Impact of wall-normal external force on particle collision dynamics in turbulent channel flow

Abstract

Particle collision in turbulent channel flow is systematically investigated by a direct numerical simulation coupled with Lagrangian particle tracking. For particles with low inertia (characterized by a viscous Stokes number St⁺≤5), the external force slightly reduces the particle clustering, while it promotes the radial relative velocity (RRV). The competition between these two mechanisms results in the collision kernel of low-inertia particles being largely unaffected by the wall-normal external force. For heavy particles with St⁺≥15, a significant reduction in the collision kernel was observed when subjected to the external force. This inhibitory effect increased with the magnitude of the external force ψ and the particle inertia. The radial distribution functions (RDF) of heavy particles display a monotonic decrease with the external force, as the force weakens the interaction between particles and the coherent flow structures. Notably, the RRV of heavy particles exhibits a novel increasing-decreasing trend with the external force. By analyzing the flow structures at the location of colliding particles, we demonstrate that the external force causes faster particles to enter ejection structures, where they collide with slower particles, resulting in an elevation of the relative colliding velocity. Finally, we quantify the contributions of RDF and RRV to the variation of the collision kernel as the external force increased for light and heavy particles.