Abstract
Estimation of force generated by dolphins has long been debated. The problem was that indirect estimates of force production for dolphins resulted in low values that could not be validated. Bubble digital particle image velocimetry (DPIV) measured hydrodynamic force production for swimming dolphins and demonstrated high force production. To validate the bubble DPIV and reconcile force production measurements, two bottlenose dolphins (Tursiops truncatus) performing tail stands were measured with bubble DPIV. Microbubbles were generated from a finely porous hose and compressed air source. Displacement of the bubbles by the propulsive motions of the dolphin was tracked with a high-speed video camera. Oscillations of the dolphin flukes generated strong vortices and a downward directed jet flow into the wake. Application of the Kutta–Joukowski theorem measuring vortex circulations yielded forces up to 997.3 N. Another video camera recorded body height above the water surface to determine the mass-force of the dolphin above the water surface. For the dolphin to hold its position above the water surface, the mass-force approximately balanced the vertical hydrodynamic force from the flukes. The results demonstrated the fluke motions generate high sustained forces roughly equal to the dolphin’s weight out of the water. Bubble DPIV validated high forces measured previously for thrust generated in swimming by animals and demonstrated a more accurate technique compared to standard aerodynamic analysis.
Highlights
Swimming by animals requires the transfer of momentum from the organism to the water
Relative to the vertical position of the eye above the surface of the water, the dolphins performed tail stands with the position of the eye at 0.74 ± 0.16 m above the surface of the water
For the six trials that were examined with bubble digital particle image velocimetry (DPIV) and aerodynamic analysis, the mean weight supported above the surface of the water was 882.2 ± 165.7 N
Summary
Swimming by animals requires the transfer of momentum from the organism to the water. The momentum imparted to the water by a propulsor is visualized in a thrust-type (reverse von Kármán street) wake with a central momentum jet, which is directed through the center of a staggered array of vortex rings [2,3,4,5,6,7]. This exchange of momentum and its measurement have been at the core of examinations of forces and energetics of swimming by animals [8]. The movement of fluid within the wake represents the physical manifestation of the momentum transfer
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