Driving physical mechanisms of flow and dispersion in urban canopies

The paper summarizes results from recent full-scale and wind-tunnel studies and discusses the complexity of urban canopy layer (UCL) flow and related challenges for urban modeling. Wind-tunnel data for idealized street-canyon intersections demonstrate that street-level flow and dispersion patterns are significantly altered for configurations with non-uniform building heights. For most wind directions, concentration maxima were upto a factor of 2 higher for cases with taller buildings near the intersection. Enhanced vertical downward mixing in the wakes of the taller buildings apparently causes higher street-level winds close by, but simultaneously, other regions become sheltered from active mixing and poor ventilation. The analysis of Joint Urban 2003 (JU2003) full-scale data also pointed out that high-momentum fluid can be effectively mixed down in the wakes of high-rise buildings, and that UCL flow is predominantly dynamically driven by roof-level winds. Building-height variability is thus, a key factor for UCL dynamics and mixing. Additionally, a linear model, which assumes that the along- and across-canyon velocity components are directly proportional to their above roof-level counterparts, has then been tested. The complexity of UCL flow patterns cannot be captured by such a simple model, but it may be useful for estimating street-level winds. For the along-canyon component, more than 70% of the calculated values were within a factor of 2 of the measured values, while for the across-canyon component less than 50% of the predictions agreed within a factor of 2. The practical applicability of the model was further assessed using JU2003 wind-tunnel data and a comparison between JU2003 wind-tunnel and full-scale flow profiles is also presented. It becomes obvious that UCL flow patterns are significantly altered by changes in upwind wind direction of less than 10°. While the field and laboratory results are qualitatively in good agreement, noticeable differences exist at certain sites.

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