Behaviour of macroscopic rigid spheres in lid-driven cavity flow

Experiments are conducted to investigate the behaviour of macroscopic rigid particles suspended in a fully three-dimensional viscous flow. The flow considered takes place in a closed cubic cavity, steadily driven along its upper face by a translating lid. Navier–Stokes computations are first performed to characterize the fluid flow, and the resulting kinematic template is checked using laser-illuminated micro-particles. Nearly neutrally buoyant rigid spheres are then inserted in the cavity, and their three-dimensional motions are tracked using stereoscopic imaging. The measured macro-particle motions are compared with those of simulated passive tracers, and their responses to changes in experimental conditions are examined. Although steric effects are observed to hinder passage through narrow throats of the flow field, macro-particle trajectories are otherwise found to align closely with passive tracer paths. The macro-particle orbits, however, are not evenly distributed within the cavity, and cluster closer to the periphery as the Reynolds and Stokes numbers increase. With support from observations of particle rotations relative to the fluid, we interpret this behaviour as resulting from weak forces pulling the macroscopic spheres towards preferential paths, similar to the Segré–Silberberg effect in Poiseuille flow.