Numerical simulation study on intermittent swimming characteristics of octopus based on the eight-arm undulation mechanism

The swimming mechanism of octopuses has been a hot topic in the field of biology in recent years. Existing research primarily focuses on the influence of morphological parameters on octopus swimming ability; the impact of kinematic parameters remains relatively unexplored. This study focuses on the swimming posture and movement characteristics of octopuses, formulating a synchronized flexible undulation equation for their eight arms. Based on this, computational fluid dynamics combined with dynamic mesh technology is employed to numerically simulate their propulsion mechanism and swimming characteristics. Furthermore, by varying the hover coast time ratio and duty cycle, the influence of different kinematic parameters on propulsion efficiency is explored (close phase after hover coast time ratio is defined as DSC; open phase after hover coast time ratio is defined as DSO, and the duty cycle is defined as DC). The results indicate that the periodic opening and closing motion of the octopus leads to the alternating generation and dissipation of reverse vortices in the wake field, with the jet effect between them being the primary source of propulsion. When DSC = 0.30, the forward speed of the octopus after 1 s is only 6% lower than that without hover coast behavior. When DSO = 0.30, the peak thrust coefficient of the octopus reaches 0.82, which is 15% higher than that without hover coast behavior, indicating the highest burst acceleration. On the other hand, when DC = 0.67, under asymmetric periodic swimming with fast-closing-slow-opening, the average thrust coefficient reaches 0.26, an increase of 23.8% compared to uniform opening-closing periodic swimming, while the average lateral force coefficient decreases by 66.7%. In this scenario, the octopus can achieve better forward propulsion and maintain stable movement. The findings of this study provide valuable insights for research on the intermittent swimming behavior of octopuses and underwater vehicles.