The findings, published in Physical Review Fluids, show that when a dolphin flaps its tail up and down, the motion pushes water backward and produces a turbulent flow containing swirling currents across a wide range of scales. The team applied numerical simulations capable of decomposing this complex flow to determine which components contribute most to forward thrust.
"Our goal is to understand which parts of the turbulent flow help dolphins swim so quickly," said lead author Yutaro Motoori. "Using a supercomputer, we can simulate and decompose the flow to determine which components play dominant roles."
The simulations demonstrated that the tail's oscillating motion creates strong large-scale vortex rings that drive water rearward and generate the majority of the propulsive force. These large vortices subsequently spawn smaller ones through a process known as the energy cascade. Despite their abundance, the smaller vortices contribute minimally to forward motion.
"Our results show that the hierarchy of vortices in turbulence is crucial for understanding dolphin swimming," explained 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."
A key advantage of the computational approach was its flexibility. The team ran multiple trials across different swimming speeds and found that the dominance of large-scale vortex rings in generating thrust held consistent across all conditions tested. This robustness strengthens confidence that the identified mechanism is a fundamental feature of dolphin locomotion rather than an artifact of specific speed regimes.
The level of fluid-dynamic detail achieved in the simulations would be extremely difficult to obtain through physical experiments alone, making the numerical approach particularly valuable for studying animal propulsion. The method provides a framework for dissecting turbulent flows in other biological and engineering contexts as well.
Beyond biology, the researchers expect the findings to inform practical applications. A clearer understanding of how oscillating surfaces generate thrust through vortex hierarchies could guide the design of faster and more energy-efficient underwater robots and inspire new strategies for controlling turbulence in engineered systems.
Research Report:Swimming mechanism of a dolphin on the basis of the hierarchy of vortices
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