ncas/april meeting on urban air quality modelling dispersion modelling at imperial college london...

NCAS/APRIL Meeting onUrban Air Quality ModellingDispersion modelling atImperial College London

Professor Helen ApSimonand Dr Roy Colvile

/N © Imperial College London

Key questions

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• What kind of modelling approach is fit for purpose? - What are the processes and mechanisms that need to be included?

• What purposes will be required of Urban Air Quality Modelling in future? - Personal exposure? Urban scale integrated assessment? Assessment of pollution control measures at an airport? Local air quality management during air pollution episodes in a mega-city?

© Imperial College London

Modelling aircraft and road traffic sources at Heathrow Airport

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• Model is sensitive to assumptions about variability in take-off trajectory in three dimensions


Work by Fernando Farias and Helen ApSimon, to be presented at 4th Urban Air Quality Conference, Prague


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Usual approach, modelling aircraft emissions as volume sources



More detailed representation, resolving three-dimensional line-sources of trajectories


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• Model is sensitive to assumptions about variability in take-off trajectory in three dimensions

• Exposure of people living near airport, to aircraft emissions, is dominated by a very small region within the model output



Relative contribution of aircraft and traffic at a hospital 5 km north-east of the airport

(1)




Relative contribution of aircraft and traffic at a school 0.5 km south of the airport

(2)


Relative contribution of aircraft and traffic at a sports ground 1 km north of the airport

(3)



Relative contribution of aircraft and traffic at a young offenders’ institute 3 km south of the airport

(4)



More detailed study of traffic sources in street canyons

• How much detail is needed?

Work by Kiki Asimakopoulos and Helen ApSimon, contribution to EUROTRAC-2 SATURN Sub-project




• Can we assume canyons are symmetrical?




• Asymmetry has an important influence on concentration



• Secondary pollutant: nitrogen dioxide(assuming 95% of NOx emissions are NO2)


Mechanisms of ventilation of street canyon systems

Y

X

Wind direction

Transfers between streets

Transfers over roofs

Quantification of fluxes at street canyon intersections byAthena Scaperdas and Alan Robins



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150

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0 5 10 15 20Time

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cent

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Mean value mean + 3 stnd devs

Wind tunnel measurements by Tom Bentham and Alan Robins



• Operational models assume all canyon ventilation is vertical from two-dimensional geometry

• Wind tunnel measurements and steady state computational fluid dynamics shows fluxes between streets are important

• Wind tunnel measurements show flow is unsteady at intersection and intermittent processes may be important in determining average pollutant concentrations

Application of adaptive mesh LES to street canyon intersections• Use of Fluidity, a general purpose CFD code which

solves the Navier-Stokes and accompanying field equations on arbitrary unstructured meshes, previously used to solve a wide range of problems from flow past ocean seamounts to fluidised beds

• The anisotropic adaptive mesh facility in FLUIDITY is an example of the state of the art in finite element methods. Work is currently under way to combine this with parallel domain decomposition

work in progress by Chris Pain, Tom Bentham et al


Application of adaptive mesh LES to street canyon intersections• In order to simulate turbulent flows, FLUIDITY has

been adapted to perform large-eddy simulations (LES)

• The sub-filter scale model is a ‘dynamic’ variant of the Smagorinsky model, in which the anisotropic filter length is linked to the local element size and shape

• The combination of adaptive mesh refinement and LES optimises resolution, increasing efficiency of computation especially in complex geometries where the flow is not known a priori. It also allows easy identification of flow structures

see http://amcg.th.ic.ac.uk/research/adaptivity Page no./ref © Imperial College London

Application of adaptive mesh LES to street canyon intersections

• Application of Fluidity to Street Canyon Intersections, first model runs to be completed during 2003

• Computational modelling carried out in conjunction with wind tunnel measurement


Conclusions

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• UK urban atmospheric science community capability to apply operational and advanced models has grown significantly in recent years

• Effort remains largely fragmented, and it is not clear exactly what future applications of models will be required, nor the extent to which operational and advanced approaches currently used or under development will be fit for purpose


Thank you

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ncas/april meeting on urban air quality modelling dispersion modelling at imperial college london...

Documents

road traffic sources

heathrow airportpage

praguemodelling aircraft

trajectoriesmodelling

airport1modelling aircraft

airport4modelling aircraft

airport3modelling aircraft

modelling aircraft emissions