Large-Eddy Simulations on the Effects of Surface Geometry of Building Arrays on Turbulent Organized Structures |
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Authors: | Manabu Kanda |
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Institution: | (1) Department of International Development Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan |
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Abstract: | Turbulent organized structures (TOS) above building arrays were investigated using a large-eddy simulation (LES) model for
a city (LES-CITY). Square and staggered building arrays produced contrasting behaviour in terms of turbulence that roughly
corresponded to the conventional classification of ‘D-type’ and ‘K-type’ roughness, respectively: (1) The drag coefficients
(referred to the building height) for staggered arrays were sensitive to building area density, but those for square arrays
were not. (2) The relative contributions of ejections to sweeps (S2/S4) at the building height for square arrays were sensitive to building area density and nearly equalled or exceeded 1.0 (ejection
dominant), but those for staggered arrays were insensitive to building area density and were mostly below 1.0 (sweep dominant).
(3) Streaky patterns of longitudinal low speed regions (i.e., low speed streaks) existed in all flows regardless of array
type. Height variations of the buildings in the square array drastically increased the drag coefficient and modified the turbulent
flow structures. The mechanism of D-type and K-type urban-like roughness flows and the difference from vegetation flows are
discussed. Although urban-like roughness flows exhibited mixed properties of mixing layers and flat-wall boundary layers as
far as S2/S4 was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers. |
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Keywords: | Building array D-type and K-type roughness LES-CITY Low speed streaks Sweep and ejection Turbulent organized structure |
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