Order in disorder: complex motions of cuboidal particles in fluid induced by inertia reduction

Through high-fidelity numerical simulations based on the lattice Boltzmann method, we investigate the complex behaviors of cuboids with varying aspect ratios, naturally suspended in simple shear flow under typical conditions, as they move from a nonequilibrium state to an equilibrium state. Several key findings emerge from this study: (1) At lower Reynolds numbers (), both fluid and particle inertia are weak, highlighting the interactions of forces on the cuboid faces, which result in a more intricate transition process compared to higher Reynolds numbers (); (2) The transition from the initial nonequilibrium state to the equilibrium state, while complex, exhibits strong regularity in the phase spaces of cuboid orientation and angular velocities, with a notable dependence of angular velocities on the orientation angles of the cuboid's principal axis; (3) When the lengths of two principal axes of the cuboid are comparable and the orientation is near the equilibrium position associated with the third axis, the angular velocities around these two axes become coupled, leading to cyclic and progressive evolutionary trajectories with intricate and elaborate structures in both the orientation and angular velocity spaces. This work provides crucial insights into the distinctive properties of faceted particles across various fields.