Propelling underwater vehicles using supersonic gas jets

Compared to air or space environments, jet propulsion in underwater environments faces greater performance issues due to the instability of the gas-liquid interface. Certain design improvements, such as nozzle configuration, have been employed to increase maneuverability and propulsion efficiency — in hopes that underwater vehicles can one day serve as better platforms for ocean exploration, resource surveying, and early disaster warning systems.

The high-speed multiphase flow involved in underwater propulsion is still elusive, however. One reason for this is the formation of a dense bubble layer that impedes optical observations of internal flow structure, so Si et al. constructed an experimental setup that overcomes this challenge.

“We adopted the approach of using two transparent acrylic plates to confine the flow field both inside and outside the nozzle,” said author Shipeng Li. “This allowed us to design a quasi-two-dimensional transparent experimental system, through which we could optically capture the internal flow field of the jet, and thereby reveal the underlying mechanisms that cause the jet’s highly complex and erratic behavior.”

The team characterized underwater jet flow produced by nozzles of different bevel angles qualitatively and quantitatively. This showed that the jets transform from a conical structure into a stabler conical structure but are affected by interface instability effects that shape their features. The researchers then created an equation that described the relationship between jet deflection, bevel angle, and the nozzle pressure ratio, which guides the engineering of underwater jet propulsion.

Li especially noted that their proposed quasi-2D visualization method has significant implications for fundamental research, enabling investigations of jet interface dynamics in optically opaque environments.

Source: “Flow mechanism of the underwater supersonic gas jet formed by the beveled nozzle,” by Pengfei Si, Ningfei Wang, Ge Jin, Shipeng Li, Fuqi Wang, and Muyang Feng, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0330595 .