Scalar mixing from concentrated sources in urban-like roughness

The prediction of scalar mixing in urban areas at the neighborhood/street-scale remains a challenging issue owing to the presence of obstacles that lead to very complex flow features (Britter & Hanna 2003). The flow complexity is such to make it impossible to apply Gaussian models in the near-field, where the scalar plume is tipically confined within the canopy (Belcher 2005). The use of refined computational models, such as LES and RANS, is thus required. The present study is motivated by the need of a coherent framework for the evaluation of computational models for short-range dispersion in urban environments (Rossi & Iaccarino 2009b). To this end, we selected the case of the scalar release from a concentrated source located within a staggered array of wall-mounted cubical obstacles of random height. In this research brief we present a DNS study of the flow setup that is used out to obtain a characterization of the mixing process in the presence of discrete roughness as well as to provide a reliable and comprehensive database for models evaluation and improvement (Rossi 2010).