Dipartimento di Scienza dei Materiali - University of Salento (ITALY)

1. AGE On the application of morphometry and fluid dynamics approaches for city management and sustainability Silvana Di Sabatino, Laura Sandra Leo, Riccardo Buccolieri Dipartimento di Scienza dei Materiali - University of Salento (ITALY) silvana.disabatino@unisalento.it laura.leo@unisalento.it riccardo.buccolieri@unisalento.it

2. Outline • Urban Morphology • Urban morphometry & DEMs • City breathability • Effect of trees on ventilation • Conclusions

3. Introduction 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… Sustainable development: “Development that meets the needs of the present without compromising the ability of future generations to meet their own needs."Brundtland Commission Urban share of world’s population Fluid dynamics contributes in various stages towards sustainability

4. Introduction THE URBAN ENVIRONMENT COMPLEX ECOSYSTEM

5. Variability in Urban Morphology

6. Urban Morphometry

7. Morphometric parameters 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) • λF (frontal area index)

8. Morphometric parameters Based on the building shape/geometry, it is possible to obtain several parameters (useful for modelling purposes): • building height variability • street canyon aspect ratio H/W (W is the width)

9. Morphometric parameters Oke (1988) reviews flow regimes associated with air flow over building arrays of increasingHEIGHT overWIDTH or using LAMBDA PARAMETERS Airflow over building arrays of cubes Sparse Canopy < 0.10 Dense Canopy Intermediate Canopy 0.10 < < 0.30 > 0.30 Oke T., 1988. Street design and urban canopy layer climate. Energy and Buildings 11, 103-113

10. Simple flow modelling Lettau (1969) MacDonald (2000) NON homogeneous spatial distribution, shape and building height variability cannot be neglected! TOO SIMPLISTIC

11. H 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 Novel morphometric approach DEM “Typical” modelling

12. 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 Novel morphometric approach

13. City breathability

14. 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. Building packing density 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.

15. Flow 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)

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

17. Flow 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: positive Air leaving the array: negative

18. a 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

19. Age of air • 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

20. Age of air • In the middle of the array, maximum values are found for the λp = 0.56 • λp = 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 constant • λp = 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.

21. Urban planning High-rise street configurations, large packing density (e.g. Hong Kong) λp = 0.57 • obstacles and pathways to the parallel approaching wind • streets 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 buildings • CITY-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

22. Urban planning High-rise street configurations, large packing density (e.g. Hong Kong) CITY BREATHABILITY CITY-SCALE streets with low buildings NEIGHBORHOOD-SCALE street 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

23. Summary • 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 exposure • urban 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.

24. Aerodynamic effects of trees in street canyons

25. 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 Trees in street canyons particle deposition on plant surfaces pollutant concentration reduced obstacles to airflow (air mass exchange reduced) pollutant concentration increased

26. Trees in street canyons Street canyon without trees (but with people…!) Street canyon with one-row trees Street canyon with two-rows trees

27. Trees in street canyons WIND TUNNEL INVESTIGATIONS Gromke, 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

28. CFD modelling • 1) Approaching flow perpendicular and inclined by 45° to street axis • Empty street canyon - W/H=2 • Street canyon with tree planting 2) Is wind direction important? Competition with aspect ratio…

29. CFD modelling Flow and dispersion in street canyons with tree planting Tree-free street canyon Street canyon with tree planting (densely filled crown)

30. 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 REAL SCENARIOS Aerodynamic 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)

31. REAL SCENARIOS Aerodynamic effects of trees in Bari (Italy) Wind dir.: 5° - street canyon NS: W/H ~ 2 - street canyon WE: W/H ~ 0.5 • 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

32. Concentration ratio REAL SCENARIOS Aerodynamic effects of trees in Bari (Italy) CFD simulations Measurements at monitoring station (~3m) • mean daily concentration ratios ranging from ~ 1.5 to ~ 2.2 during winter/spring time in the years 2005/2006 West • 10 March 2006 • Wind dir.: South • Usouth: 3.1 m/s • Csouth. 25μg/m3 • 23 March 2006 • Wind dir.: West • Uwest: 4.2 m/s • Cwest.: 27μg/m3 ~ 1.5 (MEAS.) ~ 1.1 (SIM.) Concentration ratios South

33. REAL SCENARIOS Aerodynamic effects of trees in Bari (Italy) 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.

34. Concentration ratio REAL SCENARIOS Aerodynamic effects of trees in Bari (Italy) • WEST CASE • Larger velocities • 3 times smaller concentrations at monitoring position without trees • 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 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

35. Conclusions • 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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