Numerical investigation of cephalopod-inspired locomotion with intermittent bursts

Abstract

Inspired by recent studies about the fluid dynamics of cephalopods in their escaping swimming mode, we propose a novel design of an underwater propulsion system using a deformable body with pressure chamber, which propels itself in burst-coast cycles through a combined effect of pulsed jet and added-mass related thrust. To investigate the performance of this system we create a freeswimming computational model-the body deformation is prescribed yet the forward motion is driven by hydrodynamic forces. Our focus is on a single bursting cycle, which corresponds to the case that the system rests between bursts. The results can also be applied to the starting stage of a continuous cruising motion. A numerical model using the boundary element method is developed to computationally study the swimming process and the dynamic characteristics of this system. The results show that in the bursting phase its peak speed depends on the size of the body, the deformation time, the amount of volume change during the deformation, and the size of the nozzle where the jet flow is generated. The optimal speed is found to coincide with the critical formation number, indicating that the formation of vortex rings in the wake plays a pivotal role in the dynamics of the system. The dynamics of the system in the coasting phase and the process of refilling the pressure chamber are also studied.

Cephalopod-inspired robot capable of cyclic jet propulsion through shape change

“…Pulsed jets can be 50% more efficient than a steady jet due to the additional entrainment of the surrounding fluid and higher pressures at the leading edge of the vortex ring [27][28][29]. Another key component of cephalopod-inspired jet propulsion is the added mass effect [16,30], in which a deformable body achieves an additional acceleration by reducing its overall volume (and thus its effective mass) during swimming. Lastly, to maximize propulsive efficiency, the jetted fluid needs to be entirely rolled up into a vortex ring, which occurs at an optimal ratio of the length of the jet to the diameter of the vortex ring (L/D), which is also referred to as the stroke ratio [31].…”

“…An important parameter in jet propulsion is the stroke ratio, a dimensionless parameter that is determined by the volume change of the robot and the diameter of the nozzle and is associated with vortex structure in the jet [30,31,33,36]. For our robot, the stroke ratio can be calculated as F = 4(V f − V i )/πD 3 , where V f and V i are the final and initial volumes of the body and D is the inner diameter (ID) of the nozzle [30].…”

Jet propulsion of a squid-inspired swimmer in the presence of background flow

“…Besides, they suggested the limiting vortex was "optimal" in achieving maximum impulse for given energy input. Indeed, recent studies of squid-inspired jet propulsion showed optimal propulsive performance was reached near the critical stroke ratio (Bi and Zhu, 2018;Christianson et al, 2020).…”

“…Based on the above findings, the vortex ring formation was then taken into consideration in a few studies of cephalopod-inspired jet propulsion. For example, a novel cephalopod-inspired jet propulsion system was proposed by Bi and Zhu (2018). They focused on a single bursting cycle and found the optimal speed coincides with the critical stroke ratio.…”

Pulsed-jet propulsion of a squid-inspired swimmer at high Reynolds number

“…Following these studies, a series of numerical simulations have been conducted by Bi and Zhu (2018, 2019a, 2019b to understand the combined effect of jetting and body deformation.…”