A flow duct is provided in which water is caused to flow by way of a delivery device disposed in a circulation line. In order to be able to adapt the flow inside the duct to the particular requirements, and/or achieve a laminar flow over the entire flow region, a plurality of circulation lines is provided, each having a controllable delivery device, wherein the circulation lines lead into the flow duct separately from each other, and wherein the discharge openings of the circulation lines are distributed over the face wall of the flow duct.