City breathability as quantified by the exchange velocity and its spatial variation in real inhomogeneous urban geometries: an example from central London urban area.

The breathability capacity and its spatial variation within an inhomogeneous urban area is investigated by examining the air flow and the induced flow exchange processes inside a real neighbourhood area of central London. The variation of the exchange velocity (as an index of city breathability) is interpreted in association with the local urban geometrical parameters and hence geometrical inhomogeneity. Numerical studies addressing flow exchange processes in urban areas have addressed so far rather idealised homogeneous geometries (e.g. Hamlyn and Britter, 2005; Salizzoni et al., 2009; Buccolieri et al., 2010; Hang et al., 2009 and 2010). This work analyses the results obtained from a Computational Fluid Dynamics (CFD) simulation study using a Reynolds-Average-Navier-Stokes (RANS) solver to study the flow and induced exchange processes in the area around the Marylebone Road and Gloucester Place intersection modelled at a 1:200 scale, with the wind direction blowing in the direction of the Marylebone street axis. Flow visualisations from the numerical results confirm that the particular building shapes and street canyon geometries determine the shape and size of vortical structures that are present in the flow field and thereby the exchange processes with the flow above. By considering appropriate control volumes enclosing each building, the exchange velocities, U(E), were deduced and found to range between 0.5% and 13% of the characteristic velocity above the urban canopy U(ref), which was referenced at a height 2.5 times of the building height. The range of the exchange velocity coefficient U(E)/U(ref) is higher than that observed in idealised regular cube arrays, mainly due to the enhanced flow mixing associated with the inhomogeneity of the urban geometry and particularly with tall buildings. This work may provide useful insight to urban designers and planners interested in examining the variation of city breathability as a local dynamic morphological parameter with the local building packing density.