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Flow characteristics of translating flexible wings at low reynolds numbers

Yazdanpanah, Mahdi
This study experimentally investigates the flow field around surging-translating wings that are started from rest and compares the flow field characteristics with that of surging-revolving wings. Three wings with different level of chordwise flexural stiffness (i.e., highly flexible, moderately flexible and rigid) were studied. The experiments were performed in an octagonal water tank at the Reynold number of 7360 based on the terminal velocity of 0.08 m/s, and the wing chord length of 92 mm. Two-dimensional two-component particle image velocimetry (2D2C PIV) technique was employed to obtain the planar flow fields at the 75% of wingspan position. The PIV measurements reveal a coherent leading edge vortex at the initial phases of the translating motion for all of the wings considered in the study. After approximately two chord lengths of travel, the LEV bursts in the rigid and moderately flexible wings, whereas for the highly flexible wing the LEV preserves its coherency for a longer period of motion. The comparison of flow fields between the translating and the revolving motion kinematics reveals similar behavior of the vortical structures yet the LEV circulation in the translating wings has higher values. The LEV centroid in the revolving cases stays above the leading-edge, while in the translating wings, it always remains at a lower position. The effect of high flexibility results in the retention of LEV closer to the wing surface for both translating and revolving cases.