Particle dispersion in variable density and viscosity shear flows

Abstract

The dispersion of small dense particles by an unsteady planar shear layer formed between two streams of different velocity, density, and viscosity is investigated numerically. The two-phase flow is assumed to be in the dilute regime. The Lagrangian transport element method is employed to provide two-dimensional unaveraged simulations of the carrier flow. The evolution of the particle field is captured by computing the trajectories of individual particles using a reduced form of the equation of particle motion. The analysis focuses on the impact of the densities and viscosities of the two streams on the dispersion of particles of different Stokes numbers (St). The results verify the well established behavior between St and dispersion: maximized dispersion for intermediate St due to centrifugation of particles by the vortices into the irrotational streams, reduced dispersion at low St with particles following the flow and minimal dispersion for large St with particles remaining unaffected by the flow. Variation of the viscosities of the irrotational streams does not substantially alter the above trends except for a shift in the value of St where different behaviors are experienced. This shift is linked to the existence of an effective viscosity that controls the dispersion process. This viscosity is the one characterizing the well mixed vortical structures. Variations in the densities of the two streams decreases the dispersion of intermediate St particles as compared to the uniform density case. This behavior is due to the impact of baroclinic vorticity generation that creates asymmetries in the overall vorticity field that diminish the centrifugation of the particles into one of the two streams.