

Generally, ACHPFR decreases to 44% on average. In summary, reducing the window

593

size, i.e. the area of indoor contract with outdoor environment, can decrease indoor

594

ventilation considerably but produce less effect on outdoor ventilation in street

595

canyon.

596

If window size decreases from 1.5m×1.5m to 1m×1m in extremely deep canyon

597

(H/W=5), Fig.13a verifies the velocity in the upper part of canyon remains unchanged,

598

while the flow in lower part of canyon is slightly influenced. That with small window

599

size is more in line with the previous study of ideal street canyon without coupling

600

indoor-outdoor ventilation[22]. Consequently, as shown in Fig.13b, the decreases of

601

indoor ACHPFR above the 15th floor are around 38.27% to 62.75% as window size is

602

smaller. When emphasizing the indoor ventilation in the lower part of canyon, 31

603

although ACHPFR changes a lot, the ventilation performance is still poor due to the

604

small velocity(Fig.8g). To some extent, the impact of window sizes on the airflow of

605

street canyon is more significant in the lower levels for extremely deep canyon.

606 607

5. Conclusions

608

Better understanding the impact of urban-built geometry on both outdoor

609

ventilation in street canyons and indoor ventilation in naturally-ventilated buildings is

610

becoming more significant to provide guidance for developing sustainable and healthy

611

urban-built environments. As a novelty, this paper investigates the influence of street

612

aspect ratios and window sizes on the integrated indoor-outdoor ventilation in 2D

613

street canyons with single-ventilated multi-storey buildings(shallow, deep and

614

extremely deep types as H/W=0.5-1, 2-3 and 5). Validated by wind tunnel experiments,

615

CFD simulations with RNG k-ε model are performed to solve flow fields in street

616

canyons and near-road buildings by coupling indoor-outdoor interaction. As a novelty,

617

both indoor and outdoor ventilation capacity are analyzed by multiple ventilation

618

concepts such as age of air(τ) and air change rate per hour(ACH) etc. Particularly,

619

purging flow rate(PFR) and its corresponding ACHPFR are adopted for overall indoor

620

and outdoor ventilation assessment.

621

For outdoor ventilation, there is a clockwise vortex in the street canyon as

622

H/W=0.5-3 and two counter-rotating vortices as H/W=5. Street canyons with smaller 32

623

aspect ratios(i.e. wider streets) experience more ACHs and smaller age of air for both

624

outdoor and indoor ventilation. In particular, outdoor ACHPFR goes down from

625

14.69-17.55h-1 to 3.96h-1 as H/W is from 0.5 to 3 and dramatically decreases to

626

0.76-0.91h-1 in extremely deep canyon as H/W=5. Besides, leeward-side air in street

627

canyons is older than windward-side as H/W=0.5-3, however, in lower part of canyon

628

as H/W=5, windward-side air is older owing to the weak and lower-level

629

counterclockwise vortex.

630

For indoor ventilation, in shallow streets with H/W=0.5-1(8-floor buildings),

631

ACHmean and ACHPFR in leeward 4-6th floor experience the smaller ACHs(e.g. ACHPFR

632

~1.13-1.64h-1 as H/W=1) comparing to other floors. In deep streets with H/W=2 and 3

633

(16 and 24 floors), the ACHmean and ACHPFR in leeward-side middle-floor(except the

634

top and bottom two floors) are nearly constants and much smaller(e.g.

635

ACHPFR~1.02-1.69h-1) than the windward-side(e.g.ACHPFR~1.41-4.35h-1) which

636

increase toward the upper floors. As H/W=0.5-3, windward-side rooms basically

637

attain better ventilation than leeward-side. Furthermore, the difference of indoor age

638

of air between leeward-side and windward-side rooms as H/W=2-3 is larger than

639

H/W=0.5-1. In extremely deep street(H/W=5), the lower-level vortex flow is 1-2 order

640

weaker than the upper-vortex region, producing extremely small indoor ACHPFR

641

(0.04-0.61h-1) in the corresponding low-level floors(1-13th floors). Indoor ACHs in the

642

upper-level vortex region(16-40th floors) are similar with those as H/W=3.

