On the application of morphometry and fluid dynamics approaches for city management and sustainability Dipartimento di Scienza dei Materiali - University

On the application of morphometry and On the application of morphometry and fluid dynamics approaches for city fluid dynamics approaches for city

management and sustainabilitymanagement and sustainability

Dipartimento di Scienza dei Materiali - University of Salento (ITALY)[email protected]

[email protected]@unisalento.it

Silvana Di Sabatino, Laura Sandra Leo, Riccardo Buccolieri

AGE

Iglesias (Sardinia, Italy), 28-30 April 2011 O

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Urban Morphology

Urban morphometry & DEMs

City breathability

Effect of trees on ventilation

Conclusions

OutlineOutline


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Urban share of world’s population

Sustainable development:

“Development that meets the needs of the present without compromising the ability of future generations to meet their own needs." Brundtland Commission

Fluid dynamics contributes in various stages towards sustainability

IntroductionIntroduction

The urbanisation growth is a worldwide issue. Society is facing a number of changes. We must consider if such a growth is sustainable, adaptability to new conditions (resilience), mitigation strategies…


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IntroductionIntroduction

THE URBAN ENVIRONMENT

COMPLEX ECOSYSTEM


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Variability in Urban MorphologyVariability in Urban Morphology


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Urban MorphometryUrban Morphometry


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Based on the building shape/geometry, it is possible to obtain several parameters (useful for modelling purposes):

•λP (planar area index, ratio between the area occupied by buildings and the total lot area)

T

iip,

p A

Aλ

pA

Morphometric parametersMorphometric parameters

TA

• λF (frontal area index)

FA

T

ii,f

f A

A


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•building height variability

•street canyon aspect ratio H/W (W is the width)

ii,p

ii,p i

A

AHH

1N

HHi

2

i

H

Based on the building shape/geometry, it is possible to obtain several parameters (useful for modelling purposes):


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p < 0.10 Sparse Canopy

Intermediate Canopy pp Dense Canopy0.10 < < 0.30 > 0.30

Airflow over building arrays of cubes

Oke (1988) reviews flow regimes associated with air flow over building arrays of increasing HEIGHT over WIDTHor using LAMBDA PARAMETERS

Oke T., 1988. Street design and urban canopy layer climate. Energy and Buildings 11, 103-113


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Simple flow modellingSimple flow modelling

0 5

02

0 51 1

.

d dD Fz z z. β cexp

kH H H

MacDonald (2000)

Lettau (1969) 0 0 5 fz . H

NON homogeneous spatial NON homogeneous spatial distribution, shape and distribution, shape and building height variability building height variability cannot be neglected!cannot be neglected!

1 1 Pdp

z( )

H

TOO TOO SIMPLISTICSIMPLISTIC


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Novel morphometric approachNovel morphometric approach

H

DEM“Typical” modelling

Di Sabatino, S., Leo, L.S., Cataldo, R., Ratti, C., Britter, R.E., 2010. Construction of digital elevation models for a southern European city and a comparative morphological analysis with respect to Northern European and North American cities. Journal of Applied Meteorology and Climatology 49, 1377-1396


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Novel morphometric approachNovel morphometric approach

Di Sabatino, S., Leo, L.S., Cataldo, R., Ratti, C., Britter, R.E., 2010. Construction of digital elevation models for a southern European city and a comparative morphological analysis with respect to Northern European and North American cities. Journal of Applied Meteorology and Climatology 49, 1377-1396


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City breathabilityCity breathability


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WIND

The aim is that of investigating city berathability and its link to pollutant dispersion in urban-like geometries. The focus is on pedestrian level where people live and to try to establish a good strategy to follow for new built areas to prevent poor air quality.

Building packing densityBuilding packing density

Buccolieri, R., Sandberg, M., Di Sabatino, S., 2010. City breathability and its link to pollutant concentration distribution within urban-like geometries. Atmospheric Environment 44, 1894-1903.


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Flow rate/Age of airFlow rate/Age of air

mass flow balance - flow rate through the street opening is defined as:

local mean age of air (link between a concentration level to a time scale)


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Typical CFD code setup

• RANS equations

• Turbulence model

- standard k-ε

• Second order discretization schemes

• Grid: hexaedral elements

- ~ two millions and half

- δx=δy=0.06H, δz=0.03H

- expansion rate <1.3

- ~ 4 days of simulation for each case (2 CPU)

• Turbulent Schmidt Sct = 0.7

CFD modelling setupCFD modelling setup

UH = 1.656 m/s (undisturbed wind velocity at the building height H)α = 0.35δ = 0.77 m (boundary layer depth) u* = 0.19 m s−1 (friction velocity)κ =0.40 Cμ = 0.09


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Flow ratesFlow rates

Flow enters the array from side streets in all cases with the exception λp = 0.0625.

