Particle Image Velocimetry Measurements of Turbulent Flow Within Outdoor and Indoor Urban Scale Models and Flushing Motions in Urban Canopy Layers
We investigate the spatial characteristics of urban-like canopy flow by applying particle image velocimetry (PIV) to atmospheric turbulence. The study site was a Comprehensive Outdoor Scale MOdel (COSMO) experiment for urban climate in Japan. The PIV system captured the two-dimensional flow field within the canopy layer continuously for an hour with a sampling frequency of 30 Hz, thereby providing reliable outdoor turbulence statistics. PIV measurements in a wind-tunnel facility using similar roughness geometry, but with a lower sampling frequency of 4 Hz, were also done for comparison. The turbulent momentum flux from COSMO, and the wind tunnel showed similar values and distributions when scaled using friction velocity. Some different characteristics between outdoor and indoor flow fields were mainly caused by the larger fluctuations in wind direction for the atmospheric turbulence. The focus of the analysis is on a variety of instantaneous turbulent flow structures. One remarkable flow structure is termed ‘flushing’, that is, a large-scale upward motion prevailing across the whole vertical cross-section of a building gap. This is observed intermittently, whereby tracer particles are flushed vertically out from the canopy layer. Flushing phenomena are also observed in the wind tunnel where there is neither thermal stratification nor outer-layer turbulence. It is suggested that flushing phenomena are correlated with the passing of large-scale low-momentum regions above the canopy.
KeywordsFlushing Organized structure Outdoor scale model Particle image velocimetry Urban canopy layer
Unable to display preview. Download preview PDF.
- Brown MJ, Lawson RE Jr, Decroix DS, Lee RL (2000) Mean flow and turbulence measurements around a 2-D array of buildings in a wind tunnel. In: 11th joint AMS/AWMA conference on the applications of air pollution meteorology. Long Beach, CAGoogle Scholar
- Inagaki A, Kanda M (2010) Organized structure of active turbulence over an array of cubes within the logarithmic layer of atmospheric flow. Boundary-Layer Meteorol. doi: 10.1007/s10546-010-9477-0
- Inagaki A, Maruyama A, Kanda M (2009) Spatial and temporal scales of coherent turbulence over outdoor reduced urban scale model. In: The 7th international conference on urban climate. Yokohama, JapanGoogle Scholar
- Inagaki A, Castillo MCL, Yamashita Y, Kanda M (2010) Numerical simulation of atmospheric turbulence within and above a cubical canopy. In: Ninth symposium on the urban environment. American Meteorological Society, KeystoneGoogle Scholar
- Jeong J, Hussain F, Schoppa W, Kim J (1997) Coherent structure near the wall in a turbulent channel flow. J Fluid Mech 332: 185–214Google Scholar
- Kaga A, Inoue Y, Yamaguchi K (1992) Application of a fast algorithm for pattern tracking on airflow measurement. In: Proceedings of 6th international symposium on flow visualization, pp 853–857Google Scholar
- Stull RB (1988) An introduction to boundary layer meteorology. Kluwer Academic Publishers, Dordrecht, p 666Google Scholar
- Sugawara H, Ogawa H, Hagishima A, Narita K, Tanimoto J (2006) Observation of stability-influenced canyon flow patterns. In: Proceedings of 6th international conference on urban climate. Göteborg, Sweden, pp 180–183Google Scholar
- Takimoto H, Sato A, Michioka T, Kanda M (2009) PIV measurements on the effects of fetch lengths and atmospheric stabilities on turbulent flow over building arrays. In: Proceedings of the conference on physical modelling of flow and dispersion phenomena. Sint-Genesius-Rode, Belgium, pp F5.1–F5.9Google Scholar