Modelling the urban radiant fluxes using view factors

Abstract

The exchange of radiant fluxes between different surfaces in outdoor urban environment can be described based on the theoretical framework of enclosure theory (or net-radiation method) in order to have a general understanding of the relation between different surfaces and consequent radiant fluxes and hence the Tmrt within the enclosure. To apply the theory, the required enclosure could be constructed by including the real surfaces of building facade and the imaginary surface of sky dome. The mean radiant temperature at a point within the enclosure could be determined if each surface temperature Ti is solved given a known input energy flux qi is imposed to each surface. At any instant, the surface energy balance holds for every surface of the enclosure by energy conservation. However, the enclosure theory requires certain assumption, idealizations and computations. For example, each surface of the enclosure are assumed to be gray, diffuse and at a different uniform temperature. But most of the real materials are not black, gray nor diffuse. The area of the enclosure may also be subdivided into smaller areas on a basis of uniform surface temperature over those smaller areas. It may require excessive computational time by dividing an area into too many smaller ones as for each surface there are two equations to be solved (not shown in the text). A system of 2N equations has to be solved if there are N surfaces ‘recognized’ in the enclosure, or in turn in the urban environment. This has also given rise to the difficulties in ‘recognizing’ individual ‘surface’ of uniform temperature in the real outdoor spaces surrounded by buildings with sunlit areas, tress with leaves, and so on. To put it simply, by treating the urban structure as a ‘black box’ and using regression analysis, the objective of this study is to identify and evaluate the empirical relation between radiant fluxes from directions within urban context in daytime. The urban environment will be captured in fish eye photos and decomposed into different components based on materials and properties. Radiant fluxes will be measured and regressed on those view factors. The preliminary results revealed a simple and significant correlation between view factor of materials and outdoor radiant fluxes.