Sedimentation of Nonisotropic Particles

If the velocity of a spherical particle in Stokes settling is always codirected with the gravity force, even for homogeneous axisymmetric particles the velocity is directed vertically if and only if the vertical coincides with one of the principal axes of the translational tensor K. If the angle between the symmetry axis and the vertical is p, then the velocity direction is given by the angle [94] 

If the settling direction is not vertical, this means that a falling particle is subject to the action of a transverse force, which leads to its horizontal displacement. An additional complication is that the center of hydrodynamic reaction (including the buoyancy force) does not coincide with the particle center of mass. In this case, in addition to the translational motion, the particle is subject to rotation under the action of the arising moment of forces (e.g., the somersault of a bullet with displaced center of mass). For axisymmetric particles, this rotation stops when the system the mass center + the reaction center becomes stable, that is, the mass center is ahead of the reaction center. In this case, the settling trajectory becomes stable and rectilinear. 

However, in the more general case of an asymmetric particle, the combined action of the lateral and rotational forces may lead to motion along a spatial, for instance, spiral trajectory. At the same time, a steady-state settling trajectory with helicoidal (propeller-like) symmetry remains rectilinear, notwithstanding the body rotation [179]. 

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