Overall, more air is transported into the array from the sides and leaves through the street top as the packing density increases up to λp = 0.56.

The λp = 0.69 case is indeed characterized by a vertical outflow lower than that occurring in the packing density λp = 0.56.

Air entering the array: positiveAir leaving the array: negative


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a classificationa classification

Three scenarios can be recognized: sparse, compact and very compact city.

- The sparse city (λp = 0.0625, 0.11 and 0.25), acts as a collection of obstacles, where reversed flow only occurs behind the buildings.

- The compact city (λp = 0.44 and 0.56) behaves as a unique obstacle with respect to the flow. A single wake, whose size scales with the horizontal dimension of the city, forms behind the building array. Even though a reversed flow bubble is present within the building domain, the horizontal flow rate is positive i.e. aligned with the wind direction.

- The very compact city (λp = 0.69) shows the presence of a strong reversed flow bubble. The horizontal flow rate is negative i.e. opposite to the approaching wind direction.

z = 0.5H


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large in poorly ventilated recirculation zones and in downstream regions. older in the downstream region of the array as the building packing density increases. As suggested from flow rates discussed in the previous section, low near the side openings where lower concentrations are found.larger close to the middle of the array for all cases investigated. Moreover, it increases as building packing density increases, and this occurs both in the middle and at the edge of the array.

Differences: it increases downstream in the three lowest configurations (λp = 0.0625, 0.11, 0.25), while for the most compact cases (λp = 0.44, 0.56, 0.69), it reaches a maximum and then decreases close to the end of the array. This maximum value occurs at lower distance downstream as the packing density increases.

Age of airAge of air

at pedestrian level


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Age of airAge of air

In the middle of the array, maximum values are found for the λp = 0.56λp = 0.69: the recirculation zone in the case extends over most of the building array length. In this zone, pollutants are well mixed and the local mean age of air is almost constantλp = 0.56: smaller recirculation zones, with lower flow velocities. Within regions of larger pollutant accumulations, the local mean age of air is found to be larger.


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obstacles and pathways to the parallel approaching windstreets with tall buildings may capture more rural air through the windward entry but also drive more air out of the street upwardly across street roofs when the street is too long

NEIGHBORHOOD-SCALE: being the street length limited, street with tall buildings may obtain more ambient air flushing the street for pollutant dilution, resulting in better city breathability than what occurs in streets with low buildingsCITY-SCALE: being the streets long, in streets with tall buildings, pollutant removal across street roofs is less effective than those found in streets with low buildings, thus city breathability is worse

Urban planningUrban planning

λp = 0.57

High-rise street configurations, large packing density (e.g. Hong Kong)


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Urban planningUrban planning

CITY BREATHABILITY

CITY-SCALEstreets with low buildings

NEIGHBORHOOD-SCALEstreet with tall buildings

high-packed cities (tall buildings are usually built to provide sufficient residential area)city-scale high-rise urban areas should be avoided!

High-rise street configurations, large packing density (e.g. Hong Kong)

wide canyon

neighborhhod-scale high-rise urban area

neighborhhod-scale high-rise urban area


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Maps of mean age of air can be used to derive multiple kinds of information about:

overall outdoor ventilation i.e. breathability of a given city or neighbourhood;concentration levels from a “worst case” scenario, with pollutants released everywhere;areas where there is likelihood that allowable mean concentrations are exceeded;areas where it is necessary to filtrate the intake of air to reduce exposureurban planning strategy

This is the first attempt to build a unified approach for the assessment of air quality of the total indoor and outdoor

environment.

SummarySummary


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Aerodynamic effects of trees Aerodynamic effects of trees in street canyons in street canyons


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Trees in street canyonsTrees in street canyons

obstacles to airflow (air mass exchange reduced)

particle deposition on plant surfaces pollutant concentration reduced

pollutant concentration increased

Buccolieri R., Gromke C., Di Sabatino S., Ruck B., 2009. Aerodynamic effects of trees on pollutant concentration in street canyons. Science of the Total Environment 407, 5247-5256


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Street canyon without trees (but with people…!)

Street canyon with one-row trees Street canyon with two-rows trees



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WIND TUNNEL INVESTIGATIONSGromke, C. and Ruck, B., 2009. On the impact of trees on dispersion processes of traffic emissions in street canyons. Boundary-Layer Meteorology 131, 19-34.

