The Ducted Tip—A Hydrofoil Tip Geometry With Superior Cavitation Performance

Abstract

A novel hydrofoil design, consisting of a small diameter flow-through duct affixed to the tip, has been studied. The tip vortex cavitation inception index, σi, of this hydrofoil geometry is about a factor of 2 lower than that of a conventional rounded hydrofoil tip. This inception improvement comes with little associated performance penalty. For angles of attack greater than 8 deg the noncavitating lift-drag ratio is actually superior to that of an unducted hydrofoil of equal span, although with lower wing loadings the hydrofoil performance is diminished by application of the ducted tip. The ducted tip is effective at reducing the tip vortex inception index because, in contrast with the rounded tip, for which vorticity in the Trefftz plane is confined to a line, the ducted tip shed vorticity at the trailing edge is distributed over a line and circle. Distributing the vorticity in this fashion causes the trailing vortex to roll up less tightly, and hence have a higher core pressure and lower σi, than a conventional hydrofoil tip. It is also suspected that the interaction at the microscale level between the flow through the duct, and the flow around it, makes the vortex core size larger, and therefore σi smaller. The ducted tip design has many potential marine applications, including to ship and submarine propellers, submarine control fins, and ship rudders.