The relationship between appendage geometry and propeller blade unsteady forces

Abstract

Abstract : A study was undertaken to determine the relationship between appendage geometry, unsteady propeller blade forces and the incident velocity field distribution. Axial velocity data from water tunnel tests of an axisymmetric body with appendages has been used to calculate the harmonic content of the wake and the resulting distribution of unsteady thrust and torque for a given propeller geometry. The results indicate that unsteady force reduction can be obtained by modification of the flow at the appendage/body intersection, and by modification of the wake such that flow regions having substantial harmonic content lie outside the propeller's maximum diameter. A propeller operating in the turbulent wake of a torpedo or submarine encounters wake non-uniformities which result in spatial and temporal fluctuations of blade angle-of-attack. These angle-of-attack fluctuations result in unsteady blade loadings and the generation of noise. These unsteady forces may then result, for example, in the radiation of noise from the vehicle's shell and in the generation of vehicle self-noise. The sources of propulsor generated noise can be characterized by three types of unsteady force mechanisms: a) turbulence ingestion; b) vortex shedding; and c) blade-rate. The first two mechanisms typically generate continuous spectrum (broadband) radiated noise levels while blade-rate forces generate discrete frequency noise levels at various blade-passage frequencies and harmonics.