City breathability – Aerodynamic effects of trees – Odour assessment 1 City breathability An application to urban-like geometries by mean of CFD model

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City breathabilityCity breathabilityAn application to urban-like geometries by mean of CFD modelAn application to urban-like geometries by mean of CFD model

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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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What is breathability?What is breathability?

Many researchers have studied the distribution of pollutants within street canyons and densely built-up areas, both experimentally and numerically using Computational Fluid Dynamics (CFD) models: urban street width and street building heights, wind direction and velocity, building roof geometry, tree planting, building packing density etc.

Additionally, street level concentration have been analysed using the concept of exchange velocity between the urban canopy layer and the region above. This concept, that is defined by the average velocity of mass transfer into or out of the urban canopy, is a surrogate variable for ventilation within the urban canopy. It accounts for bulk vertical exchange of air mass but does not account for other effects such as flow spatial variability.

Other ways could be explored to determine the pollutant dilution rate within an urban environment using building ventilation concepts commonly applied to evaluate indoor exposure.

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process of exchange of indoor air with outdoor air (pollutants emitted indoors are diluted and removed)

building ventilation

knowledge of external atmospheric conditions is the boundary conditions for any building ventilation system (building packing density large: reduction in pressure difference across buildings, which limits the possibility to use natural ventilation

for buildings)

flux of pollutant leaving the region and never returning again

external air polluted: it needs to be filtered to reduce or avoid exposure inside

concepts to estimate ventilation in indoor environments

dilution rate

indoor and outdoor environments: recirculating flows.

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Ventilation conceptsapplied directly to measured pollutant concentrations in the final steady state within a given flow, regardless the position of the sources and the physical mechanisms involved in the pollutant mixing and removalbased on observed pollutant dilution and in principle can be applied to complex flows with strong recirculation regionsused for indoor building ventilation

Ventilation processes outdoors and indoors can be described by the same concepts. The physical fundamental processes involved are the same: dilution, removal and recirculation of contaminantsthe scale of the individual physical processes are different, but does not depend on the scale of the processesrarely applied to the urban environment because of the objective difficulty of setting up the required boundary conditions for the equations in the open urban atmosphere

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Our intention is that of finding a new integrated approach to evaluate both indoor and outdoor urban air quality.

The interaction between the atmospheric approaching flow and a city results in complicated flow patterns between buildings, along streets, stagnant zones and wake regions. Air mass approaching a city can either enter the streets, flows above the buildings or around it.

Bulk flow over a given distribution of buildings determines its breathability, i.e. city potential of removing and diluting pollutants, heat, moisture and other scalars.

P

0P

0P

P

0P

0P

Cin

Croof

Cout

Upward flow

Inflow

Outflow

Urban canopy layer

FLOW RATE

MEAN AGE OF AIR

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Flow rateFlow rate

Quantified by means of CFD simulations and by simply applying a mass flow balance in a given control volume.

The flow rate through the street opening is defined as:

velocity vector, normal direction to street openings, A area of the street entry. The vertical flow rate through the top is expressed in a similar way.

Normalized flow rate by the reference flow rate far upstream (qref):

To estimate local variations of q* within the building array, the street top level was subdivided into 7 smaller areas

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Homogeneous emission rate method (pollutants have been generated at the same emission rate at each location within the flow domain - pollutant generation occurs everywhere).

Given the effective uniformly distributed emission rate QU, and the corresponding tracer concentration field, the local mean age of air, at a given location P, can be estimated from the local concentration cP as:

Link between a concentration level to a time scale (a poorly ventilated region implies a large age, i.e. air mass takes a long time to reach a given region and therefore pollutant removal will be slower. This, in turn, means an accumulation of pollutants in the region and larger concentration).

Urban building array: we just estimate the net transport across boundaries (different from the time that pollutants spend within the building array volume). When pollutants return across open boundaries, the effective age increases and therefore it reflects the actual contamination situation within the building array.

The age of air was normalized using only a portion of the overall gaps volume as follows:

VOL volume of the gaps within the building array from the ground to the pedestrian level.

