UC San Diego

Inspired by the locomotion method of cephalopods such as squids, we propose a novel concept of underwater propeller that utilizes pulsed jet for thrust generation. A squid-inspired robot is expected to possess multiple advantages such as mechanical simplicity, high swimming speed, and low environmental footprint. To understand the physical mechanisms of squid-like jet propulsion, computational simulations are conducted to explore the underlying fluid dynamics and fluid-structure interaction problems.

A two-dimensional fluid-structure numerical model is firstly developed by usingthe Immersed Boundary Method (IBM), which avoids the complexity of body-fitted grid regeneration, and is thus suitable for problems involving large body deformations. Through systematic simulations we demonstrate that the 2D squid-inspired swimmer is capable of long-distance swimming through cyclic deflation-inflation shape change. Through parametric studies, it is found that the body oscillation frequency is the most important parameter determining the hydrodynamics of the swimmer.

The 2D IBM-based model is then extended to an axi-symmetric numerical rendition.Based on control volume analysis, a thrust-drag decoupling strategy and a thrust decomposition method are proposed. In the thrust decomposition method the jet-related thrust is calculated as the summation of three components, the jet flux force, the exit normal stress and the flow momentum force inside the chamber. This method allows us to understand the underlying physics of force generation, e.g. the effects of jet speed profile, jet acceleration, background flow and nozzle geometry. Moreover, it enables the separation of thrust and drag forces on the body (a classical problem in free-swimming bodies) so that it leads to a novel method to calculate the propulsive efficiency.

Finally, a potential-flow-based rendition of a 3D squid-inspired propulsion systemis developed to explore the swimming process and the dynamic characteristics. 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. 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.