Turbulence and dispersion below and above the interface of the internal and the external boundary layers

Abstract

This study has looked at the development of the internal boundary layer (IBL) over a block array close to a sharp change in surface roughness and its effect on dispersion from a ground level source for ratios of the downstream distance to the roughness length of less than 300. This was done by comparing a Large--Eddy Simulation (LES) with inflow boundary conditions against a LES with inlet--outlet periodic boundary conditions and data from a wind tunnel experiment. The comparison showed that using inflow boundary conditions resulted in significantly better predictions of normal Reynolds stresses, mean concentration and scalar variance than when using periodic boundary conditions. In addition to established methods, an alternative approach based on the vertical Reynolds stress was used to evaluate the depth of the IBL as it developed over the array which enabled the location of the interface to be more clearly defined. It was confirmed that the IBL growth rate close to the change in surface roughness could be described by a power law profile, similar to the widely used power law formula with an exponent 0.8 for a ratio of the downstream distance to the roughness length greater than 1000. An analysis of mean concentration and turbulent scalar fluxes suggested that the presence of the IBL constrained the vertical development of the plume from a ground level source and so led to trapping of material in the canopy layer.

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ORCID for Zheng-Tong Xie: orcid.org/0000-0002-8119-7532

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