Research on a water-treading bionic propeller with variable pitch

Abstract

Lizards living in the rainforests of the Americas can run and tread on the water surface. It was found that a lizard's feet are constantly reorienting in the water to adapt to the flow field during treading and running, thus providing greater driving forces and faster speeds than other animals with the same mass. Inspired by this, we utilized planetary gears and a crank rocker mechanism to design a water-treading paddle propeller in order to imitate the behavior of a lizard, and we also derived a mathematical model of the propeller during motion. Using FLUENT's sliding dynamic mesh technique, the water-treading and variable-pitching propeller and a conventional fixed-pitch propeller were simulated under different operating conditions. The results showed that the performance of the newly designed propeller was better than that of the conventional propeller, mainly due to the change in the blade angle of the new propeller. The front edge of the pitching propeller touched the water surface first at the moment of water entry and generated a positive thrust, while the blade's vertical exit reduced the drag. The maximum efficiency of the pitching propeller was obtained at an advance coefficient of J=1.25. Compared with the case of J=1.85, it was found that at the moment of water entry for the case of J=1.25, the large leading-edge vortex around the blade's lower surface made the thrust coefficient higher. At the moment of water exiting, a large vortex at the front of the propeller created a low-pressure area, which caused the propeller to be subject to less drag. By considering the effect of the rotation speed on the performance of the variable-pitch propeller, it was found that the efficiency at a rotational speed ω2=4πrad/s was always higher than those at ω1=0.5ω2 and ω3=2ω2, and the thrust and lift coefficients decreased with the increase in the rotation speed.