Age of airAge of air

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Commercial CFD code FLUENT

• 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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Each case experiences an upward flow (negative flow rates) at the beginning of the array. This is due to the resistance provided by the first row of buildings.

In most cases, at the downstream end of the array, there is a downward flow due to lower resistance; therefore the flow along the street increases and air is transported downwards to “fill up” the street with more air.

In the largest packing density cases (λp = 0.44, 0.56, 0.69), large positive flow rates (air flowing downward) are visible at the downstream end.


VERTICAL FLOW RATES VERTICAL VELOCITIES

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λp = 0.69: the flow is opposite to the wind direction at the centre of the array. The reversed flow is due to a recirculation bubble.

from the ventilation perspective, a recirculation bubble is a stagnation zone, and it is a response resulting in the change from an upward flow to a downward flow over a short distance corresponding to lower vertical velocities in that region


Overall, two types of vertical recirculation zones can be identified: weak and strong. Weak recirculation zones (λp = 0.44 and λp = 0.56) do not change the direction of the flow obtained by integrating the velocity field from the ground up to the building height (the normalized horizontal flow rate is positive). Strong recirculation zones (λp = 0.69): an evident change in flow direction is present.

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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.


at pedestrian level

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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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Most suitable building arrangement which may improve ventilation and reduce pollution in sensitive areas.

1) maps of the local mean age of air for a given packing density2) by estimating an average emission rate (memiss) and by assuming that the average emission rate is

released uniformly, concentrations in the urban environments can be estimated as follows:

3) results can be used to verify whether mean values are exceeded at a specific position, or within sensitive areas, both indoors and outdoors.

- c* is larger than the maximal allowable mean concentration (C limit): consider alternative building packing density

- c* lower than Climit, the concern might be about the health effects on living beings, caused by the long term exposure to high pollutant levels. The total intake for a given period is obtained as follows:

tindoor and toutdoor time people spend indoors and outdoors, qB breathing rate

If the intake of the ventilation air to the building is not treated, the indoor concentration will be the same as the outdoor concentration. Due to the long time spent indoors this may not be acceptable. Then one can filter the intake of outdoor to lower the indoor concentration below the outdoor concentration which, in turn, implies a lower intake.

Urban planning strategy Urban planning strategy fast assessment of mean high pollution spots. useful in those cases where the legislation imposes limits for the mean value of a specific pollutant.

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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.

ConsiderationsConsiderations

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Neighborhood-scale H/W L/W

Case [2, 47.4] 2

47.4Case [2.5, 47.4] 2.5

Case [3, 47.4] 3

Case [4, 47.4] 4

Case [2, 79] 2

79Case [2.5, 79] 2.5

Case [3, 79] 3

Case [4, 79] 4

Urban planningUrban planning

City-scale H/W L/W

Case [2, 333] 2

333Case [2.5, 333] 2.5

Case [3, 333] 3

Case [4, 333] 4

Case [2, 667] 2

667Case [2.5, 667] 2.5

Case [3, 667] 3

Case [4, 667] 4

λp = 0.57

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

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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


λp = 0.57


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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!


wide canyon

neighborhhod-scale high-rise urban area

neighborhhod-scale high-rise urban area

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Aerodynamic effects of treesAerodynamic effects of treesAn application to Bari city by mean of CFD modelAn application to Bari city by mean of CFD model

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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

UC Concentration 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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Trees in urban street canyons have important aerodynamics effects (aspect ratios and wind direction are among the most important ones!) They have somehow been quantified using wind tunnel controlled experiments. Real conditions may be different. BULK effects are probably understood individually but not in combination (especially in real scenarios)… multiple canyons, neighbourhood scale

RANS CFD simulations/analyses for concentration predictions in street canyons are currently feasible with a proper turbulence closure but LES is more adequate to take into account non-stationary processes, encouraging its use as alternative tool to experiments for predicting flow fields and pollutant dispersion in city planning or policy-making projects.

We still need to account for the effect of buoyancy (Radiation Sheltering effect but buildings release heat in. Trapping effect. Warm air in the bottom part of the canyon

ConsiderationsConsiderations

Documents

City breathability – Aerodynamic effects of trees – Odour assessment 1 City breathability An application to urban-like geometries by mean of CFD model