Wake-foil interactions and energy harvesting efficiency in tandem oscillating foils

Oscillating foils in synchronized pitch/heave motions can be used to harvest\nhydrokinetic energy. By understanding the wake structure and its correlation\nwith the foil kinematics, predictive models for how foils can operate in array\nconfigurations can be developed. To establish a relationship between foil\nkinematics and wake characteristics, a wide range of kinematics is explored in\na two-foil tandem configuration with interfoil spacing from four to nine chord\nlengths separation and multiple interfoil phases. Using data from experiments\nand simulations, an in-depth wake analysis is performed and the mean velocity\nand the turbulent kinetic energy are quantified in the wake. With this energy\nquantification, the trailing foil efficiency is modified to account for the\nmean flow in addition to the energy transported by the coherent leading edge\nvortices (LEVs) shed from the leading foil. With the mean wake velocity, a\npredictive wake model is able to distinguish three regimes through analyzing\ntrailing foil efficiency profiles and the strength of the primary LEV shed from\nthe leading foil. Dividing the wake into regimes is an insightful way to narrow\nthe range of foil kinematics and configurations and improve the energy\nharvesting in a two-tandem foil array.\n