An experimental investigation is conducted to study the leading edge vortex (LEV) evolution of a simultaneously heaving and pitching foil operating in the energy harvesting regime. Two dimensional particle image velocimetry measurements are collected in a wind tunnel at reduced frequencies of k = fc/U = 0.05–0.20. Vorticity flux analysis is performed to calculate the constant C in the vortex formation number equation proposed by J. O. Dabiri [1], and it is shown that for a flapping foil operating in the energy harvesting regime, this constant is approximately equal to 1.33. We demonstrate that the optimal LEV formation number (T̂max ≈ 4) is achieved at k = 0.11, which is well within the range of optimal reduced frequency for energy harvesting applications (k = 0.1–0.15). This suggests that the flow energy extraction is closely related to the efficient evolution process of the LEV.

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