Thrust production of rigid hydrofoils in an in-line configuration

Abstract

Presented here are the results of experiments aimed at determining the performance of two quasi-two-dimensional hydrofoils arranged in an in-line configuration. The net thrust, power, and efficiency produced by each of two hydrofoils undergoing prescribed pitching motions while in such a configuration were measured directly. The phase differential between the trailing edge kinematics of the two pitching foils as well as the spacing between the foils were varied, and the performance under each condition was measured. These measurements were compared to those of a single, solitary foil. It was found that under certain spacing and phase conditions, the downstream foil can produce up to 1.6 times the thrust of a single solitary foil, and, under other conditions, produces less than half the thrust of the solitary foil. The upstream foil, however, was found to consistently produce about the same thrust as the solitary foil. Because the downstream foil performance changes significantly while the upstream foil performs similarly to a solitary foil, the downstream foil performance dictates the net performance of the tandem foil system when the two foils are relatively close together. Particle image velocimetry was conducted to investigate wake interaction associated with foil performance. It was found that destructive interaction between a vortex created at the downstream foil’s leading edge and the one forming at its trailing edge is associated with peaks in performance, and constructive interaction is associated with troughs. These vortex interaction modes were associated with distinct time-averaged flow fields.