Experimental investigation on settling behavior and inter-particle interaction of dual cubic particles released side-by-side in quiescent fluids

This study experimentally investigates the settling behavior and interparticle interactions of dual cubic particles released side-by-side in quiescent fluids at intermediate Reynolds numbers (Re = 20–400). An electromagnetic release device was used for precise initial conditions, and particle tracking velocimetry and particle image velocimetry were employed to measure particle trajectories, velocities, and flow fields. The effects of initial spacing (l0* = 0–5), Reynolds number, and particle orientation (surface, edge, and corner modes) were analyzed. Results show that the settling process can be divided into three stages, with distinct repulsion characteristics: negligible interaction (Stage 1), strong repulsion dominated by rotational motion (Stage 2), and oscillatory trajectories induced by vortex shedding (Stage 3). Cubic particles exhibit orientation-dependent rotation that enhances lateral separation, with stronger repulsion at high Re. The ratio of the terminal settling velocity of dual cubic particles to that of a single cubic particle (Ud/Us) is generally less than 1 due to the transfer of energy to horizontal motion caused by interparticle interaction. The smaller the initial spacing, the stronger the repulsion and the smaller the Ud/Us value. For Re < 50, Ud/Us increases slightly with Re. For Re > 50, Ud/Us decreases rapidly with Re. Flow field and mechanistic analyses attribute repulsion to pressure differentials between high-pressure inter-particle zones and low-pressure outer regions, while rotational instabilities stem from shear gradients and angular flow separation. These findings provide foundational insights into mesoscale interactions of non-spherical particles, with implications for sediment transport, aerosol dynamics, and industrial granular flows.