Optimal Kinematics for Energy Harvesting Using Favorable Wake-Foil Interactions in Tandem Oscillating Hydrofoils

Abstract

The energy harvesting performance of a pair of oscillating hydrofoil turbines in tandem configuration is experimentally studied to determine the optimal kinematics of the array. By characterizing interactions between the wake produced by the leading foil and the trailing foil, the kinematic configuration required to maximize array power extraction is determined. This is done by prescribing leading foil kinematics that produce specific wake regimes, identified by the maximum effective angle of attack parameter. The kinematics of the trailing foil are allowed to vary significantly from those of the leading foil. The heave and pitch amplitude, inter-foil phase, and foil separation of the trailing foil are varied within each wake regime and the system performance is evaluated. The power extracted by each foil over an oscillation cycle is measured through force and torque measurements. Wake-foil interactions that yield improvements in trailing foil performance are analyzed with time-resolved Particle Image Velocimetry. Constructive and destructive wake-foil interactions are compared, and it was determined that trailing foil performance could be improved by either avoiding interactions with wake vortices or by interacting directly with them. The latter configuration takes advantage of the wake vortex, and does not see power loss during the oscillation cycle. System power from the two foils is maximized when the leading foil is operated at an intermediate maximum angle of attack range, and when the trailing foil avoids collisions with wake vortices. This optimal array configuration sees both foils operating with different kinematics compared to the optimal kinematics for a single oscillating foil.