Highly efficient jellyfish jet propulsion

It may seem as though jellyfish navigate awkwardly through the water compared to the elegant glide of other, more streamlined aquatic animals. Jellyfish, squids, and octopuses propel themselves by opening a shell to fill a cavity with water. When they close the shell, they eject the water backward, which thrusts the body forward. Though sometimes choppy, jet propulsion is highly efficient, accelerates well, and is largely silent.

To harness this mechanism for specially engineered, bio-inspired aquatic and aerial devices, Kang et al. modeled jellyfish-like motion. By investigating the wake dynamics, cruising speed, propulsive efficiency, and cost of transport, the authors found the most efficient open-close combination.

In their model, jellyfish move in the horizontal direction. The authors determined the frequency of motion and measured the resulting propulsion and wake characteristics.

“We use the vortex dynamics theory to explain our model,” said author An-Kang Gao. “The repulsive force induced by the strain-rate field between the body and the previous vortex pair is the main driving force of the jellyfish-like motion. Capturing the previous vortex pair during the closing phase can significantly enhance the strain rate as well as the thrust.”

The authors observed that a fast-close-and-slow-open rate obtained the highest efficiency and best stability. They believe this method succeeds because it produces a symmetric wake which complements the recapture mechanism of the wake vortex.

“This study provides inspiration for the design and control of flexible jet propulsion devices like two-caudal wing robotic fish and jellyfish-like flying machines,” said Gao.

In the future, the team will expand the model into three dimensions and examine whether different, non-uniform stiffness distributions can further improve propulsion.

Source: “Propulsive performance and vortex dynamics of jellyfish-like propulsion with burst-and-coast strategy,” by Linlin Kang, An-Kang Gao, Fei Han, Weicheng Cui, and Xi-Yun Lu, Physics of Fluids (2023). The article can be accessed at https://doi.org/10.1063/5.0160878 .