Paris (France), 1-4 June, 2010 UNIVERSITY OF VENICE (ITALY) UNIVERSITY OF SALENTO (ITALY) STUDY OF TREE-ATMOSPHERE INTERACTION AND ASSESSMENT OF AIR QUALITY IN REAL CITY NEIGHBOURHOODS RICCARDO BUCCOLIERI Dipartimento di Informatica - Università “Cà Foscari” di Venezia (ITALY) Dipartimento di Scienza dei Materiali - University of Salento (ITALY) riccardo.buccolieri@unisalento.it Salim Mohamed Salim: University of Nottingham, Malaysia Silvana Di Sabatino: University of Salento (Lecce), Italy Andy Chan: University of Nottingham, Malaysia Pierina Ielpo: Water Research Institute-National Research Council, Bari, Italy Gianluigi de Gennaro, Claudia Marcella Placentino, Maurizio Caselli: University of Bari, Italy Christof Gromke: WSL Institute for Snow and Avalanche Research SLF, Switzerland Karlsruhe Institute of Technology, Germany
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 Outline I ntroduction Study of tree-atmosphere interaction and assessment of air quality - Background ideas / urban areas (buildings, trees ..) C FD s imulations / validation - Aerodynamic effects of trees in street canyons (IDEALISED) - Application to a real case scenario - Bari city (Italy) in real city neighbourhoods C onclusions and future perspective
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 Introduction Urban street canyons STREET CANYON Study of tree-atmosphere interaction and assessment of air quality aspect ratio , W/H city basic geometry unit geometries which affect flow and turbulence fields where the people and (the emissions) are in real city neighbourhoods where trees can be planted direct CFD/ LES is practicable Street canyon operational modeling is typically based on a more idealized recalculating vortex driven by a shear layer traffic pollutants released near the ground need to be “effectively” dispersed to maintain “adequate” air quality
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 Introduction Example of Urban street canyons Study of tree-atmosphere interaction and assessment of air quality Street canyon without trees in real city neighbourhoods Street canyon with one-row trees Street canyon with two-rows trees
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 Where are we? Impact of trees in urban areas on pollutant dispersion not widely considered Both experimental and numerical investigations are present in the literature Study of tree-atmosphere interaction and assessment of air quality Some of the tree effects on flow and dispersion have been considered individually in previous works, such as deposition, filtration, blockage etc. Still far from a comprehensive understanding of the overall role plaid by vegetation on urban air quality One of the most extensive review is given by Litschke and Kuttler (2008), who reported on several field studies as well as numerical and physical modelling of in real city neighbourhoods filtration performance of plants with respect to atmospheric dust. particle deposition on plant pollutant concentration reduced surfaces obstacles to airflow (air mass pollutant concentration increased exchange reduced) Litschke, T and Kuttler, W., 2008. On the reduction of urban particle concentration by vegetation – a review. Meteorologische Zeitschrift 17, 229-240.
