Quantitative Analysis of Flow Characteristics and Contaminant Dispersion in a Model Urban Canopy

Understanding scalar dispersion in an urban canopy is crucial in predicting the spread of hazardous materials released in cities. We present analysis of velocity and scalar concentration data acquired using magnetic resonance imaging techniques in simple urban canopies comprising a regular array of cubical buildings with a single central tall building. Comparisons are made between two wind directions, either aligned with the streets or skewed at 45 degrees.  A dominant feature in both orientations is the separated near wake of the tall building. Contaminant is advected upwards in the tall building wake then turns downstream forming a plume above the cubical buildings. Streamtube analysis around the contaminant release area shows that turbulent dispersion dominates in the plume. Geometrical dispersion is much more important within the canopy of short buildings.  The contaminant dispersion is highly dependent on the size and distance between separation bubbles behind the short buildings as well as the orientation of the bulk flow relative to the building canyons.