Proceedings of the 11 th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes EVALUATION OF THE TRAFFIC PRODUCING TURBULENCE WITHIN A MODELLED STREET CANYON USING COMPUTATIONAL FLUID DYNAMICS CALCULATIONS Efisio Solazzo 1 , Xiaoming Cai 1 , Sotiris Vardoulakis 2 1 School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham, U.K.; 2 Public & Environmental Health Research Unit, London School of Hygiene & Tropical Medicine, London, U.K ABSTRACT The motion of vehicular traffic has been identified as a significant source of mechanically- generated turbulent kinetic energy within urban street canyons. As the bulk of free stream wind flow is too weak to penetrate within the street canyon under calm wind conditions, the Traffic Producing Turbulence (TPT) becomes the dominant factor in mixing and diluting traffic-related pollutants. Full scale experiments can not give satisfactory insights into the TPT dynamics, because of the difficulty to study the TPT in isolation from other sources of turbulent kinetic energy within the canyon. For that reason, Computational Fluid Dynamics (CFD) calculations were adopted in order to improve our understanding of the process. Suitability of the CFD calculations in predicting real flow fields was evaluated using wind tunnel data. 1. INTRODUCTION The coupled processes of wind flow and TPT have been extensively studied in the literature by means of analytical formulations, numerical models, and wind tunnel experiments. However, most of the formulations proposed were intended for single lane and flat roadways, where the complexity of bounding walls interaction and vortical flow, typical of street canyon-type geometries, were absent (Stern and Yamartino, 2001). Moreover, studies addressing the combining effects of wind flow and TPT in urban street canyons most often adopted a “linear” method, consisting in resolving the flow field generated by the wind flow only, with TPT included as an extra Turbulent Kinetic Energy (TKE) term superimposed to the mean flow. However, it is possible to identify two main weaknesses in such approach: a) the postulated linearity, which leads to neglect the interaction between the wind flow field and the TPT. Such interaction results in the advection of flow and turbulence generated by moving vehicles toward the leeward side of the canyon, and it has been documented by several filed measurements (e.g. Vachon et al., 2002); b) the neglecting of a possible organised motion produced by the moving vehicles. Wind tunnel investigations by Kastner-Klein et al. (2001) found that the advection-type flow induced by moving vehicles outweighed the turbulent fluctuations, especially in the case of one-way traffic. The aim of this work is to model both the flow and the turbulence induced by moving vehicles, when an external wind flow is also present, using CFD calculations and adopting a cost-effective methodology to address the aforementioned points a) and b). 2. METHODOLOGY At any given point within a street canyon, mean velocity and turbulence are generated by the combined effects of wind flow and vehicular traffic (neglecting turbulence generated by thermal processes). In particular, velocity fluctuations are due to the following processes: I. turbulence in the atmosphere; II. a vehicle passing the point will generate a wake, producing deformation of the flow, and thus turbulence; III. turbulence in the wake. The organised flow is due to: IV. external wind flow; V. vehicle motion. Page 264