Osaka, Japan – Dolphins are famous for their speed and agility in the water, but what exactly allows them to swim so effectively? Scientists have been asking this question for years, hoping to learn how to optimize propulsion in fluids from these elegant creatures.
In an article published in Physical Review Fluids, researchers from The University of Osaka have uncovered a key part of the answer: large, powerful vortices created by the movement of the dolphin's tail. The research team used large-scale numerical simulations to visualize the dynamics of these vortices across a wide range of conditions, quantifying their effect on propulsion.
When a dolphin swims, it flaps its tail up and down in a kicking motion. This motion pushes water backward, generating a turbulent flow filled with swirling currents of many different sizes. Until now, it has been difficult to determine how these complex motions conspire to propel the dolphin forward.
"Our goal is to understand which parts of the turbulent flow help dolphins swim so quickly," says lead author Yutaro Motoori. "Using a supercomputer, we can simulate and decompose the flow to determine which components play dominant roles."
The numerical simulations revealed that the dolphin's oscillating tail produces strong large-scale vortex rings that push water backward and generate thrust. Then, these large vortices create smaller ones in a process known as the energy cascade. Although these smaller vortices are numerous, they contribute little to the dolphin's forward motion.
"Our results show that the hierarchy of vortices in turbulence is crucial for understanding dolphin swimming," explains senior author Susumu Goto. "The largest vortices are responsible for most of the propulsion, while the smaller ones are mainly by products of turbulent flow."
The team's approach allowed them to observe fluid motion in detail that would otherwise be nearly impossible to capture with real-world experiments. Furthermore, because a flexible computational method was used, running multiple trials at different conditions was straightforward.
"We find that our results are unchanged across a wide range of swimming speeds," says Motoori.
In the future, this understanding of the mechanics of propulsion could help guide the design of faster and more energy-efficient underwater robots as well as technologies for controlling turbulence. But for now, this research is proof that physics is a great way to make a splash.
