Effects of Spanwise Spacing on the Interaction of Tandem Pitching Hydrofoils

Abstract

The interaction between a pair of tandem in-line oscillating hydrofoils is presented. The hydrofoils undergo sinusoidal pitching about their leading edges with a fixed Strouhal number of and a Reynolds number of 10,000. The streamwise spacing, spanwise spacing, and phase offset between the hydrofoils are varied. Force measurements are employed to investigate changes in thrust, lift, spanwise force, power consumption, and propulsive efficiency. A method to mitigate confounding factors from connecting rod drag is employed using streamlined fairings. Near and far streamwise spacing regions are identified with a transition occurring near 0.875 chord lengths downstream. Decreasing streamwise spacing in the far region causes a rise in the maximum power consumption of the follower hydrofoil. Decreasing streamwise spacing in the near region results in an opposite trend, with a sharp drop in maximum average power consumption by the follower. An empirical model for power consumption of the follower is developed. Increased spanwise spacing is found to weaken the interaction between the hydrofoils, driving them toward their isolated performance. This phenomenon is related to the spanwise contraction of the wake shed by the leader and is a function of the overlap of the wake region impacting the follower.