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LES Analysis of the Aerodynamic Surface Properties for Turbulent Flows over Building Arrays with Various Geometries

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  • 1 Japan Atomic Energy Agency, Ibaraki, Japan
  • 2 Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan
  • 3 Japan Atomic Energy Agency, Ibaraki, Japan
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Abstract

This paper describes aerodynamic roughness properties for turbulent flows over various building arrays that represent realistic urban surface geometries. First, building morphological characteristics such as roughness density λf and building height variability Vh, defined respectively as the ratio of total frontal area of roughness elements to the total surface area and the ratio of standard deviation in building height to the average building height of the study site, were investigated. Next, large-eddy simulations (LESs) of turbulent flows over building arrays were performed with various surface geometries characterized by a wide range of values for both λf and Vh, based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z0 and drag coefficient were evaluated for the central Tokyo area from the values of z0 and Vh using the LES results. The values of z0 and as a function of both λf and Vh were comparable to those found in earlier studies. The values of z0 and evaluated by a conventional method using only λf were underestimated, particularly for densely built-up areas. This indicates that the present approach to estimating aerodynamic roughness parameters, taking account of both roughness density and building height variability, is more appropriate than conventional approaches when applied to actual urban areas. The roughness aerodynamic parameters as a function of λf and Vh obtained from the LES results will be useful in incorporating urban effects into weather forecasting models.

Corresponding author address: Hiromasa Nakayama, Japan Atomic Energy Agency, 2-4 Shirakatashirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. E-mail: nakayama.hiromasa@jaea.go.jp

Abstract

This paper describes aerodynamic roughness properties for turbulent flows over various building arrays that represent realistic urban surface geometries. First, building morphological characteristics such as roughness density λf and building height variability Vh, defined respectively as the ratio of total frontal area of roughness elements to the total surface area and the ratio of standard deviation in building height to the average building height of the study site, were investigated. Next, large-eddy simulations (LESs) of turbulent flows over building arrays were performed with various surface geometries characterized by a wide range of values for both λf and Vh, based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z0 and drag coefficient were evaluated for the central Tokyo area from the values of z0 and Vh using the LES results. The values of z0 and as a function of both λf and Vh were comparable to those found in earlier studies. The values of z0 and evaluated by a conventional method using only λf were underestimated, particularly for densely built-up areas. This indicates that the present approach to estimating aerodynamic roughness parameters, taking account of both roughness density and building height variability, is more appropriate than conventional approaches when applied to actual urban areas. The roughness aerodynamic parameters as a function of λf and Vh obtained from the LES results will be useful in incorporating urban effects into weather forecasting models.

Corresponding author address: Hiromasa Nakayama, Japan Atomic Energy Agency, 2-4 Shirakatashirane, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan. E-mail: nakayama.hiromasa@jaea.go.jp
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