Dynamics of semi- and neutrally-buoyant particles in thermally stratified turbulent channel flow

Stably-stratified flows are characterised by the presence of internal gravity waves (IGWs), which constitute a barrier for mass, heat, momentum and species vertical transport. In this paper, the dynamics of semi- and neutrally-buoyant particles in three-dimensional, stably-stratified turbulent channel flow is investigated, using direct numerical simulations (DNS), one-way coupled with a Lagrangian particle tracking (LPT) routine. The fluid is characterised by a shear Reynolds number Reτ=1000 and a shear Richardson number Riτ=200, such that the turbulence is sustained only in the near-wall region; whereas IGWs are observed at the core of the channel. Four sets of particles with a Stokes number St=0.044 or 0.0044 and a particle-to-fluid density ratio ρr=1.0 or 0.8, are released into the system from eleven wall-parallel planes. The particle number for each set is one million. A qualitative analysis of the particle trajectories confirms that particles released in the near-wall region are significantly influenced by the near-wall vortices and show chaotic pathlines. In contrast, particles released at the channel core exhibit smooth oscillating movements which are determined by IGWs. These different qualitative behaviours correspond to different evolution of the particle swarms: swarms of particles released at the channel core are characterised by a narrow distribution of accelerations, a slower-growing pair dispersion and an almost stationary vertical position of the swarm centre of mass compared to those released in the near-wall region.