2.5
LES STUDY ON TURBULENT ORGANIZED STRUCTURES WITHIN AND ABOVE URBAN CANOPY
Manabu Kanda, Tokyo Institute of Technology, Tokyo, Japan
Large Eddy Simulations have been performed for fully developed turbulent flow within and above explicitly resolved simple cube 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; 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 streamwise 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.
Key words: LES-CITY, urban obstacles, turbulent organized structure, surface heterogeneity, intermittency
Session 2, Air Quality Modeling and Forecasting: Part 1 (Room 611)
Monday, 12 January 2004, 4:00 PM-5:30 PM, Room 611
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