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One Fish, Two Fish
I will tell two short stories of fish hydrodynamics. First, I will argue, based on biological observations and mathematical models, that fish that actively flap their tail in concert with passive hydrodynamic forces can simultaneously improve swimming speed and efficiency. Second, I will show that passive hydrodynamic forces between pairs of flapping swimmers stabilize the pair into cohesive formations that result in significant energy savings (up to 35% energy reduction for the follower). This work opens up the prospect of establishing physics-based explanations and interpretable strategies of how fish, single or in a school, can control their body motion for maximal hydrodynamic benefits.