Wake Characteristics of Tall Buildings in a Realistic Urban Canopy

Abstract

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. We explore the aerodynamic effects of tall buildings on the microscale to local scales with a focus on the interaction between the wake structure, canopy and roughness sublayer flow of the surroundings in a realistic urban setting in central London. Flow experiments in a boundary-layer wind tunnel use a 1:200 scale model with two tall buildings (81 m and 134.3 m) for two wind directions. Large changes in mean flow, turbulence statistics and instantaneous flow structure of the wake are evident when tall buildings are part of the complex urban canopy rather than isolated. In the near-wake, the presence of lower buildings displaces 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 are 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 model refinements are needed to account for such flow-structure changes in tall-building canopies. Keywords Tall-building environments • Urban canopy • Urban flow • Wake model • Wind tunnel Nomenclature H Building (roof) height H NoTall Mean building height (No Tall) H ave Mean building height Electronic supplementary material The online version of this article (

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“…To ensure the instruments are above the roughness sub‐layer (RSL), within the inertial sub‐layer or constant flux layer, the instruments are mounted at 140 m (IAP) and 36 m (MY) above ground level. Individual tall buildings have a large influence and increase the depth of the RSL (Millward‐Hopkins et al, ; Kanda et al, ; Kent et al, ; Hertwig et al, ). Analyses suggests that the urban canopy layer at the IAP site is approximately 47–63 m thick (Peng and Hu, ) with turbulent variances at 47, 140 and 280 m levels confirming 47 m is within the RSL, while 140 and 280 m are within constant flux layer (Zou et al, ).…”

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“…Point-wise velocity measurements 41,42 obtained using a two-component laser Doppler anemometry system, 43 are analysed. In each experiment, neutral atmospheric stability conditions (i.e.…”

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