Dispersion within and above an array pf blocks in a spatially developing boundary layer

Abstract eng:
Flow over an array of cuboids mounted on a wall provides an excellent test case for validation of Large-Eddy Simulation (LES). A fully developed rural boundary layer approaching a neighbourhoodscale urban region is simulated by using Large-Eddy Simulation and the Inflow Boundary Condition (IBC) technique [5]. One particular urban-like geometry is considered: an array (29h x 12h) of 48 aligned blocks of equal height, where h is defined as the cuboid’s height of 70 mm. The results are first validated by comparing mean velocities and the Reynolds stress tensor against the available wind tunnel measurements (EnFlo, University of Surrey). Subsequently, the analysis is centred on the discrepancies (mainly above the canopy) observed in LES simulations based on using periodic boundary conditions at the inlet and outlet [2]. This comparison is found to be useful to understand the interaction of the locally generated inner boundary layer and the upstream boundary layer. By using the method proposed by Antonia & Luxton [1] we found that the edge of the internal layer is evident. Lastly, inflow turbulence effects on dispersion from a ground-level point source are investigated. Mean concentration of the scalar is measured horizontally and vertically, downstream of the source's location, and compared with wind-tunnel data. Results show that the IBC method accurately predicts point-source dispersion. The current study represents a further step towards confirming that LES can provide accurate numerical predictions of dispersion in urban boundary layers.

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