Stokes number and coupling effects on particle interaction behavior in turbulent channel flows

Abstract

The effects of Reynolds number (Reτ=180 and 300), particle Stokes number (St+=0.5, 50, and 92), and fluid–solid phase coupling level (one-way, two-way, and four-way) on particle behavior in turbulent channel flows has been investigated using direct numerical simulation and Lagrangian particle tracking. Previous studies have used all these levels of coupling, but in terms of those employing four-way coupling, no consideration is given as to how emergent phenomena due to collision dynamics within a flow affect the way in which particles impart feedback to the continuous phase. In the present work, we relate the particle–particle interaction to particle–fluid coupling, as well as in assessing its relation to the Stokes number. As the Reynolds number increases and the turbulent region narrows, fewer particles retain their velocity as they migrate to the wall-region leading to reduced streamwise velocity fluctuations and preferential concentration. It is also evident that low Stokes number particles are capable of minor wall-accumulation at Reτ=300. At this increased Reynolds number, four-way coupled simulations performed with moderate Stokes number particles (St+=50) are shown to diminish the effects of particle–fluid feedback, leading to similar fluid and particle statistics as the one-way coupled simulations. It is concluded that turbophoretic and preferential concentration effects are responsible for this phenomenon, since the increased collision rates due to larger concentrations of particles and velocity fluctuations in the wall-region correlate directly with the impact on the two-way coupling flow modifications. Analysis of the collision dynamics also indicates particles colliding with increased relative velocities and angles, which cause larger momentum transfer and directional redistribution, increasing and redirecting slip velocities. It is concluded that for midrange Stokes numbers, four-way coupling is imperative to increase simulation accuracy beyond that obtained assuming one-way coupling.