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling Validation studies (W/H=2) 1) Approaching flow perpendicular and inclined by 45° to street axis Study of tree-atmosphere interaction and assessment of air quality Empty street canyon - W/H=2 Street canyon with tree planting in real city neighbourhoods 2) Is wind direction important? Competition with aspect ratio …
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling Objectives: Validation studies / speculative approach Example of a typical CFD simulation setup • commercial CFD-Code • RANS-Equations 8H Study of tree-atmosphere interaction and assessment of air quality WIND • turbulence closure schemes - RSM at least! 8H 30H • second order discretization schemes • grid: hexahedral elements - ~ 400,000 – 1,000,000 - δ x =0.05H, δ y =0.25H, δ z =0.05H INLET - expansion rate <1.3 2 3 • turbulent Schmidt number Sc t = 0.7 u ( z ) z u z u z k ( 1 ) ε ( 1 ) δ u H C κz δ in real city neighbourhoods μ H turbulent vis cos ity t Sc t D turbulent diffusivit y t u H =4.7 m/s : undisturbed wind speed at the building height H α=0.30 : power law exponent Dimensionless concentrations c+ u =0.52 m/s : friction velocity * c m measured concentration κ=0.40: von Kàrmàn constant c u H u ref reference velocity m ref c C μ = 0.09 Q l H building height T Q T /l strength of line source
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling porous tree crowns A cell zone is defined in which the porous media model is applied and the pressure loss in the flow is determined Study of tree-atmosphere interaction and assessment of air quality The porous media model adds a momentum sink in the governing momentum equations : Si: source term for the i-th (x, y, or z) momentum equation v : magnitude of the velocity viscous loss term + inertial loss term D and C: prescribed matrices This momentum sink contributes to the pressure gradient in the porous cell, creating a pressure drop that is proportional to the fluid velocity (or velocity squared) in the cell. The standard conservation equations for turbulence quantities is solved in the porous medium . Turbulence in the medium is treated as though the solid medium has no effect on the turbulence generation in real city neighbourhoods or dissipation rates . permeable zone LOOSELY FILLED: λ = 80 m -1 with the same loss coefficient λ as in DENSELY FILLED: λ = 200 m -1 wind tunnel experiments
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling STREET CANYON WITH TREES Validation studies (W/H=2) wind direction : perpendicular Loosely filled crown WT CONCENTRATIONS ( λ = 80 m -1 , P Vol = 97.5 %) Study of tree-atmosphere interaction and assessment of air quality Wall A Wall B in real city neighbourhoods Measured concentrations Relative deviations [%] in respect of tree-free street canyon Concentration increase in proximity of wall A and decrease near wall B Maximum concentrations at pedestrian level in proximity of wall A Differently to the tree-free street canyon case, less direct transport of pollutants from wall A to wall B occurs
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling STREET CANYON WITH TREES Validation studies (W/H=2) wind direction : perpendicular z=0.5H y=1.25H WIND 6 6 5 5 0.4 0.4 Study of tree-atmosphere interaction and assessment of air quality 1.2 4 4 1.2 0.3 0.3 1 1 3 3 0.8 0.8 2 2 0.2 0.2 0.6 0.6 z/H z/H 1 1 0.4 0.4 0.1 WIND y/H 0.1 y/H 1 1 0.2 0 0 0.2 0.9 0.9 0 0 0.8 -1 0.8 -1 -1 -0.6 -0.2 0.2 0.6 1 -1 -0.6 -0.2 0.2 0.6 1 x/H x/H 0.7 0.7 -2 -2 0.6 0.6 0.5 0.5 -3 -3 Increases in concentrations in 0.4 0.4 -4 -4 0.3 0.3 0.2 0.2 proximity of wall A and decreases near -5 -5 0.1 0.1 -6 -6 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 wall B x/H x/H Differently to the tree-free street canyon The pollutants are advected towards the in real city neighbourhoods case, less direct transport of pollutants from leeward wall A, but, since the circulating wall A to wall B occurs fluid mass is reduced in the presence of tree planting, the concentration in the Most of the uprising canyon vortex is intruded uprising part of the canyon vortex in into the flow above the roof level. Here, it is front of wall A is larger diluted before partially re-entrained into the canyon. As a consequence, lower traffic exhaust concentrations are present in proximity of wall B
13th Int. Conf. on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Paris (France), June 1-4 CFD modelling STREET CANYON WITH TREES Validation studies (W/H=2) wind direction : perpendicular Wall A Wall B CFD - WT CONCENTRATIONS Study of tree-atmosphere interaction and assessment of air quality street canyon model – wind tunnel street canyon model – CFD in real city neighbourhoods calculated concentrations relative deviations [%] in respect of measurements CFD simulations were successful in predicting an increase in concentrations in proximity of wall A and a decrease near wall B and the relative deviations in respect of tree-free street canyon As in the tree-free case, it slightly underestimated experimental data
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