Large-Eddy Simulations of Transient Flow and Scalar Dynamics in Idealised Urban Topographies


Student thesis: Doctoral Thesis

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Award date1 Dec 2017


The evolution of the urban atmosphere depends on lateral boundary conditions, initial conditions and basic states. In the urban computational fluid dynamics (CFD) literature, however the sensitivity of the turbulence and scalar dynamics to changing boundary conditions or different initial pollutant distributions or thermal stability has been largely ignored. This thesis investigates the robustness of numerical simulations of urban flow and dispersion from three different perspectives.

First, the sensitivity of flow and ventilation to time-periodic inflow perturbations is studied in a unit-aspect-ratio street canyon. For a neutral boundary layer, the response depends non-monotonically on the perturbation frequency and is maximised for perturbation periods comparable to the timescale of the mean canyon circulation. Frequency analyses indicate that the behaviour arises from a resonance between the inflow forcing and the mean motion around closed streamlines. A linear model for the error evolution is derived. At resonance, the error kinetic energy attains its maximum and the ventilation improves by ~10%.

Second, the sensitivity of the scalar dynamics to different initial conditions is investigated. The decay rates of the mean are similar implying the ventilation rate is independent of the initial conditions. However, on account of the open boundary and inhomogeneous flow, the sensitivity of the scalar field to the initial conditions persists in the long-time limit. The decay rate of the variance indicates that the mixing is inhomogeneous and strongly influenced by the streamline geometry. Following established theoretical predictions, the mixing rates are related to the divergence of Lagrangian trajectories and effective diffusion across closed streamlines.

Third, the sensitivity of turbulent flow and ventilation to the boundary-layer stability is examined for a regular building array. With the bulk Richardson numbers between -0.51 ~ 0.20, ventilation timescale changes can be as large as ~60% with respect to the neutral condition. When the flow becomes very unstable, the roof-level shear layer is no longer preserved; however building vortices persist for z/H ≲ 1.

The findings of this thesis offer a new perspective on incorporating mesoscale aerodynamic and thermodynamic information into urban CFD models. Frequency spectra suggest that effects of perturbed flow can be modelled as a superposition on top of the control simulation. Based on the resonance mechanism, coupling efforts on incorporating the mesoscale perturbations over the entire frequency spectrum may be diverted to motions having the largest impact on the urban system. The scalar dynamics are found to be sensitive to the initial conditions and open boundaries, which differs from the behavior in closed domains. These results indicate that well-known predictions from the fluid dynamics literature may not apply exactly to urban boundary layers.