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Virtually all animals swim unsteadily. They oscillate appendages, undulate, and produce periodic propulsive forces so that the velocity of some part of their bodies changes in time. Because of their unsteady motion, animals experience a fluid force in addition to drag—the acceleration reaction. The acceleration reaction dominates the forces resisting rapid accelerations of animals and may be responsible for generating thrust in oscillating appendages and undulating bodies. The ever-present unsteady nature of animal swimming implies diverse applications of the acceleration reaction.
Thomas L. Daniel (Wed,) studied this question.