INTRODUCTION TO POLLUTANT DISPERSION IN URBAN STREET CANYONS MUHAMMAD NOOR AFIQ WITRI

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1 INTRODUCTION TO POLLUTANT DISPERSION IN URBAN STREET CANYONS MUHAMMAD NOOR AFIQ WITRI 1

2 CONTENTS 1.Motivation 2.Introduction 3.Street canyon characteristics 4.Wind field models 5.Dispersion models 1.Future Works 2.Gantt Chart 2

3 MOTIVATION Air Quality Issues 3

4 INTRODUCTION There are three scales used for different atmospheric layers a) Mesoscale: planetary boundary layer (PBL); b) Local scale: Surface layer; c) Microscale: Urban canopy layer (UCL) & Roughness sublayer (RSL) 4

5 INTRODUCTION (cont.) SPATIAL SCALE OF THE PROBLEM 5

INTRODUCTION (cont.) Street canyon generally refers to a relatively narrow street in-between buildings that line up continuously along both sides.

6 INTRODUCTION (cont.) Street canyon generally refers to a relatively narrow street in-between buildings that line up continuously along both sides. It has a distinct climate where micro-scale meteorological processes dominate (Oke, 1988) and the air ventilation and pollutant removal are solely through the roof level. 6

7 INTRODUCTION (cont.) Field Observations Street Canyon Research Approaches CFD Modelling Laboratory Modelling NEXT 7

8 SOLUTION APPROACH: Field Observations ADVANTAGES 1. Can provide useful information on airflow characteristics and pollutant distribution within street canyons. DISADVANTAGES 1. Low spatial resolution 2. Uncontrollable meteorological conditions 3. Complex building configuration RETURN 8

9 SOLUTION APPROACH: Laboratory Modelling ADVANTAGES 1. Fully controllable upwind boundary conditions, airflow and street-canyon geometries. DISADVANTAGES 1. High cost. 2. Only limited cases in detail settings. RETURN 9

3. Controlled conditions: allows for chemistry,

10 SOLUTION APPROACH: CFD Modelling ADVANTAGES 1. Economic approach. 2. High spatio-temporal resolutions. 3. Controlled conditions: allows for chemistry, depositon, multi-phase DISADVANTAGES 1. Inaccuracy for validation RETURN 10

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12 STREET CANYON CHARACTERISTICS: Canyon Geometry Simplifications in street canyon studies: SIMPLIFICATION Real street canyon (non-uniform) A street canyon model in wind tunnel study (uniform) 12

13 STREET CANYON CHARACTERISTICS: Canyon Geometry Windward side Street canyons are usually characterized by their aspect ratio (AR), which is the ratio of building height, H to the canyon width, W. 13

14 STREET CANYON CHARACTERISTICS: Canyon Geometry A street canyon is called shallow canyon if its AR 1, while AR 2 represents a deep canyon. Urban streets are also classified as symmetric canyons, if buildings have approximately the same height, and asymmetric, if buildings have different heights. 14

15 STREET CANYON CHARACTERISTICS: Wind Flow Field Wind flow pattern inside street canyons depends on their geometry : aspect ratio (AR). Oke (1988) identified three flow regimes for wind direction perpendicular to the street axis. 15

16 STREET CANYON CHARACTERISTICS: Pollutant Dispersion According to Riain et al., the exchange of canyon air with the free stream wind flow and the connecting streets controls dispersion of pollutants in street canyons. Hunter et al. reported that in skimming flows, minimum exchange of air occurs, which leads to ineffective pollutant removal from street canyons. 16

17 STREET CANYON CHARACTERISTICS: Pollutant Dispersion DePaul & Sheih, and Qin & Kot performed a series of field measurements that showed the concentrations of pollutants increase on the leeward wall of buildings and decrease with height above the street on both side of canyons. 17

18 STREET CANYON CHARACTERISTICS What s the previous studies focus on? Wind flow (velocity magnitude, direction); Aspect ratio (build-height-to-street-width); Building roof shape; Vehicle induced turbulence; Tree plantings. 18

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20 Is Atmospeheric Flow Laminar or Turbulent? According to Tennekes & Lumley (A First Course in Turbulence), the turbulence flow has following characters: Diffusive 3D vorticity fluctuations Continuum Large Re no. 20

WIND FIELD MODELS: Categories of Turbulence Models Currently, three types of models are commonly used for resolving/modeling fluid turbulence.

21 WIND FIELD MODELS: Categories of Turbulence Models Currently, three types of models are commonly used for resolving/modeling fluid turbulence. k-ε model (RANS based) Large-eddy simulation (LES) Direct numerical simulation (DNS) Model k-ε model LES DNS Accuracy Lower Higher Computational cost Relatively cheap Expensive Very expensive

22 WIND FIELD MODELS : Past Studies Turbulent Model Author Standard k-ε model Sini et al., 1996; Johnson and Hunter, 1998; Baik and Kim, 1999; Huang et al., 2000; Chan et al., 2003; Assimakopoulos et al., 2003 RNG k-ε model Tsai and Chen, 2004; Kim and Baik, 2004; Li et al., 2005 Realizable k-ε model Jicha et al., 2000; Sagrado et al., 2002 Reynolds stress model and Chang and Meroney, 2003 Spalart Almaras model Hamlyn & Britter, 2005 LES Ca et al., 1995 Chabni et al., 1998 Walton & Cheng, 2002 Walton et al., 2002 DNS Coceal et. al, 2006 Orlandi & Leonardi,

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24 DISPERSION MODELS Dispersion Models Categories Operational models CFD models Box models Gaussian plume models Langrangian puff models Langrangian Eulerian 24

25 DISPERSION MODELS The Eulerian method treats the particle phase as a continuum and develops its conservation equations on a control volume basis. Concentration field is determine from continuous distribution. The Lagrangian method considers particles as a discrete phase and tracks the pathway of each individual particle. Particle concentration can be calculate by studying the statistics of particle trajectories. 25

26 DISPERSION MODELS Eulerian VS Lagrangian Eulerian Lagrangian Accuracy Low High Computational effort Low High 26

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28 Urban Flow Dynamics Turbulent model (RANS/LES) Future Works Pollutant dispersion (Lagrangian model) Non- Uniform street canyon 28

29 Gantt Chart (sem1 2011/2012) Topics/Month Sept Oct Nov Dec Jan Feb N.S. Eqn Non-uniform street canyon Pollutant dispersion model Turbulence models 29

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Effects of Buildings Layout on the Flow and Pollutant Dispersion in Non-uniform Street Canyons ZHANG Yunwei, PhD candidate

Effects of Buildings Layout on the Flow and Pollutant Dispersion in Non-uniform Street Canyons ZHANG Yunwei, PhD candidate May 11, 2010, Xi an, Shannxi Province, China E_mail: zhangyunwe@gmail.com 1 Contents: