Velocity relaxation of a particle in a confined compressible fluid

TL;DR

This study investigates how a spherical particle's velocity relaxes in a confined compressible fluid, revealing the complex interplay of viscous diffusion and sound wave reflection influenced by wall spacing.

Contribution

It provides new insights into the effects of confinement and compressibility on particle relaxation dynamics using direct numerical simulations.

Findings

Backtracking time varies non-monotonically with compressibility.

Wall spacing influences the dominant relaxation mechanism.

Sound waves significantly affect particle motion in confined fluids.

Abstract

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.