THE UNSTEADY MOTION OF SOLID BODIES IN CREEPING FLOWS

In treating unsteady particle motions in creeping flows, a quasi-stead y approximation is often used, which assumes that the particle's motio n is so slow that it is composed of a series of steady states. In each of these states, the fluid is in a steady Stokes flow and the total f orce and torque on the particle are zero. This paper examines the vali dity of the quasi-steady method. For simple cases of sedimenting spher es, previous work has shown that neglecting the unsteady forces causes a cumulative error in the trajectory of the spheres. Here we will stu dy the unsteady motion of solid bodies in several more-complex flows: the rotation of an ellipsoid in a simple shear flow, the sedimentation of two elliptic cylinders and four circular cylinders in a quiescent fluid and the motion of an elliptic cylinder in a Poiseuille flow in a two-dimensional channel. The motion of the fluid is obtained by direc t numerical simulation and the motion of the particles is determined b y solving their equations of motion with solid inertia taken into acco unt. Solutions with the unsteady inertia of the fluid included or negl ected are compared with the quasi-steady solutions. For some flows, th e effects of the solid inertia and the unsteady inertia of the fluid a re important quantitatively but not qualitatively. In other cases, the character of the particles' motion is changed. In particular, the uns teady effects tend to suppress the periodic oscillations generated by the quasi-steady approximation. Thus, the results of quasi-steady calc ulations are never uniformly valid and can be completely misleading. T he conditions under which the unsteady effects at small Reynolds numbe rs are important are explored and the implications for modelling of su spension flows are addressed.