A cleaning system (200) for a combine harvester (20) includes a fan (42), at least one duct (220) having an interior passageway that is positioned to receive an air stream from the fan, and a surface (230, 232, 302, 402, 504) defined within the interior passageway of the at least one duct that is configured to restrict a volume of the flow of air passing through the at least one duct and increase a pressure of the air stream passing through the duct.
