Short range dispersion in urban areas Alan Robins, University of - - PowerPoint PPT Presentation

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Short range dispersion in urban areas Alan Robins, University of - - PowerPoint PPT Presentation

Dec 01, 2022 189 likes •648 views

Short range dispersion in urban areas Alan Robins, University of Surrey DAPPLE team Surrey - wind tunnel modelling, analysis Bristol - tracer studies, analysis Cambridge - tracer studies, modelling & applications Imperial - personal

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Short range dispersion in urban areas

Alan Robins, University of Surrey

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Surrey - wind tunnel modelling, analysis Bristol - tracer studies, analysis Cambridge - tracer studies, modelling & applications Imperial - personal exposure, modelling & applications Leeds - traffic movement, emissions, pollution & wind field measurement Reading - meteorology, wind field & modelling Golder Associates (UK) Ltd.

DAPPLE team

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 From canyons to intersections - early work  Some features of dispersion in cities using London as an example  Some sensitivity studies  Implications for regulatory/emergency response modelling

Content

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Four blocks

Athena Scaperdas

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Four blocks - velocity vectors

Measured Velocity Vectors (LDA) z/H = 0.16, Orientation = 10 degs U/Uref

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Four blocks - exchanges

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Streets and intersections

Where are the classical street canyons?  The city as a set of short streets between intersections.  Concentrate

  • n the

intersections.  Use London as an example (of a certain class of cities). AURN Site

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A simple model for generic use

Initial wind tunnel model

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Flow field

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Horizontal mean velocity vectors at heights of 5 and 20 m

Mean flow field at an intersection

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Mean velocity vectors at heights of 5 m

Mean flow field at an intersection

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Mean velocity vectors at heights of 5 m

Mean flow field at an intersection

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Flow visualisation

Flow ‘switching’ with diagonal winds - bimodal pdfs? Light sheet at about z/H = 0.3

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Flow visualisation

Horizontal light sheet near surface Vertical light sheet along centre of road Vertical light sheet along centre of road

Source

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Light sheet along Marylebone Road

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Light sheet along Marylebone Road

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Vertical sheet along Marylebone Rd

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Vertical sheet along Marylebone Rd

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Vertical sheet along Gloucester Pl

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Vertical sheet along Gloucester Pl

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Flow visualisation and LES

  • 1. Exchanges through a street

network

  • 2. Loss/gain to flow above roof

level.

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Network models - SIRANE

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∫ ∫ ( U C + < u c > ) d A

Flux balances - mean and turbulent

?

U, UT, UN

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SIRANE

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Some empirical results

C* = CU(H)H 2 / Q c'* = c'U(H)H 2 / Q Separation R* = R / H Review C *(R / H), c'*(R / H) Ground level source in a boundary layer C* (R / H)N, N = 1.5 1.8 c'/ C 0.5 0.7

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Mean concentration

C* = 12/R*2 downwind sector only

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Salt Lake City

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Concentration fluctuations

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Fluctuation intensity

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Single events (realisations) may differ greatly from the mean of a large number (ensemble) of repeated events. Field studies provide a collection of realisations … that are not part of a common ensemble.

Receptor position 1- plume edge

ensemble mean in white, realisations in red

Variability amongst realisations

Receptor position 6- plume centre

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Scatter plot for individual releases

1:1 correspondence

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Sensitivity studies

Monitor air quality where?

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Comparison of results from low and higher resolution models; significant differences confined to short ranges.

Model detail

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Comparison of results from low and higher resolution models; significant differences confined to short ranges.

Model detail

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Generic upwind building arrangement

Upwind model detail

Source ‘X’

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Upwind model detail

Concentrations, C*, across the main intersection for source at X, wind direction 51˚

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Upwind model detail

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Reference; blockage at A, B or C; blockage at A, B & C. Green, reference; blue, blockage at A; red, at D; yellow at A & D. Wind direction

  • 90, degs

Wind direction

  • 45, degs

Blocked streets

Concentrations along Marylebone Road

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Car speed profile during one experiment Roadside sampler locations

Moving source

Not a line source - a set of point sources.

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Moving source

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Decay law C* = 12 R H

  • 2

Threshold CL* = 10 RL H

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Contaminated region RL = H 10 CL *

  • 1/2

RL RL RL

“Source” street

Empirical modelling

Upwind spread

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Regulatory level modelling

 Exchanges at intersections are a fundamental feature  Near-classical canyon conditions within about a street width  Substantial fluxes above roof level (even at short range)  Substantial parameter data base to acquire and test  Then add isolated tall buildings, open spaces, etc.

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Advanced level modelling

 Relatively weak sensitivity to model detail  Generic upwind conditions appear adequate  Substantial unsteadiness  Intermittent dispersion process important  Provision of parameters for regulatory level models

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Some questions

 Choice of reference wind  Street roughness elements (buses, trees …)  Evaluation against field data  Unsteady source terms  Thermal effects, light winds, traffic

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One person’s experience of an urban concentration field