The Experimental Findings

Results and flow physics from tandem flipper propulsion experiments.

Results

Data from simulations show that the thrust of the hind flippers can be up to twice as much as a single flipper. This performance augmentation depends on the phase and spacing between the flippers.

Simulation result figure 1
Simulation result figure 2

Experimental data at a spacing of three chord lengths show thrust increases of around 40%, which is understandably lower than the simulations due to three-dimensional and higher Reynolds number effects. Comparable increases in efficiency are also observed.

Forget the Matrix: Meet the Vortex

A vortex is a region of rotating fluid. Picture water going down a plug hole and you are seeing a vortex in operation. Flapping wings create alternating vortices, which produces thrust. A second wing behind the first wing is able to extract energy from vortices produced by the first wing, and can create thrust more efficiently than a single wing. This increase in efficiency may have given plesiosaurs an evolutionary advantage.

Vortex illustration 1
Vortex illustration 2

Flow visualisation reveals that the hind flipper has high performance (high thrust and efficiency) when it weaves between the vortices shed from the fore flipper.

High performance flow visualisation
High performance: hind flipper weaves between vortices

Low performance is observed when the hind flipper intercepts these vortices.

Low performance flow visualisation
Low performance: hind flipper intercepts vortices

Research Summary

To summarise, the four-flipper system of plesiosaurs enabled substantially higher thrust and efficiency, giving a potential evolutionary advantage. This research provides the first quantitative data on the hydrodynamics of plesiosaurs, advances understanding of fundamental foil–wake interaction mechanisms, and supports the design and development of underwater vehicles and energy extraction systems that use tandem flapping foils.