Hydrodynamics study on a traveling wave-based undulating surface of a hydrofoil in a free-stream flow

A fluid-structure interaction study on free-stream flow across a NACA0012 hydrofoil, with a traveling wave-based surface undulation, is numerically studied at a constant wave number K=10 of the traveling wave and phase difference of 180o between the top and bottom surface undulations. Effect of phase speed c∗ (1-10) of the wave, local amplitude-thickness ratio AL (0.05-0.25), and Reynolds number Re on the near-wake flow characteristics and propulsive performance are investigated. For the various values of the nondimensional governing parameters, the present results revealed two types of vortex patterns: an almost steady vortex sheet and periodic vortex street. Further, forward and reverse types of both vortex sheets and vortex streets are found behind the hydrofoil. A novel two-pair of reverse vortex streets and reverse vortex sheets are found at larger values of c∗, AL, and Re. Surface undulations cause a high-pressure and high-velocity zone near the troughs of the wave. Consequently, a pressure-suction mechanism in the leeward and forward side of each wavy section produces a spatially varying thrust. The spatial variation of the thrust is attributed to the variation of wave amplitude along the foil. A threshold value of c∗, Re, and AL exists to produce thrust. A scaling analysis is presented with a power-law scaling for the mean thrust coefficient (CT) as a function of K, c∗, AL, with 3, 2, and 3 as the respective exponents. The proposed scaling is found to be consistent with the computations. The present biomimetic fish-inspired study can lead to design and development of a need-based autonomous underwater vehicle or energy harvesting device.