  Oscillating foils in synchronized pitch/heave motions can be used to harvest
hydrokinetic energy. By understanding the wake structure and its correlation
with the foil kinematics, predictive models for how foils can operate in array
configurations can be developed. To establish a relationship between foil
kinematics and wake characteristics, a wide range of kinematics is explored in
a two-foil tandem configuration with inter-foil spacing from 4-9 chord lengths
separation and multiple inter-foil phases. Using data from experiments and
simulations, an in-depth wake analysis is performed and the mean velocity and
the turbulent kinetic energy are quantified in the wake. With this energy
quantification, the trailing foil efficiency is modified to account for the
mean flow in addition to the energy transported by the coherent leading edge
vortices (LEVs) shed from the leading foil. With the mean wake velocity, a
predictive wake model is able to distinguish three regimes through analyzing
trailing foil efficiency profiles and the strength of the primary LEV shed from
the leading foil. Dividing the wake into regimes is an insightful way to narrow
the range of foil kinematics and configurations and improve the energy
harvesting in a two-tandem foil-array.
