Investigating Wake Characteristics of Tall Buildings in a Realistic Urban Canopy Using Wind Tunnel Modelling and Doppler Lidar Measurements

The presence of tall buildings in cities affects momentum and scalar exchange within and above the urban canopy. As wake effects can be important over large distances, they are crucial for urban-flow modelling on and across different spatial scales. This study explores the aerodynamic effects of tall buildings on the microscale to local scales with a focus on the interaction between the wake structure and urban canopy flow of the surroundings in a realistic urban setting. The MAGIC project (www.magic-air.uk) site at London Southbank University in central London, UK, was chosen for study due to a number of tall buildings in the vicinity. Two methods were used: a) a wind tunnel model of the site at the EnFlo atmospheric boundary layer wind tunnel at University of Surrey, UK, and b) Doppler lidar observations from a moderately high roof-top at the site (27 m agl).

For the wind tunnel work, a 1:200 scale model of the site, representing a 700 m diameter study area with a mean canopy height of 14.7 m, was constructed. Wakes from two tall buildings (81 m and 134.3 m) within the domain for two wind directions were measured, both isolated and embedded in the urban canopy. Large changes in mean flow, turbulence statistics and instantaneous flow structure of the wake were evident when tall buildings were part of the complex urban canopy rather than isolated. In the near-wake, the presence of lower buildings displaced the core of the recirculation zone upwards, thereby reducing the vertical depth over which flow reversal occurs. This amplifies vertical shear at the rooftop and enhances turbulent momentum exchange. In the near part of the main wake, lateral velocity fluctuations and hence turbulence kinetic energy were reduced compared to the isolated building case as eddies generated in the urban canopy and roughness sublayer distribute energy down to smaller scales that dissipate more rapidly. Evaluation of a wake model for flow past isolated buildings suggests that model refinements are needed to account for such flow-structure changes in tall-building canopies.

For the ongoing Doppler lidar work, continuous measurements of vertical wind profiles have shown strong shear in the wake of local tall buildings, relaxing to a power law profile above. Stability plays a role in modifying the wake characteristics, and the night-time boundary layer is occasionally of a similar depth to the taller study building. Results from horizontal velocity scans through building wakes (Velocity Azimuth Display) will be shown, testing whether stability causes significant changes to wake structure and characteristics.