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Title:Role of pavements in urban energetics
Author(s):Sen, Sushobhan
Director of Research:Roesler, Jeffery
Doctoral Committee Chair(s):Roesler, Jeffery
Doctoral Committee Member(s):Al-Qadi, Imad; Masud, Arif; Gregory, Jeremy; Harvey, John
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):urban heat island
computational fluid dynamics
thermal conductivity
heat capacity
thermal diffusivity
canopy level UHI
air temperature
urban form
land cover heterogeneity
wind direction
Abstract:The Urban Heat Island (UHI) is a sustained increase in temperature in urban areas as compared to adjacent rural areas. This effect is partially due to the replacement of vegetated surfaces with lower-albedo impervious surfaces (primarily made of asphalt and concrete) to make way for urban infrastructure, and partially due to the decreased wind speed in urban areas as a result of increased turbulent dissipation. The UHI effect can be studied at a variety of spatial and temporal scales. A bulk of the literature has focused on the traditional, mesoscale definition, which compares an urban area to an adjacent rural area. However, the heterogeneity of the UHI at smaller scales has received less attention. The present study considers the distribution of energy (and hence temperature) within a city, which can be described as the study of urban energetics. The term ’microscale UHI’ is adopted to highlight this difference and is studied in terms of differences in air temperature at 2 m height averaged over an hour of the day. To determine the contribution of pavements to the development of microscale UHI, two research aims were pursued: (a) the thermal and optical characterization of asphalt and concrete, and the factors that influence them; and (b) the development and validation of an urban climate model to determine the wind speed and air temperature in a sub-area of a city. Asphalt field cores were characterized by a Transient Plane Source (TPS) technique for thermal properties and a spectrophotometer for optical properties. A new bilinear aging albedo model was developed, which can be easily calibrated. Lab concrete specimens of varying proportions but fixed constituents were cast, and the optical and thermal properties were found to be most sensitive to the volume of paste (VP) and the ratio of fly ash to cementitious materials (FA/CM). Finally, to rapidly measure the albedo of pavements over a large road network, a new instrument called the Discrete SPectrAl RefleCtometer (D-SPARC) was developed, which could measure the albedo of pavements during any time of the day or night approximately four times faster than the current standard technique, with a reasonable error of 0.02-0.06. Next, to simulate the urban microclimate, two numerical models were developed and validated: a 1-D pavement thermal model, called the Illinois Thermal Analysis Program Finite Volume (ILLITHERM-FV); and a 3D Computational Fluid Dynamics (CFD) RANS-based urban canyon model. These models were first tested on a hypothetical urban environment (3 by 3 city block) using representative meteorological data from Chicago, IL, with an extrusion-exclusion technique being used to generate a high-quality hexahedral mesh of the city. The urban canyon model was implemented using the open-source solver OpenFOAM and run with the pavement model in uncoupled and coupled modes. Results from the uncoupled showed that while pavements with higher albedo and thermal diffusivity could mitigate UHI by up to 0.7°C, the extent of mitigation varied spatially, depending on the urban form and wind direction. Interior urban canyons, which are away from the boundaries of the city, had a higher UHI than ones closer to the boundary. The uncoupled model underestimated the UHI by 2 to 10% as compared to the coupled model but took approximately three times less time to compute. The uncoupled model was then used to perform a case study on the Power Ranch community in suburban Phoenix, AZ where significant meteorological data was available. Changing the existing aged asphalt pavements to typical concrete pavement had a negligible impact on the UHI, but a reflective pavement decreased the air temperature by 0.2-0.4°C. A combination of cool pavements, roofs, and walls (reflective surfaces) led to a significant decrease of 0.8-1.0°C. However, the heterogeneous land cover in the area limited the spatial extent to which these strategies were effective, indicating that careful modeling of individual urban areas is needed before selecting any cool pavement strategy.
Issue Date:2019-12-04
Rights Information:Copyright 2019 Sushobhan Sen
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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