Computational Fluid Dynamics Modelling of Aerosol Dispersion and Processes within Urban Street Canyons Bee Kiat Tay 1 Martin Gallagher 1 , Gordon McFiggans 1 & Paul Watkins 2 University of Manchester, UK 1. School of Earth Atmospheric and Environmental Sciences, Centre of Atmospheric Science 2. Mechanical, Aerospace and Civil Engineering 31 st March 2009
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Contents • Background of Project • Code Description & Design of Computational Domain • Research Themes – Extent of heterogeneities of flow properties – Investigation of flux from street canyons – Urban canyon aerosol processes Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Background of Project • Vehicle emission is a major source of aerosols in the urban environment – Adverse health effects are attributed to exposure to aerosols (fine/ ultrafine) • Nature of urban street canyon dispersion is influenced by both micrometeorological factors & urban geometry • Urban aerosol distribution – Size spans across several orders of magnitude – Multi-modal in nature/ Lognormal – Modified by aerosol processes ( e.g. condensation, coagulation) • Research Questions – Structure of aerosol dispersion within street canyons – Sensitivity of aerosol flux to various meteorological factors – Importance of in-canyon aerosol processes Coupling aerosol dynamical model with Computational Fluid Dynamics to model urban aerosol transport in street canyon Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Code Description • In-house 2-Dimensional CFD code (Finite Volume Method) 1 – Features • Incompressible flow ( constant density ) • Buoyancy represented by Boussinesq approximation Turbulent flow represented by standard k- � turbulence model • – High Reynolds Number wall function to characterise shear at the walls • SIMPLE ( Semi Implicit Method for Pressure Linked Equation ) algorithm to obtain velocity vectors Dispersion patterns validated with wind tunnel dispersion data 2 – • Euler-Euler multiphase approach • Discrete phase represented by moments, integral property of size distribution • Modal Method: 3 moments (0 th ,3 rd ,6 th ) transported to facilitate lognormal closure of distribution • Low Volumetric Loading: – Discrete phase carried by flow field of the continuous phase – One-way coupling ( aerosol phase has insignificant influence on continuous phase) D. P. Jones, A. P. Watkins; Spray Impingement Model based on Method of Moments; 22 nd European Conference on Liquid Atomisation and Spray Systems 1. 2. M. Ketzel, P. Louka, P. Sahm, E. Guilloteau, J.-F.; Sini The use of computational fluid dynamics in modelling air quality in street canyons. Chapter 2.2 of the TRAPOS summary report Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Schematic Diagram of Computational Domain Symmetry Inflow Outflow Velocity and 5H Turbulence Intensity* Wall (Smooth) 2H 10H H/W= 1, 2 H Emission (0.3m above street) *ratio of the root-mean-square of the velocity fluctuations, u', to the mean flow velocity W Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Underlying Assumptions Steady state • 2 D (perpendicular wind), ignoring lateral effects • – Assuming a long canyon Vehicular turbulence may be neglected • – For wind speeds >2.5 m/s Aerosols treated as inert scalar (advection and turbulent diffusion) • Constant concentration of aerosols at “emission point”, approximately 1m from • the tailpipe – Valid for fast traffic movement Assumed distribution at emission point: • Aitken mode Accummulation Lognormal Parameters Lognormal Parameters mode Number Concentration N o 1x10 11 1x10 10 (particle/ m 3 ) D GN 15 150 Geometric Mean Diameter (nm) � g Standard Deviation 1.5 1.6 Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Investigation of the Vertical Structure �������������������� ������������������������ ��������������������� ��������������������� ������������������ ������������������� ������������������ �������� Normalised Concentration Prashant Kumar, Andrew Garmory, Matthias Ketzel, Ruwim Berkowicz, Rex Britter; Comparative study of measured and modelled number concentrations of nanoparticles in an urban street canyon; Atmospheric Environment, Volume 43, Issue 4, February 2009, Pages 949-958 Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Aerosol Dispersion Particle Number Concentration ( p m -3 ) Frame 001 26 Mar 2009 continuity Frame 001 26 Mar 2009 continuity Wind Sharper concentration gradient Leeward Windward 1x10 8 1x10 8 1 5 x 1 0 8 1 1 5 x 1 x 1 0 8 0 9 1 x 1 0 9 0.8 1.5x10 9 0.8 0.6 0.6 Z/H Z/H 2 x 1 0 9 0.4 0.4 1.5x10 9 0.2 0.2 2x10 9 0 0 0 0.5 1 0 0.2 0.4 0.6 0.8 1 X/W Aspect Ratio: 1 Aspect Ratio:2 X/W Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Leeward Vertical Profile 11 10 9 Height of Canyon (m) 8 7 10 m/s Low TI 10 m/s Mid TI 6 10 m/s High TI 5 2.5 m/s Low TI 4 2.5 m/s Mid TI 3 2.5 m/s High TI 2 1 0 0.00E+00 4.00E+08 8.00E+08 1.20E+09 1.60E+09 2.00E+09 Particle Concentration (p/m 3 ) Comparable with previous measurement results Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Windward Vertical Profile 11 10 9 Height of Canyon (m) 8 7 10 m/s Low TI 10 m/s Mid TI 6 10 m/s High TI 5 2.5 m/s Low TI 4 2.5 m/s Mid TI 3 2.5 m/s High TI 2 1 0 0.00E+00 1.00E+08 2.00E+08 3.00E+08 4.00E+08 5.00E+08 6.00E+08 Particle Concentration (p/m 3 ) Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Results Summary Wind Venting from canyon Bulk entrainment of air (AR=1) into canyon Turbulent Shear Layer ( Sharper negative concentration gradient ) Decreasing Concentration concentration increasing with height at the lower part of with height. Gradient canyon, then proportional to turbulent decreasing with viscosity. height. Windward Increasing Leeward Centre of Atmospheric Science Concentration with height
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Flux from Street Canyon Inflow Condition: Wind speed: 2.5- 10 ms -1 & Turbulence Intensity (TI): 0.05 to 0.26) Aspect Ratio: 1- 2 Transport and β = β + β dispersion NET Turbulent advective of aerosols due to z � is horizontally integrated flux advection F is flux and turbulent C is concentration of aerosol diffusion K c is turbulent diffusivity x � β = β + β = + ( F F ) dx NET Turbulent advective t m L ∂ ___ C � � = + = − + ( cw CW dx ) ( K CW dx ) ∂ c z L L Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay flux1.xls : (1)Sheet1, X , Y , Z Rank 4 Eqn 151232673 lnz=a+blnx+cy r^2=0.99861805 DF Adj r^2=0.99778888 FitStdErr=6006567.1 Fstat=2167.842 a=16.960305 b=0.92517512 c=3.7288871 ASPECT RATIO: 1 6e+08 6e+08 5e+08 5e+08 4e+08 4e+08 3e+08 Flux Flux 3e+08 2e+08 2e+08 1e+08 1e+08 0 0 9 8 0.25 7 0.2 6 0.15 5 U 0.1 4 3 0.05 TI 2 0 Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay flux3.xls : (1)Sheet1, X , Y , Z Rank 4 Eqn 151232673 lnz=a+blnx+cy r^2=0.99897237 DF Adj r^2=0.99835579 FitStdErr=3319762.7 Fstat=2916.3289 a=16.880619 b=0.95533697 c=1.9685648 ASPECT RATIO: 2 3.5e+08 3.5e+08 3e+08 3e+08 2.5e+08 2.5e+08 2e+08 Flux Flux 2e+08 1.5e+08 1.5e+08 1e+08 1e+08 5e+07 5e+07 9 8 0.25 7 0.2 6 0.15 5 U 0.1 4 0.05 3 TI 2 0 Centre of Atmospheric Science
Modelling Aerosol Dispersion and Processes in Urban Street Canyons B K Tay Discussion • Turbulent flux dominates the ventilation process at the roof level – Turbulent flux is of higher magnitude than advective flux – Advective flux is negative (reintroduction of aerosols into the canyon) and turbulent flux is positive (vents aerosols). – Net flux is positive • Exponential relationship between TI and net flux • A parameterisation is proposed for a perpendicularly blowing wind and when buoyancy effects are unimportant β = + + exp( ln ) a b U cTI N in Positive Exponential Relation Relation Centre of Atmospheric Science
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