Hydrofoil Drag Reduction by Partial Cavitation

Partial cavitation reduces hydrofoil friction, but a drag penalty associated with unsteady cavity dynamics usually occurs. With the aid of inviscid theory a design procedure is developed to suppress cavity oscillations. It is demonstrated that it is possible to suppress these oscillations in some range of lift coefficient and cavitation number. A candidate hydrofoil, denoted as OK-2003, was designed by modification of the suction side of a conventional NACA-0015 hydrofoil to provide stable drag reduction by partial cavitation. Validation of the design concept with water tunnel experiments has shown that the partial cavitation on the suction side of the hydrofoil OK-2003 does lead to drag reduction and a significant increase in the lift to drag ratio within a certain range of cavitation number and within a three-degree range of angle of attack. Within this operating regime, fluctuations of lift and drag decrease down to levels inherent to cavitation-free flow. The favorable characteristics of the OK-2003 are compared with the characteristics of the NACA-0015 under cavitating conditions.