Abstract: A system for controlling fluid turbulence inside and just outside of a cay adjacent to an opening in a wall, the wall having a forward lip which defines the leading edge of the opening and an aft lip having a leading edge which defines the trailing edge of the opening, the forward lip terminating in a sharp edge lying generally in the plane of the outer surface of the wall. The aft lip has a length of 10 to 50% of the length of the opening and extends into the opening at an angle of 30 to 60 degrees to terminate in an arcuate portion having a radius of 2 to 6% of the length of the opening.