643

Finally, the indoor ventilation capacity and efficiency vary significantly with the 33

644

change of window size. Decreasing window size from 1.5m×1.5m to 1m×1m will

645

reduce ACHPFR by 19.38%~88.28%, while little influence outdoor ventilation of street

646

canyon. In general, ACHPFR is greater than ACHmean but smaller than the sum of

647

ACHmean and ACHturb for both outdoor and indoor ventilation.

648

Since the indoor-outdoor ventilation in shallow 2D street canyons(H/W=0.5-1)

649

are similar but become much worse in deep 2D streets as H/W=2-3 and extremely

650

weak as H/W=5, H/W=1 could be the optimized considering both indoor-outdoor

651

ventilation and land resources. Nevertheless, too large aspect ratio (e.g. H/W=5)

652

should not be recommended to avoid the two-main-vortex situations and ensure

653

required ventilation. As shown in Fig. A1d in Appendix, in deep streets with weak

654

indoor and outdoor airflow, the influence of buoyancy force induced by solar shading

655

and air-wall temperature difference will be further considered. In addition, the

656

realistic wind speed and directions may vary with time, so the impacts of unsteady

657

boundaries on indoor and outdoor ventilation will be further taken into account.

658

Although it still requires further investigations before providing practical

659

guideline for sustainable urban-built ventilation design, this paper provides an insight

660

and scientific foundation on integrated indoor-outdoor ventilation with various

661

effective ventilation indexes and proposes effective methodologies for indoor-outdoor

662

ventilation assessment in more complicated urban-built configurations.

34

663 664

Acknowledgments This study was financially supported by National Key R&D Program of China

665

[2016YFC0202206, 2016YFC0202205 and 2016YFC0202204], National Natural

666

Science Foundation--Outstanding Youth Foundation (No. 41622502), STINT (dnr

667

CH2017- 7271) and the National Natural Science Foundation of China (No.

668

51811530017 and 41875015) as well as the Key projects of Guangdong Natural

669

Science Foundation (No 2018B030311068).

670 671 672

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870 871

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872

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874

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Environment 2008;43:1991-2004.

876 877 878

Figure list

879

Fig. 1 (a) Details of the computational domain and boundary conditions, (b)

880

dimensions of the computational model and grids arrangements on the ground, (c) the

881

measured lines of the velocity profiles.

882 883

Fig. 2 Vertical profiles of Ux/Uref along lines at the centre section of building model

884

for windward and/or leeward single-sided ventilation: (a-b) x=-H/25, (c-d) x=H/2, (e-f)

885

x=3H/2.

886 887

Fig. 3 (a) Descriptions of computational domain and target street canyon; (b) Grid

888

arrangements with minimum size of 0.1m in Case [2, w1.5]. 46

889 890

Fig. 4 The illustration of the age of air, PFR, Qmean and Qturb calculation.

891 892

Fig. 5 Normalized velocity (U/Uref) and turbulent kinetic energy (k/ Uref) in (a-b) Case

893

[0.5, w1.5], (c-d) Case [0.67, w1.5], (e-f) Case [1, w1.5].

894 895

Fig. 6 The outdoor age of air (τ, unit:s) in (a) Case [0.5, w1.5], (b) Case [0.67, w1.5],

896

(c) Case [1, w1.5].

897 898

Fig. 7 (a) Velocity vector and normalized stream-wise velocity within and near the

899

rooms in all floors of leeward and windward buildings in Case [0.5, w1.5]. Indoor

900

ventilation indices in all leeward and windward rooms in Case [0.5, w1.5], Case [0.67,

901

w1.5], Case [1, w1.5]: (b) age of air, (c) ACHmean, (d) ACHturb, (e) ACHPFR.

902 903

Fig. 8 Normalized velocity (U/Uref) and outdoor age of air (τ, s) in (a-b) Case [1,

904

w1.5], (c-d) [2, w1.5], (e-f) [3, w1.5] respectively. Horizontal profiles of (g)

905

normalized velocity and (h) outdoor age of air (τ, unit:s) at z=1.5m.

906

47

907

Fig. 9 (a) Indoor age of air (τ, unit:s) and ventilation indices including ACHmean,

908

ACHturb, ACHPFR of all leeward rooms and windward rooms in (b) Case [1, w1.5], (c)

909

[2, w1.5], (d) [3, w1.5].

