Experimental study of the transfer velocity for urban surfaces with a water evaporation method
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A major problem in urban climate modelling is determining how the heat fluxes from various canyon surfaces are affected by canyon flow. To address this problem, we developed a water evaporation method involving filter paper to study the distribution of the convective transfer velocity in urban street canyons. In this method, filter paper is pasted onto a building model and the evaporation rate from the paper is measured with an electric balance. The method was tested on 2D (two-dimensional) street canyon models and 3D model arrangements. Moreover, in this technique, it is easy to restrict the flux within an arbitrary surface in question. That is, the evaporation distribution on a surface can be studied by using several small pieces of filter paper. In the 2D case, the wall transfer velocity was strongly dependent on the canyon aspect ratio for perpendicular wind directions and it varied widely with height within both windward and leeward wall surfaces. For 3D cubic arrays, the relation to canyon aspect ratio was largely different from that of the 2D canyon. And, as a case study, the variation of wind direction was investigated for a city-like setting. The area-averaged transfer velocity was insensitive to wind direction but its local deviation was significant. Finally, we measured the transfer velocity for a clustered block array surrounded by relatively wide streets. The effect of spatial heterogeneity on the transfer velocity was significant. Moreover, for a fixed total building volume, the transfer velocity was considerably larger when the building height varied than when it was uniform. Therefore, the water evaporation method with filter paper is expected to be useful for studying the transfer velocity and ventilation rates in urban areas with various canyon shapes.
KeywordsSensible heat flux Spatial heterogeneity Turbulent transfer Urban canopy layer Urban climate
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- Barlow JF, Harman IN, Belcher SE (2004) Scalar fluxes from urban street canyon. Part I: laboratory simulation. Boundary-Layer Meteorol 113:369–385Google Scholar
- Harman IN, Barlow JF, Belcher SE (2004) Scalar fluxes from urban street canyon. Part II: Model. Boundary-Layer Meteorol 113:387–410Google Scholar
- Incropera F, DeWitte D (1996) Fundamentals of heat and mass transfer, 4th edn. Wiley, New York, 981 ppGoogle Scholar
- Ito N, Kimura K, Oka J (1972) A field experiment study on the convective heat transfer coefficient on exterior surface of a building. ASHRAE Trans 78:184–191Google Scholar
- Masson V, Grimmond CSB, Oke TR (2002) Evaluation of the town energy balance (TEB) scheme with direct measurements from dry districts in two cities. J Appl Meteorol 41:1011–1026Google Scholar
- Narita K (2004) Effects of building-height heterogeneity on area-averaged transfer velocity in the street surface – wind tunnel experiments using salinity change technique. Proc of 5th Symposium on the Urban Environment. AMS, Vancouver, CanadaGoogle Scholar