Meeting Abstract
The ability to accelerate quickly is essential for survival, as it underlies critical behaviours such as prey capture and predator evasion. Far from being an extension of steady swimming, whereby the tailbeat (tb) amplitude is approximately constant at 0.2 body length (L) and swimming speed is modulated primarily by increasing tb frequency, we argue here that acceleration is a distinct type of locomotion that has distinct optimization parameters. We suggest that this phenomenon is generalized to all fishes, given that we observed similar acceleration kinematics in more than 50 species of fishes with vastly different body shapes, swimming modes and ecological habitats. We found during acceleration tb amplitude is 25% higher than during steady swimming. To uncover how elevated tb amplitude relates to thrust production and propulsive efficiency, we used a combination of flow visualization experiments on live fish and experiments with soft-bodied robotic models. Results from particle image velocimetry show that rainbow trout (Oncorhynchus mykiss, L = 22.4±2.0 cm) can reach a maximum acceleration rate of 15 L s-2 from initial swimming speed of 3 L s-1 by generating thrust 4 times higher than that required to maintain the initial speed. To do this, fish entrain more fluid around the body anterior to the tail, and exploit these fluids with synchronized tail movements to enhance vortex shedding by 100%. Complementary experiments with a 3D-printed robotic fish model show that increasing tb amplitude during acceleration can increase propulsive efficiency up to 50%. Our results reveal that optimum tb amplitude values for acceleration and steady swimming do not overlap, and fishes adopt acceleration kinematics that are tuned for high propulsive efficiency.
