Modelling Pollution Dispersion in Urban Areas Silvana Di Sabatino Universita’ di Lecce, Dipartimento Scienza dei Materiali, Via Arnesano, Lecce (I) University of Cambridge, Engineering Department, Trumpington Street, Cambrige, (UK)

INCREASING URBANISATION MOBILITY WIND ENVIRONMENT / LOADING / PEDESTRIAN / COMFORT URBAN CLIMATOLOGY URBAN AIR QUALITY INDOOR AIR QUALITY/VENTILATION ENERGY USAGE WEATHER MODELLING HAZARDOUS RELEASES

SPATIAL SCALE OF THE PROBLEM REGIONAL up to 200km-larger CITY up to 20km NEIGHBOURHOOD up to 2km STREET (CANYON) up to 0.2km POLLUTANT SOURCE e.g. TAILPIPE km WE SEEK GENERALISED SOURCE-RECEPTOR RELATIONSHIPS IN THE FORM OF MODELS

STREET (CANYON) SCALE WHERE THE PEOPLE AND (THE EMISSIONS) ARE DIRECT CFD/LES IS PRACTICABLE OPERATIONAL MODELLING IS TYPICALLY BASED ON A MORE IDEALISED RECIRCULATING VORTEX DRIVEN BY A SHEAR LAYER EXCHANGE BETWEEN THE CANYON AND FLOW ABOVE IS IMPORTANT DIFFICULTIES IN CRITICAL SITUATIONS WHEN THE WIND SPEED IS LOW; TRAFFIC PRODUCED TURBULENCE BECOMES IMPORTANT

TRAFFIC-PRODUCED TURBULENCE MODEL

The general form for the dissipation of turbulent kinetic energy per unit mass is given by Light traffic density - no flow interaction among the vehicles

where is the cross-section area in the canyon in which TPT is active describes turbulence averaged over the traffic layer describes turbulence averaged over the whole canyon the number of vehicles can be expressed as Intermediate traffic density - interaction between the vehicle wakes

Large traffic density – strong flow interaction among the vehicles Comparison with Experimental Data km/h Full scale

Estimated normalised TKE t scale in the central plane of an idealised street canyon without external wind-driven flow.

Conclusions Traffic produced turbulence (TPT) is important for estimation of pollution concentration levels in streets; A theoretical framework for parameterisation of turbulent transport by traffic induced air motions in street canyons is established; The analysis distinguishes between three traffic flow conditions: (i) light traffic conditions (isolated vehicles, non-interacting vehicle wakes); (ii) moderate traffic conditions corresponding to non- isolated (interacting) vehicle wakes; and (iii) heavy (congested) traffic conditions characterised by strongly interacting wakes; The analysed experimental data give indications of different scaling laws for TPT and resulting concentration fields under different traffic conditions.