<p>Many swimming fishes can be considered as underwater bio vehicles with high hydrodynamic efficiency. Depending on their propulsion mechanisms they can use flapping tails and/or other fins to generate thrust, which seems to be connected with generation of a reverse von Kármán wake. It is essential to investigate vortices in the wake for a better understanding of this form of propulsion with the objective of creating innovative solutions for greener and efficient marine transportation. The Strouhal number, defined as the product of the flapping frequency and characteristic wake width divided by the advancing speed U, i.e. Sr=fA/U, is employed to characterize the features of fish generated wake and so the fish thrust. Laboratory and theoretical studies suggest the optimal propulsive performance incurred by a flapping foil is within a narrow interval 0.2&lt;Sr&lt;0.4, which is consistent with the range adopted by many aquatic animals regardless of the sizes and species. In the present work, the flow past a 2D flapping foil is simulated by solving incompressible Navier-Stokes equations in the open-source OpenFOAM platform to analyze the transition of vorticity types in the wake as well as drag-thrust transition. Overset grid method is used herein to allow the foil to move without restrictions. The comparison between numerical results and available experimental data shows that this methodology can be applied to realize large amplitude motions with adequate efficiency and accuracy. The transitions in the types of wake are consistent with the benchmark experimental data, while the drag-thrust transition of the pitching foil does not coincide with von Kármán-reverse von Kármán wake transition, indicating that more attention should be paid in this discrepancy region to ensure the gain of propulsive forces.</p>

2D Numerical Study on Wake Scenarios for a Flapping Foil

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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