Flapping foils are studied to achieve an efficient propeller. The performance of the flapping foil is influenced by many factors such as oncoming vortices, heaving amplitude, and geometrical parameters. In this paper, investigations are performed on flapping foils to assess its performance in the wake of a D-section cylinder located half a diameter in front of the foil. The effects of heaving amplitude and foil thickness are examined. The results indicate that oncoming vortices facilitate the flapping motion. Although the thrust increases with the increasing heaving amplitude, the propelling efficiency decreases with it. Moreover, increasing thickness results in higher efficiency. The highest propelling efficiency is achieved when the heaving amplitude equals ten percent of the chord length with a symmetric foil type of NACA0050 foil. When the heaving amplitude is small, the influence of the thickness tends to be more remarkable. The propelling efficiency exceeds 100% and the heaving amplitude is 10% of the chord length when the commonly used equation is adopted. This result demonstrates that the flapping motion extracts some energy from the oncoming vortices. Based on the numerical results, a new parameter, the energy transforming ratio (R_ET), is applied to explicate the energy transforming procedure. The R_ET represents that the flapping foil is driven by the engine or both the engines and the oncoming vortices with the range of R_ET being (0, Infini) and (−1, 0), respectively. With what has been discussed in this paper, the oncoming wake of the D-section cylinder benefits the flapping motion which indicates that the macro underwater vehicle performs better following a bluff body.

Recently, inspired by the flippers of humpback whales, researchers have been widely studying leading-edge tubercles for use as passive flow control devices. In this research, we numerically investigated the effects of leading-edge tubercles on a three-dimensional flapping foil coupled with rolling and pitching motions. Appropriate spanwise flexibility is considered to mimic the real flapping motion of humpback whales, and the profile of the angle of attack was analyzed in a representative section under the effects of spanwise flexibility. The motion of flexible foils was decomposed into rigid motion and flexible deflection by using the sliding mesh and dynamic mesh methods, respectively. Then, the hydrodynamic performance of the flexible flapping foils was estimated by solving the unsteady Reynolds Averaged Navier–Stokes equations. The effects of the shape and kinematic parameters on thrust, power consumption, and propulsive efficiency were studied and the mechanism behind these effects was investigated. A maximum efficiency loss of 19.4% was observed for the sharpest tubercle shape. Although the hydrodynamic advantages of leading-edge tubercles were not observed in the present study, the tendency of flow separation over peaking sections was suppressed under low angles of attacks. The results suggest that leading-edge tubercles are more suitable for foils with steady or quasi-steady motions, such as propellers or turbines.