Application of a Numerical Optimization Technique to the Design of Cavitating Propellers in Nonuniform Flow

Abstract

High-speed propulsor blades often experience moderate to substantial amounts of unsteady cavitation, and up to now have been designed via design methods for noncavitating blades combined with methods for the analysis of cavitating flows in a trial-and-error manner. In this paper, a numerical nonlinear optimization algorithm is developed for the automated, systematic design of cavitating blades. The method is first applied to the design of propeller blades in uniform flow. The blade mean camber surface is defined via a cubic B-spline polygon net in order to facilitate the handling of the geometry, and to reduce the number of the design parameters. Noncavitating blade geometries designed by the present method are directly compared with those designed via an existing lifting-line/lifting-surface design approach. Finally, the optimization algorithm is applied to the design of cavitating blades in nonuniform flow. The objective of the design is to obtain maximum propeller efficiency for given conditions by allowing controlled amounts of sheet cavitation. Several constraints on the unsteady cavity characteristics, such as the area of cavity planform and the amplitudes of the cavity volume velocity harmonics, are incorporated in the optimization technique. The effect of the constraints on the efficiency of the propeller design is demonstrated with various test cases.