910 911

Fig. 10 (a) Flow field, (b) turbulent kinetic energy, (c) age of air (τ, unit:s) in outdoor.

912

(d) Indoor ventilation indices in all leeward and windward rooms in Case [5, w1.5].

913 914

Fig. 11 Normalized velocity (U/Uref) and outdoor age of air (τ, unit:s) in (a,c) Case [1,

915

w1.5] and (b,d) Case [1, w1] respectively.

916 917

Fig. 12 Comparisons of indoor age of air (τ, unit:s) between different window size in

918

(a) leeward rooms and (b) windward rooms as H/W=1-3. Percentage of ACH:

919

ACH(1m×1m) /ACH (1.5m×1.5m) in (c) H/W=1, (d) H/W=2, (e) H/W=3.

920 921

Fig. 13 (a) Comparison of flow field, (b) percentage of ACHPFR in H/W=5.

48

922

Appendix A. Flow pattern validation for 2D street canyon with H/W=2 and 3 by

923

scale-model outdoor experiments

924

As displayed in Fig.A1b, an outdoor scale-model filed experiments was carried

925

out by Zhang et al.[22] to study the flow patterns in two-dimensional (2D) street

926

canyon with various street aspect ratios (building height H=1.2 m; H/W=1,2,3; street

927

length L=12.5m>10H). For each type of streets canyon (H/W=1, 2, 3), a set of 3D

928

ultrasonic anemometers were used to measure the temporal profiles of velocity

929

components (Ux, Uy and Uz) and turbulence at five different heights (z=0.3, 0.6, 0.9,

930

1.44, 2.4 m) (Fig.A1b). The sampling rate was 20 Hz for all anemometers. As a

931

example, Fig.A1c presents the experimental profiles of stream-wise velocity (Ux, i.e.

932

perpendicular to the street axis) in street canyon with H/W=2 and 3. In both cases, the

933

wind-driven dynamic force dominates urban airflow and Reynolds number

934

independence requirement is fully satisfied, as the Reynolds number is large

935

(Re~1.5×105≫11000 as Uref~2.0 m·s-1) and buoyancy force is relatively weak (i.e.

936

Froude number Fr =

937

U ref 2 gH (∆T / Tref )

~10.2 as △T=10 K and Uref = 2.0m s-1).

As shown in Fig.A1c and A1d, it is obvious that regardless of aspect ratio , the

938

stream-wise velocities at z=0.25H are positive while those at z=0.75H and z=2H are

939

negative, confirming that there is only one main vortex in such 2D deep street

940

canyon(H/W=2 and 3) in the field measurement. The findings are consistent with the

941

flow patterns of CFD results in this paper. More detailed experimental setups can 49

942

referred to Zhang et al.[22].

50

(a)

(b)

(c)

Fig. 1 (a) Details of the computational domain and boundary conditions, (b) dimensions of the computational model and grids arrangements on the ground, (c) the measured lines of the velocity profiles.

1

experiment data CFD results by RNG k-ε model with 1.8 fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4

2.0

1.2

1.2

experiment data CFD results by RNG k-ε model with fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4 1.8

y/H

y/H

2.0

1.0

1.0

0.8

0.8

0.6

0.6 0.4

0.4

wind

0.2

-H/25

0.0 -0.4

-0.2

0.0

0.2

wind

0.2

0.4

0.6

-H/25

0.0

0.8

1.0

-0.4

1.2

-0.2

0.0

0.2

0.8

1.0

1.2

1.2

(b)

2.0

experiment data CFD results by RNG k-ε model with fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4

2.0

1.8

1.8

1.2

1.2

experiment data CFD results by RNG k-ε model with fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4

y/H

y/H

(a)

1.0

1.0

0.8

0.8

0.6

0.6 0.4

wind

0.2

wind

0.2

H/2 -H/2

0.0 -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

H/2

0.0

1.0

1.2

-0.4

-0.2

0.0

0.2

U/Uref

(c)

0.6

0.8

1.0

0.6

0.8

1.0

(d)

experiment data CFD results by RNG k-ε model with fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4

