3D MRI measurements of the effects of wind direction on flow characteristics and contaminant dispersion in a model urban canopy

Three dimensional velocity fields and contaminant dispersion within an evenly spaced orthogonal array of cubic buildings (height = H) with a central tall building (height = 3H) were studied to examine the influence of tall buildings and wind orientation within an urban canopy. Mean velocity and contaminant data were collected using magnetic resonance velocimetry and magnetic resonance concentration methods. Two building orientations, each angled at 0° and 45° with respect to the bulk flow, were examined. The single tall building strongly influenced the distribution of the streamwise and vertical mass fluxes in both wind orientations. In particular, the tall building wake strongly influences how mass and momentum are transported downward into the building canopy. The contaminant dispersion pattern was dependent on the size of separation regions behind buildings and the wind orientation relative to the street canyons. A plume of contaminant trailed behind the tall building and was dispersed by the building wake turbulence. This dispersion was more rapid for the 45° wind orientation likely due to the wider wake in this case. The complex mean flow within the canopy plays a major role in controlling ground level dispersion when the wind is not aligned with the street canyons. The relatively simple geometry of the canopy and the detailed full field velocity and concentration data make this an ideal case for testing simulations.