Air rings blown by dolphins and smoke rings from jet engines are examples of vortex rings, structures that have been studied for their role in propulsion and biological locomotion. Researchers at New York University and NYU Shanghai have discovered a new property of these rings, shedding light on how they behave when moving through water to air.
The team found that when a vortex ring travels sideways and upward through water to the surface, it can rebound while largely maintaining its shape, similar to a tennis ball bouncing off a wall. This reflection results in only a small loss of energy. However, if the ring moves directly upward, it breaks apart instead of bouncing.
This discovery helps understand the power and limits of vortex rings in underwater propulsion and oceanographic phenomena like underwater volcanic eruptions. “Since Hermann von Helmholtz first mathematically analyzed vortex rings in the mid-1800s, scientists have sought to unravel the mysteries of their properties,” says Jun Zhang, professor at New York University and NYU Shanghai.
To study this behavior at the water-air interface, researchers conducted experiments using a piston submerged in a tank to generate vortex rings with different strengths and paths. The movement was traced with fluorescent dye and recorded by high-speed cameras.
The outcomes varied: some dissipated or rebounded while maintaining shape; others broke apart or transformed into jets depending on strength and direction.
