Geometry (GEO) is one of the urban morphological variables that has effects on microclimate within the urban canopy level (UCL). It is usually quantified in previous researches by different aspect ratios, which have a basis on the buildings’ individual height (H), width between each other (W) and their buildings’ longitudinal length (L), e.g. H/W, L/H or L/W. In this paper, the impact of different morphological scenarios for GEO variation on external ventilation levels within a typical high-rise Housing and Development Board (HDB) residential estate (or precinct) in Singapore, is analyzed through a parametric study exercise. The GEO values are quantified by using the author-proposed Maximum Hydraulic Diameter (HDMax). Two types of common high-rise HDB block types are examined – point and slab blocks, in two types of configurations: (i) geometrical height variation and (ii) geometrical width variation. Numerical studies are done by utilizing a commercial computational fluid dynamics (CFD) code named Star-CCM+. External ventilation levels are quantified by using the area-averaged Wind Velocity Ratio (VR) index, an indication of the average outdoor ventilation potential within an estate at a certain level above ground. Measurements were taken at both the pedestrian and mid-levels under different wind orientations. The final results indicated that in general, GEO is positively related to VR and concluded the usefulness of using HDMax instead of the common aspect ratios of canyons proposed by previous researchers, of which the scale and absolute size were not reflected and produced different gradients of relationship with VR under different geometry range values. Case studies were also included at the later part of this paper to verify this positive relationship between GEO and VR.
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