An interesting aspect of particle-laden flows is preferential migration of particles to certain regions within the fluid flow. This is often characterized by the Stokes number (St) of the particles. At low St, particles tend to act as tracers and are uniformly distributed. At high St, particles are heavy and are influenced less by the fluid and more by its inertia. At intermediate St, particles are affected by both the fluid motion and its inertia, which gives rise to several interesting behaviors. This is especially true in wall-bounded flows where there is a velocity gradient near the wall.
One of the earliest works describing preferential migration is the experimental work of Segre and Silberberg. They showed that a neutrally buoyant particle in a laminar pipe flow comes to an equilibrium position between the wall and the axis. This is referred to as the Segré–Silberberg effect. Saffman explained this in terms of the force acting on the particle when it experiences a velocity gradient across it. Feng et al. have studied this through detailed direct numerical simulations and have elaborated on the physical mechanism of this migration.
Recently it was found that even for non-neutrally buoyant particles similar preferential migration occurs
. At low St, the particles tend to settle at an equilibrium position while for high St, the particles begin to oscillate about the center of the channel.
The behavior becomes even interesting in turbulent flows. Here, the turbophoretic force (transport of particles down gradients of turbulent kinetic energy) causes a high concentration of particles near the walls. Experimental and particle-resolved DNS studies have explained the mechanism of this migration in terms of the Saffman lift and the turbophoretic force
. These preferential migration are of significant importance to several applications where wall-bounded particle-laden flows are encountered and is an active area of research.