Turbulent dispersion from an elevated line source: measurements of wind-concentration moments and budgets

Abstract

Wind and tracer-concentration fluctuations, and hence the budgets for tracer variance, vertical flux and streamwise flux, have been measured in the dispersing plume from an elevated lateral line source in an equilibrium turbulent surface layer, using heat as a passive tracer. The results are analysed by testing closure assumptions for models of turbulent dispersion at first and second order. Except close to the source, a first-order (gradient-diffusion) model satisfactorily predicts both the vertical and streamwise tracer fluxes.The tracer-variance budget is essentially a balance between advection and dissipation, with production becoming significant as fetch increases. The vertical and streamwise heat-flux budgets have advection and turbulent-transport terms which are in balance (almost exactly for the vertical flux, only approximately for the streamwise flux), leaving balances between local production and pressure-gradient interaction. The turbulence-interaction component of the pressure term cannot be modelled as $-\overline{u^{\prime}_{i}\theta^{\prime}}/\tau, \overline{u^{\prime}_{i}\theta^{\prime}}$ being the flux vector and τ a scalar timescale.

Improved Mathematical and Numerical Modelling of Dispersion of a Solute from a Continuous Source

“…We compare our numerical results for the steady problem (i.e., (2a-2b) but with the time derivative omitted) with experimental results of [9]. A heat source was treated as the passive tracers and the wind flow maintained in such a way that the flow yielded an approximate logarithmic velocity profile u * = u * log(z * /z * 0 )/κ with roughness height z * 0 = 0.12 mm and κ = 0.38.…”

“…Measurements of the concentration are available from [9] at four down-stream locations: x = 0.2778, 0.8333, 1.6667, and 3.3333. These are shown, left to right, as circles in Figure 3 below.…”

“…This is because we wish to compare our numerical results with experimental data. Since a reliable and well-tested set is available in the literature [9] for the steady case, we have opted to restrict our attention to that. We note that the issues concerning transformation from the semi-infinite domain to a finite one, and resolution of the interior layer, still feature.…”

“…One such, widely cited study is [9]. However, those experiments are for a situation that is not as general as the model presented here allows; it corresponds to the steady case.…”

Quality Measures of Mixing in Turbulent Flow and Effects of Molecular Diffusivity

“…A cloud of particles or scalar markers resulting from an instantaneous release from a point or a line is called a puff (for example, consider a puff of smoke that comes out of the exhaust pipe of a car or a puff of smoke exhaled by a smoker), while a cloud of particles that results from a continuous release of particles is called a plume [13,14]. The behavior of a puff is the most elementary dispersion in a turbulent flow field, and can be used to investigate and understand quite fundamental mechanisms of turbulent dispersion [15][16][17], even though it is not easy to study experimentally, where the study of plumes is easier [18][19][20][21][22][23][24][25].…”