GOLDMAN, J.A.: The effect of the free water surface on animals that jump out of water

As an animal such as a dolphin, fish, or penguin jumps out of water, it passes through a complex hydrodynamic regime where the viscous, inertial, and gravitational forces act on its body in an uncertain way. I explored two lines of evidence that suggest that an animal’s overall resistance increases as it approaches the free water surface to perform a jump: (1) an animal’s resistance to horizontal motion increases with decreasing depth in the region near the free surface, and (2) an animal jumping out of water creates waves and sprays, which results in ancillary resistance. In addition, I directly tested the hypothesis that an animal’s resistance increases as it approaches the free surface at an angle by recording the time-varying force on model animals as I towed them at constant speeds through the free surface. Results of horizontal towing experiments do not support the first line of evidence. Force coefficients of spheres, disks, and streamlined bodies of revolution towed horizontally below the free surface did not vary with depth. Videotapes of model animals emerging vertically from water and photographs of actual animals jumping out of water, however, did show surface waves and sprays upon emergence. Results of experiments in which towed models passed through the free surface revealed that hydrodynamic resistance remained constant on approach to the surface and decreased to a negligible level during the transition from water to air. The results, which were robust over a range of shapes, speeds, and angles, suggest that the free water surface does not alter an animal’s resistance when swimming near or jumping through it. The waves and spray are likely due to the inertia of the water carried along with the submerged animals and not an additional resistance generating mechanism.