Force Production Mechanisms of a Heaving and Pitching Foil Operating in the Energy Harvesting Regime

The influence of vortex dynamics on the force production of a heaving and pitching foil operating in the energy harvesting regime is studied experimentally using 2C-PIV. Results are obtained for reduced frequencies in the range of k = fc/U = 0.06 to 0.14. The flow induced vertical force-time history during cyclic operation is evaluated from PIV data by using the impulse-based derivative moment transformation method. The contribution of each term in the equation is investigated. The results show that the leading edge vortex has the largest contribution to the total force, whereas the trailing edge vortex is shown to contribute negatively. In addition, the dynamics of the leading edge vortex are further analyzed by measuring the circulation and trajectory relative to the foil. It is shown that the force is not only dependent on the circulation magnitude, but also on the LEV foil-normal and chord-wise trajectories. The force-time history for all reduced frequencies exhibit two main distinct peaks. The primary peak is generated when the leading edge vortex forms. The secondary peak, on the other hand, is formed when the chord-wise convection of the leading edge vortex increases, as well as when the LEV is closer to the foil surface during the reversal of pitching motion.