An investigation of three-dimensional characteristics of flow regimes within the urban canyon

Canyon geometry is an important determinant characteristic airflow regimes observed within urban canyons. Three principal flow regimes are: “skimming” flow, “wake interference” flow and “isolated roughness” flow, following the nomenclature of Oke (1987, Boundary Layer Climates, 2nd edn, Methuen, London). The transition between flows is determined by canyon geometry and can be described in terms of threshold height/width (H/W) ratios for an arbitrary length/height (L/H) ratio.

The determination of threshold H/W ratios has previously relied on repeated wind tunnel experiments or repeated runs of a numerical model, with canyon geometry altered until the observed flow regime changes. The present numerical investigation of typical three-dimensional flows within urban canyons identifies the key parameters which mark transition between flow regimes. On this basis it is possible to establish the geometric thresholds between regimes with analysis of a horizontal cross-section of a few simulated flows.