LAUSR

Abstract We study the dynamics of dilute, slightly negatively buoyant, millimetre-size spherical particles fully suspended in a smooth-wall open channel flow. The Reynolds number is $Re_{\tau } = 570$ and the particle Stokes number is ${{\textit {St}}}^{+} = 15$. Particle image velocimetry and tracking are used to obtain simultaneous, time-resolved flow fields and particle trajectories. Particles travel at a lower mean velocity than the fluid: in the log layer this is due to the oversampling of slow fluid regions, but closer to the wall the cause is instantaneous slip between particles and fluid. The particle Reynolds stresses exceed those of the fluid. Near the wall, the particle streamwise diffusivity is larger than the momentum diffusivity, while the opposite is true for the wall-normal component. The particle transport is strongly linked to ejections, while the role of sweeps is marginal, and there is no evidence of turbophoresis. The concentration profile follows a power law with a shallower slope than predicted by equilibrium theories that neglect particle inertia. Upward-/downward-moving particles display positive/negative mean streamwise acceleration due to the particle–fluid slip. The particles that contact the wall are faster than the local fluid both before reaching the wall and after leaving it. Therefore, they are decelerated by drag and pushed downward by shear-induced lift. The durations of wall contact follow exponential distributions with characteristic time scale close to the particle response time. Lift-offs coincide with particles meeting a fluid ejection. These observations emphasize the competing effects of inertia and gravity.