Tail of a shark creates double jets by actively changing the tail's rigidity in mid swing.

"[R]esearchers have discovered that as a shark’s tail swings from side to side, it creates twice as many jets of water as other fishes’ tails, smoothing out the thrust and likely making swimming more efficient. Sharks do this by stiffening the tail midswing, a strategy that might one day be applied to underwater vehicles to improve their performance." (Pennisi 2011)


"Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation...The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns." (Flammang et al. 2011: 3670)

"As the tail crosses the midline, the radialis muscles within the tail are actively stiffening the tail against this increased hydrodynamic loading. And it is precisely at this time of maximum expected stiffness and greatest drag that the first vortex is produced (figure 3), resulting in a jet with strong lift and thrust components (table 2). The remaining vorticity is shed as the tail is cupped slightly and continues laterally until it changes direction at maximum lateral excursion" (Flammang et al. 2011: 3674)

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Journal article
Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structureProceedings of the Royal Society B: Biological SciencesApril 5, 2011
B. E. Flammang, G. V. Lauder, D. R. Troolin, T. Strand

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