Turbulent dispersion of a passive scalar in a smooth-wall turbulent boundary layer

Abstract

The scalar dispersion of a ground-level point-source plume in a smooth-wall turbulent boundary layer is experimentally investigated using simultaneous particle image velocimetry and planar laser induced fluorescence techniques. In the near-source region, the viscous sublayer is observed to trap dye, while in the far-field, the half-width, vertical profiles and peak decay of the mean concentration and concentration variance exhibit self similar behaviour and collapse with empirical relations. Full two-dimensional maps of the turbulent scalar fluxes show a net transport direction of upward and towards the incoming flow, with the vertical profiles collapsing well with Weibull-type exponential functions and the decay of peaks following power laws. Using the first-order gradient transport to model the turbulent scalar fluxes, maps of the anisotropic turbulent diffusivity tensor and an effective turbulent diffusivity coefficient are calculated. The streamwise and wall-normal turbulent scalar fluxes are driven dominantly by the wall-normal concentration gradient. The turbulent Schmidt number, relating the turbulent diffusivity and the turbulent (eddy) viscosity calculated using the Boussinesq’s hypothesis, varies with wall-normal position with values on the order of unity in the logarithmic layer.

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ORCID for Christina Vanderwel: orcid.org/0000-0002-5114-8377

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