Mechanisms influencing the efficiency of aquatic locomotion

The optimal aquatic locomotion has previously been associated with a narrow St(= fA/u) number range of 0.2–0.4. We present how animals tune their Strouhal (St) number to this range to reveal the mechanisms influencing efficiency. The self-propelled swimming of a 2D swimmer is simulated using an immersed boundary method. The locomotion kinematics is controlled by two variables, [Formula: see text] and frequency f. We show that only when animals constrain their [Formula: see text] = 0.125–0.25, their St number can fall into the optimal St range. When [Formula: see text] Hz, the St number is independent with frequency. Although different combinations of f and [Formula: see text] can achieve a same cruising velocity, high-f and low-[Formula: see text] motions are more efficient. This can be linked to its larger lateral velocity component in the proto-vortex region and the transition of the tail vortices into small eddies.