RANS modeling of scalar dispersion from localized sources within a simplified urban-area model

The use of RANS models in the analysis of the urban climate and in air quality studies has become increasingly popular when predictions have to deal with complex flows, such as those arising at the street-scale. The need for model assessment and validation has thus also become crucial towards establishing the reliability of RANS simulations of heat and mass transport in urban areas. In this research brief we evaluate RANS models for predicting dispersion from a localized source within the urban-canopy layer using a DNS database. The work is a follow-up of the DNS study of the same numerical experiment (Rossi & Iaccarino 2010), specifically designed to provide reliable concentration measurements and turbulence statistics for a simplified, but sufficiently challenging, urban-area model. The work is focused towards a comparative study of anisotropic Algebraic Scalar Flux models (AFMs) versus the widespread, though isotropic, standard gradient-diffusion model (Rossi & Iaccarino 2009b). In the ongoing research project algebraic models have been successfully applied to the release of a contaminant in the wake of isolated obstacles (Rossi & Iaccarino 2009a; Rossi et al. 2010) as well as to the injection of a passive tracer in a separated flow over a wavy wall (Rossi 2010), showing improved accuracy over the standard model for the same flow and Reynolds stress predictions. In the present analysis we seek to complete the evaluation study of algebraic models for predicting dispersion in realistic urban-area models.