Near-field wake characterization of tandem flexible flapping fins

This study characterizes the effects of passive flexibility on the wake of a tandem bio-inspired flapping fin. Soft fins made of polydimethylsiloxane (PDMS) with 10:1 and 20:1 mixing ratios, corresponding to elastic moduli (E) of 820 kPa and 380 kPa, respectively, were evaluated in comparison to a rigid fin of identical geometry made of nylon (E = 50 MPa). To investigate how material stiffness and tandem fin phasing influence wake flow structure, we measured thrust and conducted two-dimensional (2D) particle image velocimetry (PIV). Experiments were performed at three phase offsets (-112\degree, 0\degree, 67\degree) and at three wake planes behind the front fin (x/c = 2.1, 5 and 8, normalized by the chord length). The results showed a substantial effect of the fin stiffness on the wake strength and expansion rate downstream. An intermediate level of passive flexibility produced higher thrust and greater wake coherence, whereas both the rigid and highly flexible fins performed poorly in comparison. These findings indicate the existence of a 'goldilocks' range of fin flexibility and further clarify how fin stiffness and tandem phasing shape the downstream wake signature.