THE DUCTED TIP - A HYDROFOIL TIP GEOMETRY WITH SUPERIOR CAVITATION PERFORMANCE

A novel hydrofoil design, consisting of a small diameter flow-through duct affixed to the tip, has been studied The tip vortex cavitation in ception index, sigma(i), of this hydrofoil geometry is about a factor of 2 lower than that of a conventional rounded hydrofoil tip. This inc eption improvement comes with little associated performance penalty. F or angles of attack greater than 8 deg the noncavitating lift-drag rat io is actually superior to that of an unducted hydrofoil of equal span , although with lower wing loadings the hydrofoil performance is dimin ished 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 distribu ted over a line and circle. Distributing the vorticity, in this fashio n causes the trailing vortex to roll up less tightly, and hence have a higher core pressure and lower ai, than a conventional hydrofoil tip. It is also suspected that the interaction at the microscale level bet ween the flow through the duct, and the flow around it, makes the vort ex core size larger, and therefore sigma(i) smaller The ducted tip des ign has many potential marine applications, including to ship and subm arine propellers, submarine control fins, and ship rudders.