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We study the dispersion characteristics of slightly buoyant droplets in aturbulent jet using large eddy simulations (LES). The droplet number densityfields are represented using an Eulerian approach with the dispersed phasemodelled using the Fast-Eulerian method (Ferry & Balachandar 2001) thatincludes the droplet rise velocity. Radial concentration profiles and turbulentconcentration fluxes for droplets of different sizes are analyzed to quantifythe "trajectory crossing effect", when relative motions between particles andturbulent eddies tend to reduce turbulent diffusion. For finer LES gridresolutions, the model captures the differential, size based dispersioncharacteristics of the droplets with the transverse dispersion of the largerdroplet sizes suppressed, since trajectory crossing effects are explicitlyresolved in LES. We examine a similarity solution model for the size dependentradial concentration profiles based on a modified Schmidt number derived fromthe theory of turbulent diffusion of particles in the atmosphere proposed byCsanady 1963. The results are validated with the high resolution LES data andshow good agreement. Then, the size dependent Schmidt number model isreformulated as a model for unresolved subgrid-scale trajectory crossingeffects and used to calculate the subgrid concentration flux in a coarse LES ofa turbulent jet, with slightly buoyant droplets injected at the centerline inthe self-similar region of the jet. The results are compared to a simulationwith higher grid resolution and a coarse simulation with a constant Schmidtnumber SGS model. We find that the subgrid model enhances the predictionaccuracy of the concentration profiles and turbulent concentration flux for thecoarse LES.

Resolved and subgrid-scale crossing trajectory effects in Eulerian large eddy simulations of polydisperse droplet transport