Numerical Analysis of the Effect of the Non-Sinusoidal Trajectories on the Propulsive Performance of a Bionic Hydrofoil

Aquatic animals usually generate the effective propulsive force via non-sinusoidally flapping their fins. Inspired by the kinematics of fish, the propulsive characteristics of a NACA012 hydrofoil is numerically studied in this paper. The combination of non-sinusoidal heaving and pitching motions is adopted in the twodimensional hydrofoil kinematics parameters. The elliptic function and the flattening parameter S are introduced to achieve the varieties of non-sinusoidal periodic motions. The numerical model is established by using the commercial computational fluid dynamic solver STAR-CCM+, and the code is verified by comparing with the published experimental results. The Reynolds number is fixed at 40,000 in all the numerical simulations. The results show that the non-sinusoidal trajectories affect the propulsive performance by affecting the angle of attack (AOA), the hydrodynamics of the foil and the flow structure behind the foil. The non-sinusoidal flapping trajectories can improve significantly the thrust coefficient at the same kinematics parameters compared with the sinusoidal motions in most cases. However, they may reduce the propulsive efficiency. When the values of S are greater than 1, the improvement of thrust coefficient acquired with the non-sinusoidal motions is more obvious. The wake pattern is also discussed which indicates that the strong leading-edge vortices results in the decrease of the propulsive efficiency acquired by the non sinusoidal trajectories. It is possible to apply the non-sinusoidal motions of a flapping foil to improve propulsive performance of the underwater bionic machine.