The effect of surface flexibility at the leading and trailing edge on the flow dynamics in the near wake region of a heaving foil is investigated using 2D particle image velocimetry measurements. The experiments were conducted in a closed loop wind tunnel at a Reynolds number of 25,000 and at a range of reduced frequencies from 0.09 to 0.2. Swirling strength analysis and velocity triple decomposition are used to quantify the vortical structures and their influence on momentum entrainment, respectively. The results show at large reduced frequencies, flexibility at the leading edge generates a significantly different wake structure when compared to the flexible trailing edge and rigid foils, by allowing a large scale vortex to form and shed from the leading edge. The swirl strength is shown to decrease with increasing reduced frequency due to vortex compression from the three dimensional effects. In addition, it is observed that the dominant mechanism of momentum transfer is associated with the leading edge vortex.
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