Direct numerical investigations of dilute dispersed flows in homogeneous turbulence

Abstract

The motivation for the present work is to investigate particle-laden turbulent flows using accurate numerical simulations. In the present work, the carrier phase is modeled using direct numerical simulations (DNS) and the particles are tracked in a Lagrangian sense. Investigations of both one-way and two-way coupled particulate flows in homogeneous isotropic turbulence have been carried out. The phenomenon of interest in one-way coupled simulations is preferential accumulation, which refers to the tendency of heavy particles in isotropic turbulence to collect in regions of high strain and low vorticity. Several measures and mechanisms of accumulation have been reported in the literature often showing conflicting scaling with particle and fluid parameters. In the present study, accumulation has been quantified using several indicators to give a unified picture. The present work addresses the scaling of preferential accumulation with Reynolds number and suggests that while the spacing between particle clusters does exhibit a dependence on Reynolds number, the structure of particle clusters as viewed by individual particles shows little dependence on Reynolds number. The effect of adding a gravitational settling force on the particles has also been explored. While the gravity force tends to homogenize the particle distribution at low Stokes numbers, at high Stokes numbers it tends to arrange the originally random distribution into streaks in the direction of gravity. The ability of the Lorentz force to limit preferential accumulation has been the focus of the next part of the study. Charges are placed on particles to produce an electric field when the particles are inhomogeneously distributed. The electric field and thereby the Lorentz force tend to homogenize the particle distribution. It is interesting to note that the particle distribution attains a stationary state determined by the total amount of charge contained in the domain. It is demonstrated that in the presence of gravity, less amount of charge is required to homogenise particle distribution. Good agreement is observed for simulations of settling charged particles with experimental work. The modification of carrier phase turbulence by particles is studied formono-sized particles. The non-uniform modification of the fluid energy spectrum by particles has been demonstrated. It is seen that there is an increase in energy at high wave numbers for microparticles (St k < 1), whereas for high Stokes number particles, energy is damped at all scales. The effect of incorporating two way coupling on particle distribution has also been reported. It is noted that increasing mass loading leads to attenuation of accumulation at low Stokes numbers while the effect is reversed at higher Stokes numbers.

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