  The velocity relaxation of an impulsively forced spherical particle in a
fluid confined by two parallel plane walls is studied using a direct numerical
simulation approach. During the relaxation process, the momentum of the
particle is transmitted in the ambient fluid by viscous diffusion and sound
wave propagation, and the fluid flow accompanied by each mechanism has a
different character and affects the particle motion differently. Because of the
bounding walls, viscous diffusion is hampered, and the accompanying shear flow
is gradually diminished. However, the sound wave is repeatedly reflected and
spreads diffusely. As a result, the particle motion is governed by the sound
wave and backtracks differently in a bulk fluid. The time when the backtracking
of the particle occurs changes non-monotonically with respect to the
compressibility factor and is minimized at the characteristic compressibility
factor. This factor depends on the wall spacing, and the dependence is
different at small and large wall spacing regions based on the different
mechanisms causing the backtracking.
