Impact of building separation on natural ventilation behaviour and performance for low-rise structures

Natural ventilation, a readily available natural resource, should be better utilized for integrating the concept of sustainability in our built environment. Obviously, the clearance among buildings is a crucial factor affecting the natural ventilation performance. This paper attempts to reveal the ventilation behaviour in urban geometry of different building spacing. Large-eddy simulations (LES) were performed with the one-equation subgrid-scale (SGS) model for the unresolved turbulent kinetic energy. Three evenly separated two-dimensional (2D) hypothetical buildings were aligned in the streamwise direction that made up the computational domain. Cross-ventilation was enabled by opening up the lower halves of the windward and leeward facades. The Reynolds number was prescribed at around 50,000 to ensure fully developed turbulent flow. The sensitivity of the ventilation rate and the flow pattern in and around the buildings to building separation was examined. Fresh air entrainment from the shear layer aloft was significantly suppressed when the building separation was small. Reversed flow and recirculations were dominated near the ground and within the building envelope. Changes in the flow pattern, such as the positions, the sizes and the intensities of the recirculations, were observed with increasing separation. The contribution of turbulence to the total ventilation rate differed by various extents. When the separation approached 1.5 times the building height, the mean ventilation flux across the building envelope dropped to almost zero while the turbulent flux dominated the ventilation.