2004 Fiscal Year Final Research Report Summary
Large Eddy Simulation Model for Complex Urban Surfaces and the Implication to Surface Similarity Law
| Project/Area Number |
15360262
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
水工水理学
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| Research Institution | Tokyo Institute of Technology |
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Principal Investigator |
KANDA Manabu Tokyo Institute of Technology, Graduate School of Science and Engineering, Associate Professor, 大学院・理工学研究科, 助教授 (90234161)
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| Co-Investigator(Kenkyū-buntansha) |
MORIWAKI Ryo Tokyo Institute of Technology, Graduate School of Science and Engineering, Research Associate, 大学院・理工学研究科, 助手 (10302952)
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| Project Period (FY) |
2003 – 2004
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| Keywords | LES / Urban Boundary Layer / Atmospheric Boundary Layer / Turbulent Organized Structures / Urban Canopy / Tower / Scale model / Urban Meteorology |
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| Research Abstract |
Large Eddy Simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cubic arrays. The model (LES-CITY) was validated in comparison with laboratory experiments. The systematic influence of cubic density on turbulent flow characteristics was investigated through numerical experiments in a wide range of cubic area density (0 % to 44%). The following results were obtained, (1)The influence of the heterogeneity of surface geometry : the dispersive momentum flux is quite large within the canopy layer due to a mean stream such as re-circulation, while it is small but not negligible above the canopy due to Turbulent Organized Structures(TOS). The spatial variation of temporally averaged momentum in the roughness sub-layer is as large as 10-20 % of the total kinematic surface drag. (2)The intermittency of flow : the temporally and spatial-ensemble averaged flow structure confirm the existence of conventionally described canyon flow regimes ;
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isolated, interfacial and wake. However, the intermittency of the canyon flow for any cubic densities is quite large and the stream patterns are never persistent. (3)TOS : TOS similar to those observed in turbulent surface layer flows are simulated, which are characterized by longitudinally-elongated low speed streaks and the corresponding shorter stream wise vortices. The streaks in sparse and dense canopy flows are likely to be aligned to the street-line and to the roof-lines, respectively. Such heterogeneity of TOS partially accounts for the large spatial variation of momentum flux. (4)The difference from vegetation flow : the TOS and the resulting turbulent statistics of urban flow above the canopy resemble those in surface layers, contrast to the mixing layer analogy of vegetation flows. The re-circulation within the canopy makes significant influence on the turbulent statistics such as negative Reynolds stress, large dispersive flux and standard deviations, and the up-shift of the displacement height.
Turbulent organized structures above building arrays were investigated using a large eddy simulation(LES) model for a city (LES-CTTY). Square and staggered building arrays produced contrasting behavior in terms of turbulence that roughly corresponded to the conventional classification of ‘D-type' and ‘K-type' roughness, respectively, (1)The drag coefficients at the building heights for staggered arrays were sensitive to building area density, but those for square arrays were not (2)The relative contributions of ejections to sweeps ( ) at the building height for square arrays were sensitive to building area density and nearly equaled 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 was concerned, the turbulent organized structures of urban-like roughness flows resembled those of flat-wall boundary layers. Less
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