2.0

1.2

1.2

experiment data CFD results by RNG k-ε model with 1.8 fine grid (0.05m×0.05m×0.05m) medium grid (0.1m×0.1m×0.1m) 1.6 coarse grid (0.2m×0.2m×0.2m) 1.4

y/H

1.8

1.0

1.0

0.8

0.8

0.6

0.6 0.4

0.4

wind

0.2

-3H/2 3H/2 -0.2

0.0

0.2

0.4

U/Uref

wind

0.2

0.0 -0.4

0.4

U/Uref

2.0

y/H

0.6

U/Uref

U/Uref

0.4

0.4

0.6

0.8

1.0

1.2

3H/2

0.0 -0.4

-0.2

0.0

0.2

0.4

1.2

U/Uref

(e) (f) Fig. 2 Vertical profiles of Ux/Uref along lines at the centre section of building model for windward and/or leeward single-sided ventilation: (a-b) x=-H/25, (c-d) x=H/2, (e-f) x=3H/2.

2

(a)

(b) Fig.3 (a) Descriptions of computational domain and target street canyon; (b) Grid arrangements with minimum size of 0.1m in Case [2, w1.5].

3

Fig. 4 The illustration of the age of air, PFR, Qmean and Qturb calculation.

4

(a)

(b)

(c)

(d)

(e) (f) Fig. 5 Normalized velocity (U/Uref) and turbulent kinetic energy (k/Uref) in (a-b) Case [0.5, w1.5], (c-d) Case [0.67, w1.5], (e-f) Case [1, w1.5].

5

(a)

(b)

(c) Fig.6 The outdoor age of air (τ, unit:s) in (a) Case [0.5, w1.5], (b) Case [0.67, w1.5], (c) Case [1, w1.5].

6

(a)

(b)

7

8 7

Floor No.

6

ACHmean

0.26h-1

5

1.04h-1

4

1.02h-1

Leeward Windward

0.46h-1 3

Case [0.5,w1.5] Case [0.67,w1.5] Case [1,w1.5]

2 1 0

1

2

3 4 ACHmean (h-1)

5

6

7

8

(c) 8 7

Floor No.

6

ACHturb

5 4 3

Leeward Windward Case [0.5,w1.5] Case [0.67,w1.5] Case [1,w1.5]

2 1 0

4

8

12

16 20 ACHturb (h-1)

24

28

32

36

(d) 8 7

Floor No.

6 5

ACHPFR

4 3

Leeward Windward Case [0.5,w1.5] Case [0.67,w1.5] Case [1,w1.5]

2 1 0

1

2

3

4

5 6 7 ACHPFR (h-1)

8

9

10

11

12

(e) Fig. 7 (a) Velocity vector and normalized stream-wise velocity within and near the rooms in all floors of leeward and windward buildings in Case [0.5, w1.5]. Indoor ventilation indices in all leeward and windward rooms in Case [0.5, w1.5], Case [0.67, w1.5], Case [1, w1.5]: (b) age of air, (c) ACHmean, (d) ACHturb, (e) ACHPFR. 8

(a)

(b)

(c)

(d)

9

(e)

(f) 6

101

10

10

Normalized velocity at z=1.5m Case [1,w1.5] Case [3,w1.5] Case [2,w1.5] Case [5,w1.5]

Age of air at z=1.5m Case [1,w1.5] Case [2,w1.5]

Case [3,w1.5] Case [5,w1.5]

105

0

10

-1

10

-2

10

-3

10

-4

10

-5

H/W=5 Age of air (s)

U/Uref

H/W=1 H/W=2 H/W=3

104

H/W=3

103

H/W=2 H/W=5

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.0

x/W

(g)

0.1

0.2

0.3

0.4

0.5

H/W=1

2

10 0.6

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

x/W

(h)

Fig. 8 Normalized velocity (U/Uref) and outdoor age of air (τ, s) in (a-b) Case [1, w1.5], (c-d) [2, w1.5], (e-f) [3, w1.5] respectively. Horizontal profiles of (g) normalized velocity and (h) outdoor age of air (τ, unit:s) at z=1.5m.

10

0.6

(a) 8

7

Floor No.

6

5

Case [1,w1.5]

0.47h-1

ACHmean 4

3

2

leeward windward ACHturb leeward windward ACHPFR leeward windward

1 10-1

100

101

102

ACH (h-1)

(b)

11

Floor No.

