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The Influence of Building Packing Densities on Flow Adjustment and City Breathability in Urban-like Geometries
School of Amopheric Sciences, Sun Yat-sen University, HaiZhu, Guangzhou, China.
School of Amopheric Sciences, Sun Yat-sen University, HaiZhu, Guangzhou, China.
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system.
University of Gävle, Faculty of Engineering and Sustainable Development, BMG laboratory.
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2017 (English)In: Procedia Engineering, ISSN 1877-7058, E-ISSN 1877-7058, Vol. 198, p. 758-769Article in journal (Refereed) Published
Abstract [en]

City breathability refers to the air exchange process between the flows above and within urban canopy layers (UCL) and that of in-canopy flow, measuring the potential of wind to remove and dilute pollutants, heat and other scalars in a city. Bulk flow parameters such as in-canopy velocity (Uc) and exchange velocity (UE) have been applied to evaluate the city breathability. Both wind tunnel experiments and computational fluid dynamics (CFD) simulations were used to study the flow adjustment and the variation of city breathability through urban-like models with different building packing densities. We experimentally studied some 25-row and 15-column aligned cubic building arrays (the building width B=72 mm and building heights H=B) in a closed-circuit boundary layer wind tunnel. Effect of building packing densities (λp=λf=0.11, 0.25, 0.44) on flow adjustment and drag force of each buildings were measured. Wind tunnel data show that wind speed decreases quickly through building arrays due to strong building drag. The first upstream building induces the strongest flow resistance. The flow adjustment length varies slightly with building packing densities. Larger building packing density produces lower drag force by individual buildings and attains smaller velocity in urban canopy layers, which causes weaker city breathability capacity. In CFD simulations, we performed seven test cases with various building packing densities of λp=λf=0.0625, 0.11, 0.25, 0.36, 0.44 and 0.56. In the cases of λp=λf=0.11, 0.25, 0.44, the simulated profiles of velocity and drag force agree with experiment data well. We computed Uc and UE, which represent horizontal and vertical ventilation capacity respectively. The inlet velocity at 2.5 times building height in the upstream free flow is defined as the reference velocity Uref. Results show that UE/Uref changes slightly (1.1% to 0.7%) but Uc/Uref significantly decreases from 0.4 to 0.1 as building packing densities rise from 0.0625 to 0.56. Although UE is induced by both mean flows and turbulent momentum flux across the top surface of urban canopy, vertical turbulent diffusion is found to contribute mostly to UE.

Place, publisher, year, edition, pages
Elsevier Ltd , 2017. Vol. 198, p. 758-769
Keywords [en]
Building packing densiy, City breahability, Exchange velocity, Flow adjustment, In-canopy velocity, Atmospheric thermodynamics, Boundary layer flow, Boundary layers, Computational fluid dynamics, Drag, Velocity, Wind, Wind tunnels, Boundary layer wind tunnel, Computational fluid dynamics simulations, Turbulent momentum fluxes, Vertical ventilation, Wind tunnel experiment, Buildings
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:hig:diva-25416DOI: 10.1016/j.proeng.2017.07.127ISI: 000425682900067Scopus ID: 2-s2.0-85029853562OAI: oai:DiVA.org:hig-25416DiVA, id: diva2:1150010
Note

Funding details:

- MOE, Ministry of Education;

- Grant no: 2013U-5, THU, Tsinghua University

- Grant no: 51478486, NSFC, National Natural Science Foundation of China

Available from: 2017-10-17 Created: 2017-10-17 Last updated: 2018-03-20Bibliographically approved

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Sandberg, MatsClaesson, Leif

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