CODASC, 2008. Concentration Data of Street Canyon, internet database, http://www.codasc.de. Institute for Hydromechanics - University of Karlsruhe (GERMANY)

Flow and concentration fields in urban street canyons of different aspect ratios with various avenue-like tree planting configurations

Tree planting characteristics: influence of crown shape, diameter, height, porosity and planting density

FLOW: air exchange and entrainment conditions considerably modified, resulting in lower flow velocities and in overall larger pollutant charges inside the canyon.

DISPERSION: increases in pollutant concentrations at the leeward and decreases at the windward

street canyon with miscellaneous tree arrangements



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1) Approaching flow perpendicular and inclined by 45° to street axis

Empty street canyon - W/H=2

Street canyon with tree planting

CFD modellingCFD modelling

2) Is wind direction important? Competition with aspect ratio…


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Tree-free street canyonTree-free street canyon

Street canyon with tree planting Street canyon with tree planting (densely filled crown)(densely filled crown)

CFD modelling CFD modelling Flow and dispersion in street canyons with tree planting


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REAL SCENARIOSREAL SCENARIOSAerodynamic effects of trees in Bari (Italy)

2 street canyons and 1 junction

Hmax~46m, Hmean~24m

“repetition unit”, i.e. representative of the urban texture of a larger portion of the city.

4 tree rows avenue-like tree planting of high stand densities, i.e. with interfering neighbouring tree crowns.

Bari (ITALY)

Buccolieri R., Salim S.M., Leo L.S., Di Sabatino S., Chan A., Ielpo P., de Gennaro G., Gromke C., 2011. Analysis of local scale tree-atmosphere interaction on pollutant concentration in idealized street canyons and application to a real urban junction. Atmospheric Environment 45, 1702-1713


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Wind meandering, buoyancy effects, background concentrations and other variables limit the comparison between monitored and simulated data to a rather qualitative analysis of the concentration levels at the monitoring positions since CFD simulations are typically done assuming a constant wind direction and without thermal stratification. CFD simulations aim at providing an example of how numerical tools can support city planning requirements Computational cells: three millions and a half (cell dimensions δxmin = δymin = 1m, δzmin = 0.3m until the height of 4m). 4 days simulation time with 2 processors

Wind dir.: 5°


- street canyon NS: W/H ~ 2- street canyon WE: W/H ~ 0.5


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•23 March 2006

•Wind dir.: West

•Uwest: 4.2 m/s

•Cwest.: 27μg/m3

•10 March 2006

•Wind dir.: South

•Usouth: 3.1 m/s

•Csouth. 25μg/m3

Measurements at monitoring station (~3m)


Concentration ratio

mean daily concentration ratios ranging from ~ 1.5 to ~ 2.2 during winter/spring time in the years 2005/2006

CFD simulations

~ 1.5 (MEAS.)

~ 1.1 (SIM.)

southsouth

westwest

UC

UC

South

West

UCConcentration ratios


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CFD results provide a basis to interpret the monitored data

WEST CASE: due to the interaction with the buildings and tree planting arrangement, the resulting flow is channelled along the street canyon NS (wider canyon), predominately blowing from North to South.

SOUTH CASE: wind blows predominately along the approaching direction which is from South to North.


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SOUTH CASE

•Slightly larger velocities (channelling along tree spaces transports more pollutant away from monitoring position)

•1.3 times larger concentrations at monitoring position without trees

West/South concentration West/South concentration ratioratio

TreeTreeMeasurement: ~1.5

Simulation: ~1.1

Tree-freeTree-freeMeasurement: N/ASimulation: ~0.3

WEST CASE

•Larger velocities

•3 times smaller concentrations at monitoring position without trees

Without trees the situation is reversed!


Simulations show that it has been crucial to consider the effect of trees on pollutant dispersion to explain qualitative difference between the two cases

Concentration ratio

southsouth

westwest

UC

UC


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ConclusionsConclusions

Understanding fluid dynamics of urban flows through field and laboratory experiments and numerical modelling allows us to construct to put in place mitigation strategies in our cities

Some examples have shown that MORPHOMETRY ANALYSES offer a valid tool for wind and dispersion modelling in urban areas .

The combination of experimental and numerical approaches can provide a strategy for planning and re-development of urban areas

aiming in pedestrian exposure mitigation but also put the basis for urban effect understanding on climate!

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Documents

On the application of morphometry and fluid dynamics approaches for city management and sustainability Dipartimento di Scienza dei Materiali - University