16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Case [2,w1.5]

0.20h-1

ACHmean leeward windward ACHturb leeward windward ACHPFR leeward windward

10-1

100

101

102

-1

ACH (h )

(c) 24 22 20 18

0.19h-1

Floor No.

16 Case [3,w1.5]

14

ACHmean

12

leeward windward ACHturb

10 8

leeward windward ACHPFR

6 4

leeward windward

2 0 10-1

100

101

102

ACH (h-1) (d) Fig. 9 (a) Indoor age of air (τ, unit:s) and ventilation indices including ACHmean, ACHturb, ACHPFR of all leeward rooms and windward rooms in Case (b) [1, w1.5], (c) [2, w1.5], (d) [3, w1.5].

12

(a)

13

(b) 40

(c) Case [5,w1.5] ACHmean

36

leeward windward ACHturb

32 28

leeward windward ACHPFR

Floor No.

24 20

leeward windward

16 12 8 4 0 10-3

10-2

10-1

100

101

102

-1

ACH (h )

(d) Fig. 10 (a) Flow field, (b) turbulent kinetic energy, (c) age of air (τ, unit:s) in outdoor. (d) Indoor ventilation indices in all leeward and windward rooms in Case [5, w1.5].

14

(a)

(b)

(c) (d) Fig.11 Normalized velocity (U/Uref) and outdoor age of air (τ, unit:s) in (a,c) Case [1, w1.5] and (b,d) Case [1, w1] respectively.

15

(a)

(b)

16

8 7 Percentage of ACH as H/W=1: ACH(1m×1m) / ACH(1.5m×1.5m)

Floor No.

6 5

ACHmean

4

leeward windward ACHturb

3

leeward windward ACHPFR

2

leeward windward

1 0

10

20

30

40 50 60 70 ACH(1m×1m) / ACH(1.5m×1.5m)（ %）

80

90

100

(c) 16 14 12

Percentage of ACH as H/W=2: ACH(1m×1m) / ACH(1.5m×1.5m) ACHmean leeward windward ACHturb

Floor No.

10 8

leeward windward ACHPFR

6 4

leeward windward

2

0

10

20

30

40 50 60 70 ACH(1m×1m) / ACH(1.5m×1.5m)（ %）

80

90

100

(d) 24 22 20 18

Percentage of ACH as H/W=3: ACH(1m×1m) / ACH(1.5m×1.5m)

Floor No.

16

ACHmean

14

leeward windward ACHturb

12 10

6

leeward windward ACHPFR

4

leeward windward

8

2 0

10

20

30

40 50 60 70 ACH(1m×1m) / ACH(1.5m×1.5m)（ %）

80

90

100

(e) Fig. 12 Comparisons of indoor age of air (τ, unit:s) between different window size in (a) leeward rooms and (b) windward rooms as H/W=1-3. Percentage of ACH: ACH(1m×1m) /ACH (1.5m×1.5m) in (c) H/W=1, (d) H/W=2, (e) H/W=3. 17

(a) 40

Percentage of ACHPFR in H/W=5:

36

ACH(1m×1m) / ACH(1.5m×1.5m)

32

leeward

28

windward

Floor No.

24 20 16 12 8 4 0 0

25

50

75

100

125

150

175

ACH(1m×1m) / ACH(1.5m×1.5m) (%)

(b) Fig. 13 (a) Comparison of flow field, (b) percentage of ACHPFR in H/W=5.

18

(a)

19

Location of 3D supersonic anemometer

z=2.4m=2H

z=1.44m=1.2H

H=1.2m

z=0.9m z=0.6m z=0.3m=0.25H (b)

20

-5

AR=H/W=2 (H=120cm)

-4

u velocity (North-South) at 240cm 90cm 30cm

u (m/s)

-3

-u=south wind

-2

z=2H

-1.747m/s

-1

N

S -0.162m/s 0.145m/s

0

z=0.25H

1 10 52 35 18 00 43 26 09 51 34 32: 31: 31: 31: 31: 30: 30: 30: 29: 29: 10: 10: 10: 10: 10: 10: 10: 10: 10: 10:

time -5

AR=H/W=3 (H=120cm) -4

u velocity (North-South) at 240cm 90cm 30cm

-u=south wind

u (m/s)

