Turbulent flow over a rough wall with suction or blowing is an industrially important fluid mechanics problem. In the screening of wood pulp fibre suspensions, for example, turbulent flow is induced by a rotor adjacent to a slotted screen cylinder. To better understand the complex hydrodynamics in the critical region between the pulp screen rotor and the slotted screen wall, the stream-wise velocity and aperture velocities were measured using particle image velocimetry. The vortex generated above the aperture is shown to be strongly dependent on aperture velocity and wall roughness. The vortex diminishes in size at higher aperture velocities and increased exit layer height. The experiments also show that the reversal flow in the slot decreases with lower rotor speeds and increased mean slot velocities. This observation challenges the existing models of apertures being cleared by flow reversal driven by a Bernoulli-type suction pulse. In its place, this paper identifies elements of a more sophisticated flow model that considers the depletion of the zone below the rotor as well as the flow in the wake of the foil.