How motion trajectory affects the energy extraction performance of an oscillating foil?

The energy extraction from oscillating plunging/pitching hydrofoils with sinusoidal or non-sinusoidal pitching motions is investigated numerically. The study encompasses the Strouhal number St from 0.05 to 0.35 and the maximum effective angle of attack (AOA) αmax from 10 to 20 degree. Two types of non-sinusoidal pitching profiles are imposed to examine the effect of motion trajectory on the energy extraction performance of oscillating foils. The first type is derived from a cosine effective AOA and the second type is the trapezoidal-like pitching motion with several adjustable parameters. Computation was conducted by solving unsteady Navier-Stokes equations for laminar flows. For both the sinusoidal and non-sinusoidal pitching, a larger αmax results in a higher extraction power (Cp ) and total efficiency ( ηT ). For the pitching motion derived from the cosine effective AOA, the time-mean Cp and ηT is much beyond those for the sinusoidal one at relatively high St. With the trapezoidal-like motions, the significant increase of Cp and ηT can be observed over a certain range of St depending on the adjustable parameters. The study shows that an optimum trapezoidal-like profile may increase the output power coefficient and total output efficiency as high as 50% over a wide range of St. Copyright © 2010 by the authors.