CFD investigations of evolution and propulsion of low speed vortex ring

Abstract

Vortex ring thruster (VRT) is a new type of propulsion device which adopts impulse propulsion mode. The idea of impulse propulsion comes from bionics. It imitates cephalopods like squid and can achieve high efficiency. Vortex ring is a typical phenomenon during the pulse process and it can help improve the propulsion efficiency. Many previous studies believe that VRT can obtain large thrust and efficiency but few articles study the efficiency problem systematically. In addition, experiment is the main research method adopted at present. In this paper, an axisymmetric piston-nozzle computational model is established to simulate the thruster. Fluid is solved by CFD method based on the open source platform OpenFOAM. Dynamic mesh technology is adopted to control piston motion. This paper focus on two aspects: vortex ring evolution and propulsion efficiency characteristics. In the first part, simulations of pulse process with different strokes are carried out and compared with standard experiments. The two results are in good agreement. The vortex ring evolution is then studied with different background flows. In the second part, a reasonable open-water characteristics calculation method for VRT is proposed. Thrust coefficient and propulsion efficiency are obtained in the calculation. The results indicate that the VRT can achieve high efficiency near to 1 in the case of high advance ratio. In addition, efficiency comparison is carried out between four kinds of asymmetric velocity programs and three kinds of symmetric velocity programs to find out a more reasonable velocity program design. Finally, the impact of piston stroke is investigated. Results show that smaller stroke can obtain larger efficiency because pulsation effect is more prominent.