Reduction of unsteady underwater propeller forces via active tail articulation

Abstract

This study investigates the use of biologically-inspired tail articulation as a means to reduce unsteady propeller forces and by extension, noise due to stator wake blade interaction. This study is experimental in nature and testing was completed in a closed-channel water tunnel at the Naval Undersea Warfare Center in Newport, RI. A propeller-force measurement apparatus was designed and built to measure the forces and moments created by a spinning propeller behind a life scale stator model. Tests were conducted at a Reynolds number of 75,000 and stator tail articulation was carried out in the range of Strouhal number 0.0 < St < 0.13. A variety of non-lifting propellers were used to investigate sinusoidal articulation profiles in the range of am- plitudes (2°,5°,10°), and phase angles between propeller blades and stator (0°- 360°). It was found that stator articulation is capable of reducing the RMS of both unsteady thrust force and its time derivative as compared with a baseline static stator wake by choosing a suitable Strouhal number and phase angle. Tail articulation at St < 0.08 showed reduced unsteady forces for certain phase angles, while other phase angles demonstrated unsteady forces greater than the baseline wake.

(cont.) Articulation at St > 0.08 also showed unsteady forces that varied with phase but the associated unsteady forces were greater than the baseline wake for all phase angles. Similar results were obtained from spectral analysis where blade rate harmonics showed decreased magnitudes for certain phase angles at St < 0.08. A reduced order wake model was used to calculate the relative position of wake vortices and propeller blades which was used in turn to visualize the effect of phase angle on propeller blade-wake interaction.