INFLUENCE OF GEOMETRY ON THE MEAN FLOW WITHIN URBAN STREET CANYONS – A COMPARISON OF WIND TUNNEL EXPERIMENTS AND NUMERICAL SIMULATIONS A. KOVAR-PANSKUS 1 , P. LOUKA 2 , J.-F. SINI 2 , E. SAVORY 3∗ , M. CZECH 1 , A. ABDELQARI 2 , P. G. MESTAYER 2 and N. TOY 1 1 Fluids Research Centre, School of Engineering, University of Surrey, Guildford, Surrey, U.K.; 2 Ecole Centrale de Nantes, Lab de Mecanique des Fluides, Nantes, France; 3 Department of Mechanical and Materials Engineering, Faculty of Engineering, University of Western Ontario, London, Ontario, Canada ( ∗ author for correspondence, e-mail: esavory@eng.uwo.ca) Abstract. A comparison between numerical simulations and wind tunnel modelling has been per- formed to examine the variation with streamwise aspect ratio (width/height, W/H) of the mean flow patterns in a street canyon. For this purpose a two-dimensional (2-D) cavity was subjected to a thick turbulent boundary layer flow perpendicular to its principal axis. Five different test cases, W/H = 0.3, 0.5, 0.7, 1.0 and 2.0, have been studied experimentally with flow measurements taken using pulsed- wire anemometry. The results show that the skimming flow regime, with a large vortex in the canyon, occurred for all the cases investigated. For the cavities with W/H≤0.7 a weaker secondary circulation developed beneath the main vortex. The narrower the canyon, the smaller the wind speed close to the cavity ground, giving increasingly poor ventilation qualities. The corresponding numerical results were obtained with the Computational Fluid Dynamics (CFD) code CHENSI that uses the standard k-ε model. The intercomparison showed good agreement in terms of the gross features of the mean flow for all the geometries examined, although some detailed differences were observed. Keywords: 2-D cavity, canyon, numerical simulation, pulsed-wire-anemometry, standard k-ε model, ventilation, vortex Nomenclature d = Displacement height (m); H, W = Height and width of canyon (m); k = Turbulent kinetic energy (m 2 s -2 ); L = Spanwise length of canyon (m); Re = Reynolds number, U r ef H/ν ; U = Mean velocity in X direction (m s -1 ); U r ef = Freestream velocity (m s -1 ); U ∗ = Friction velocity (m s -1 ); Water, Air, and Soil Pollution: Focus 2: 365–380, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.