Particle dispersion in a double-diffusive turbulent layer

Abstract

We simulated the flow in a double diffusive turbulent layer using a periodic two-dimensional square computational domain. The conditions of the simulation correspond to those typically found in oceans (Pr = 7 and Sc = 1000) and results are in good agreement with the literature. When the flow was in statistically steady state we tracked the paths of particles with different inertia assuming that the concentration of particles is small and their mutual interaction and their effect on the carrying fluid can be neglected (one-way coupling). Numerical experiments were carried out to reveal the effect of the different terms of the particle force balance and the particle inertia in the preferential concentration of the particles. The different numerically obtained particle concentration distributions were analyzed using the cluster index or lacunarity, the tortuosity and the Voronoï analysis. The departure of the instantaneous particle distributions from random distributions using different box sizes for the calculation of the lacunarity of the distributions show maxima at box sizes corresponding to the Kolmogorov length scale. The novel application of the concept of tortuosity to the analysis of the instantaneous particle distributions reveals the non-isotropic character of the particle distributions. The Voronoï analysis indicates larger areas of the voids and clusters for the simulations considering only the drag force in comparison with simulations considering all the terms of the particle force balance. These observations are consequence of the role of the stress gradient term which generates high particle concentrations along the boundaries of the ascending and descending concentration plumes and relatively low particle concentrations in the tail and head of the plumes.