Concentration fluctuations and fluxes in plumes from point sources in a turbulent boundary layer

Abstract

Measurements have been made of concentration fluctuations and turbulent fluxes for two passive plumes from an elevated and a ground-level source in a turbulent boundary layer. For the concentration fluctuations, results are presented for the variance, the intermittency, peak values of concentration, probability-density functions and spectra. The balance of terms in the variance transport equation is examined, as is the overall level of fluctuations along the plume. It is shown that most of the production of fluctuations occurs very near the source. Then, the level of fluctuation decays, roughly in accordance with a balance between advection and dissipation. For the turbulent fluxes of concentration, results are presented for the vertical and lateral fluxes, with the associated behaviour of the vertical and lateral eddy diffusivities. The balance of terms in the transport equations for the fluxes is examined. The essential differences between vertical diffusion from ground-level and elevated sources and between near-field and far-field behaviour are shown to be due to the relative importance of the advection and diffusion terms in these equations.

Structure of turbulent plumes from a momentumless source in a smooth bed

“…The dispersion of ground level and elevated scalar releases has also been studied in wind tunnels over a rough bed (6,7). A summary of the findings from these studies is given in Rahman and Webster (5).…”

The role of advection of fluxes in modelling dispersion in convective boundary layers

“…As pointed out by Fackrell and Robins (1982), the advection terms and the diffusion terms are quite large close to the source (for a neutral boundary layer up to three times the boundary height). They also found that the two terms are almost equal but with opposite sign.…”

“…According to measurements by Fackrell and Robins (1982) in a neutral boundary layer these terms are as important as other terms close to the source (three times the boundary-layer height). Therefore, in the present model the terms are retained and the consequence of neglecting any of the terms will be discussed in the sensitivity tests.…”

“…In that version all tendency equations for the second-order moments were included, but with the horizontal advection terms and the horizontal transport terms neglected. Close to the source-up to three times the boundary-layer height with a neutral boundary layer-these terms have been shown to be as important as other terms in the equations (Fackrell and Robins 1982). In Enger (1990b) these terms were neglected by supposing that the horizontal advection and horizontal dispersion balanced each other.…”

Wind tunnel dispersion modelling some recent and not so recent achievements

“…This example [9] is perhaps one of the best with which to illustrate how wind tunnel experiments can stimulate modelling developments. The research arose because of a need to model short duration emissions from nuclear power plant-this prompted development of the experimental techniques [7] and triggered the basic research, undertaken both to provide a reference framework and because of its inherent interest.…”