Luke Edward
M U S C U T T
Luke Edward
M U S C U T T
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.
The experimental data at a spacing of three chord lengths show thrust increases of 40%, which is understandably lower than the simulations due to 3D and higher Reynolds number effects. Comparable increases in efficiency are also observed.
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.
Flow visualisation reveals that the hind flipper has high performance (high thrust and efficiency) when it weaves in between the vortices that are shed from the fore flipper...
High performance: hind flipper weaves between vortices
... but low performance when it intercepts these vortices.
Low performance: hind flipper intercepts vortices
To summarise, the four-flipper system of plesiosaurs enabled them to generate substantially higher thrust and efficiency, giving them a crucial evolutionary advantage. Not only does this research provide the first quantitative data on the hydrodynamics of plesiosaurs, but it also advances our knowledge of the fundamental mechanisms of foil/wake interactions, whilst aiding the design and development of underwater vehicles and energy extraction systems that use tandem flapping flippers.