CONCERNING THE EFFECTS OF MEAN SHEAR ON THE FINE-SCALE STRUCTURE OF CONCENTRATION FLUCTUATIONS IN A PLUME DISPERSING IN THE ATMOSPHERIC SURFACE-LAYER

The effect of mean shear (large-eddy motion) on the small-scale plume concentration structure is investigated experimentally using high-reso lution plume concentration fluctuation measurements made simultaneousl y at two different heights in a plume dispersing in the very high-Reyn olds number turbulent flow of the atmospheric surface layer (where the Taylor microscale based Reynolds number Re-lambda approximate to 5200 ). The generalized dimension spectra of a one-dimensional surrogate of the concentration variance dissipation rate are presented as a functi on of the mean shear. It is shown that the mean shear (due to the larg e-eddy motions in the flow) have a persistent effect on the small-scal e plume concentration structure even at very high Reynolds numbers. In particular, it is demonstrated that increasing mean shear tends to re duce small-scale spatial intermittency in the concentration fluctuatio ns (viz., the higher the mean shear, the more space-filling and less s ingular the associated local dissipation rate of concentration varianc e becomes).