The flow of suspensions through tubes: VIII. Radial Migration of Particles in Pulsatile Flow

Abstract

The radial migration of rigid spheres, rods, and discs and deformable fluid drops was studied in oscillatory and pulsatile flow over a range of the alpha parameter from 2 to 20 and at particle to tube diameter ratios ranging from 0.04 to 0.]1. Rigid spheres and cylinders exhibited the tubular pinch effect with a single equilibrium radial position when α < 5 which was displaced away from the wall with increasing particle size and toward the wall with increasing frequency of oscillation. At α > 5 more than one equilibrium position was observed. The inward migration rates from the wall were maximum intermediate between the initial and final positions and greater than the outward migration rates. Increasing the amplitude of oscillation increased the migration rate but did not affect the equilibrium position. An empirical relation between migration velocity, tube Reynolds number, particle size, and radial position similar to that previously obtained in steady flow was found. During migration, rigid discs and rods, unless initially very close to the wall, drifted into orbits corresponding to maximum energy dissipation in steady flow. Deformable fluid drops, however, always migrated inward in agreement with theory and when of low viscosity, at much faster rates than rigid spheres of comparable size and at the same α. The migration velocities of viscous drops were smaller owing to a time lag between the oscillating velocity gradient and the particle deformation.