Salp-like swimmers inflate, deflate to jet along

Soft-body robots are promising tools for underwater exploration, environmental monitoring, and other aquatic operations. However, their movement is limited in comparison to traditional robots because they cannot impart large forces on the surrounding fluid. Jet propulsion via body deformation may provide a viable solution.

Bi et al. numerically modeled the fluid dynamics of a swimmer based on salps — clear, barrel-shaped marine invertebrates. The creatures expand to suck water through the oral siphon into a pressure chamber, then shrink to push water out through the atrial siphon and form a pulsed jet. In the model, the inlet and outlet are represented by valves.

In existing prototypes of bio-inspired robots with jet propulsion, the water inlet and outlet are located at the same end of the body, wasting the kinetic energy of the refilling flow. Instead, the salp design features an inlet and outlet at opposite ends, which is expected to reduce energy waste and enhance performance.

In terms of speed and efficiency, the salp-like swimmer outperforms existing designs with inlets and outlets on the same end.

“The locomotion method is feasible. We are just duplicating what nature has been doing for hundreds of millions of years,” said author Qiang Zhu. “And if properly designed, the valves can be passively activated by the flow field so that there is no need for control or actuation systems. This can greatly simplify the design.”

The team plans to conduct more numerical studies to explore the underlying physics and optimize the swimmer’s motion. They are also interested in developing a mechanical prototype using this propulsion.

Source: “Valve-mediated flow control in salp-like locomotion,” by Xiaobo Bi, Hui Tang, and Qiang Zhu, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0096004 .