Japanese researchers have discovered that the reason dolphins swim so fast is because they create whirlpools from the movement of their tails, which will open the door for scientists to design faster and more efficient aquatic robots.
A team of researchers from Osaka University in Japan attempted to unpack this mechanism at the level of fluid flows using large-scale digital simulation.
Their research, published in the journal Physical Review Fluids, revealed a key to solving that problem: intense vortices that provide the primary thrust during the movement of the dolphin's tail.
When a dolphin swims, its tail makes vertical oscillating movements, a motion reminiscent of leg kicks. The water is not only pushed backward but also begins to behave in a more complex way as spiral flow structures, or eddies of varying scales, are formed.
Until now, it has been difficult to understand how this "turbulent mixture" transforms into directed forward motion. Digital calculations have allowed researchers to analyze the flow into its components and track which ones are actually working to propel the animal forward.
Simulations conducted by scientists revealed that large-scale vortex rings play a key role. These rings form immediately after the tail flaps and effectively push water backward, creating a reactive thrust. A cascade of events then begins where these large structures break down into smaller vortices, a process known as an energy cascade. However, the contribution of these smaller vortices to the forward motion proved negligible.
"Our goal was to understand the components of the turbulent flow that help dolphins swim at this speed," explained Yutaro Mutori, one of the co-authors of the research. "Using a supercomputer, we were able to simulate the flow and break it down into components to determine which ones play a dominant role."
The study's other author, Susumu Goto, said: "Our results show that the hierarchy of vortices in turbulence is crucial to understanding dolphin swimming. Larger vortices are responsible for most of the movement, while smaller vortices are essentially byproducts of turbulent flow. In other words, the efficiency of movement is not ensured by small, chaotic vortices, but by large, organized structures that arise from the tail movement."
The model developed by the researchers allowed for monitoring fluid dynamics in a detail unattainable by real-time experiments. Furthermore, it can be easily scaled up to encompass different motion patterns, including varying swimming speeds.
"We have found that our results remain consistent across a wide range of swimming speeds," said Moturi.
This data can be used in the future during the design of underwater robots and systems where minimizing energy loss in a turbulent environment is important.
