Student project

Turbulent flow in and over an urban canopy in Vancouver, Canada

In this Master’s thesis, turbulent flow of the atmosphere over the urban canopy of the ‘Sunset’ neighbourhood in Vancouver was studied using a Large Eddy Simulation (LES) model. A high precision Light Detection and Ranging (LiDAR) dataset was employed to represent the urban building geometry that served as a boundary condition and to implement the form drag force of trees as a function of the height-dependent Leaf Area Density (LAD) in this LES model. Eddy flux measurements of the Urban Climate Research Tower of UBC located in the study domain were sampled for wind direction and neutral atmospheric conditions in order to validate the modelling results. The simulations were carried out on a subset of 512 × 512 m with flat topography, regular building patterns and an areal fraction of trees of λv = 0.12. Modelling resolution was 2 m and model runs were carried out once without trees (only buildings) and once with trees. Velocity spectra from conditionally sampled tower measurements and from modelling results both show typical slopes of the inertial subrange (-5/3) and the production range (-1) as well as a slope change at the same frequency, which validates the modelling results. Time correlations of the longitudinal velocity u for varying lag and height show that characteristic length and time scales are larger when trees are present than if there are only buildings. Wind speeds for the simulation with trees are slightly lower than for the simulation without trees, while more small scale Turbulent Kinetic Energy (TKE) is generated in the Roughness Sublayer for the simulation with trees. The aerodynamic roughness length z0 of the modelling subset was determined to be in the range of 0.36 - 0.40 m for a hypothetical simulation without trees, and in the range of 0.42 - 0.46 m for a simulation taking into account the pressure drag on trees. The displacement height d was estimated to be d = 3.25 m. A blending height hb for the Roughness Sublayer (RSL) of hb = 11.24 m was determined for the modelling subset, taking into account the drag force exerted by trees.

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