-3

z=2H

-2 -1

S

N

-0.045m/s

0

z=0.25H 1 :50 :56 14

:0 :57 14

7

:24 :57 14

:42 :57 14

:59 :57 14

:16 :58 14

:34 :58 14

:51 :58 14

:0 :59 14

8

:25 :59 14

time

(c)

(d) Fig. A1. (a) Model setups of scale-model outdoor experiment on street canyon models with H/W=1, 2 and 3. (b) View of 3D ultrasonic anemometer locations. Example profile of streamwise velocity (Ux, m s-1) in street canyon with (c) H/W=2 and 3. (d)Future outdoor field measurement of urban-built ventilation by coupling indoor and outdoor airflow. 21

Table.1 Result of the flow statistical analysis of simulation values against with wind tunnel data from [64] x=-H/25 Fine grid

Windward single-sided ventilation

Leeward single-sided ventilation

x=H/2

Medium grid Coarse grid

Fine grid

x=3H/2

Medium grid Coarse grid

Fine grid

Medium grid Coarse grid

NMSE

0.017

0.017

0.017

0.019

0.023

0.015

0.015

0.015

0.018

FB

-0.116

-0.120

-0.122

-0.022

-0.008

-0.034

-0.101

-0.107

-0.107

R

0.998

0.998

0.997

0.986

0.982

0.988

0.997

0.997

0.996

NMSE

0.034

0.034

0.036

0.073

0.061

0.080

0.020

0.020

0.022

FB

0.206

0.207

0.210

0.261

0.249

0.239

0.162

0.170

0.175

R

0.999

0.998

0.997

0.963

0.971

0.956

0.997

0.997

0.996

Table.2 Case studies investigated

H/W

H(m)

0.5 0.67

W(m) 48

24

1

Window size (Opening ratio)

36

1.5m×1.5m (20.83%) 0.3m

24

0.8m 1.25m

2

48

3

72

24

Window

0.9m 4m

5

1m×1m (9.26%)

1.25m

1.5m

Window

1.5m

2.7m

0.9m 4m

120

Different cases with various building configurations are named as Case [H/W, w(window size)]. Where ‘H/W’ represents the various building heights and street canyon width. ‘w(window size)’ is employed to discriminate the two window size. For example, when building heights and street canyon width of 24m and H/W=1, window size=1.5m×1.5m is named as Case [1, w1.5].

Table. 3 Air change rates across street roof due to mean flows (ACHmean) and turbulent diffusion (ACHturb), and the overall ventilation capacity (ACHPFR) in window size 1.5m×1.5m, H/W=0.5, 0.67, 1, 2, 3,5 H/W

H

0.5 0.67

24m

W

ACHmean

ACHturb

ACHmean+ ACHturb

ACHPFR

48m

6.07

34.59

40.66

14.69

36m

7.98

22.94

30.92

13.95

5.93

15.15

21.08

10.78

1.51

17.11

18.62

6.08

1 2

48m 24m

3

72m

1.02

11.52

12.54

3.96

5

120

0.61

6.91

7.52

0.76

Table. 4 ACHPFR and ACHmean, ACHturb across street roof due to mean flows and turbulent diffusion in window size 1m×1m and 1.5m×1.5m, H/W=0.5, 0.67, 1, 2, 3,5 Case Name

ACHmean

ACHturb

ACHmean+ ACHturb

ACHPFR

[ 0.5 ,w1.5]

6.07

34.59

40.66

14.69

[ 0.5 ,w1]

5.88

36.48

42.36

17.55

[0.67,w1.5]

7.98

22.94

30.92

13.95

[0.67,w1]

7.98

23.88

31.86

15.11

[1,w1.5]

5.93

15.15

21.08

10.78

[1,w1]

6.01

14.83

20.84

10.58

[2,w1.5]

1.51

17.11

18.62

6.08

[2,w1]

1.51

17.11

18.62

6.09

[3,w1.5]

1.02

11.52

12.54

3.96

[3,w1]

1.02

11.52

12.54

3.97

[5,w1.5]

0.61

6.91

7.52

0.76

[5,w1]

0.61

6.91

7.52

0.91