LESによる都市建物群内外の数値シミュレーション Large Eddy Simulation of turbulent organized structures witin and above explicitly resolved cube arrays

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Abstract

近年、都市の幾何形状が上空大気に与える影響が問題となっており研究がなされてはいるが、体系的には理解されていない。また、都市キャノピー層における乱流輸送過程には、都市キャノピー層内部の乱流だけでなく大気境界層の大スケール乱れが影響を及ぼしていることが示唆されている。これらの解決のためには、都市キャノピー層から大気境界層にわたる広範囲な流れの解明が必要である。本研究では、LESモデルを用いて建蔽率を変化させた数値実験を行い、都市キャノピー内部から建物高さの3倍~4倍程度の高さに至る乱流の組織構造を検討した。

Large-eddy simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube arrays. The results from our model, hereafter LES-CITY, are shown to agree with laboratory experiments. We investigated the systematic influence of cube density on turbulent flow characteristics by performing numerical experiments for cube areal densitiesfrom 0 to 44%. The following results were obtained: (1) The dispersive momentum flux was quite large within the canopy layer due to a mean stream re-circulation, whereas it was smaller above thecanopy. The spatial variation of temporally averaged momentum in the roughness sub-layer was 20% or less of the total kinematic surface drag. (2) The temporally and spatially-averaged flow structure confirmed the existence of conventionally described canyon flow regimes; isolated, interfacial, and wake. However, the intermittency of the canyon flow for all cube densities was quite large and the stream patterns were never persistent. (3) Turbulent organized structures (TOS) similar to those observed in turbulent surface-layer flows were simulated, which are characterized by longitudinallyelongated low speed streaks and the corresponding shorter streamwise vortices. The streaks in sparse and dense canopy flows were 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) In contrast to the mixing layer analogy of vegetation flows, the TOS and the resulting turbulent statistics of urban flow above the canopy resembled those in surface layers. The recirculation withinthe canopy significantly influenced the turbulent statistical properties.

Journal

  • Proceeding of Annual Conference

    Proceeding of Annual Conference 16(0), 266-267, 2003

    THE JAPAN SOCIETY OF HYDROLOGY AND WATER RESOURCES

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