Abstract
Negatively buoyant freely swimming crustaceans such as krill must generate downward momentum in order to maintain their position in the water column. These animals use a drag-based propulsion strategy, where pairs of closely spaced swimming limbs are oscillated rhythmically from the tail to head. Each pair is oscillated with a phase delay relative to the neighbouring pair, resulting in a metachronal wave travelling in the direction of animal motion. It remains unclear how oscillations of limbs in the horizontal plane can generate vertical momentum. Using particle image velocimetry measurements on a robotic model, we observed that metachronal paddling with non-zero phase lag created geometries of adjacent paddles that promote the formation of counter-rotating vortices. The interaction of these vortices resulted in generating large-scale angled downward jets. Increasing phase lag resulted in more vertical orientation of the jet, and phase lags in the range used by Antarctic krill produced the most total momentum. Synchronous paddling produced lower total momentum when compared with metachronal paddling. Lowering Reynolds number by an order of magnitude below the range of adult krill (250–1000) showed diminished downward propagation of the jet and lower vertical momentum. Our findings show that metachronal paddling is capable of producing flows that can generate both lift (vertical) and thrust (horizontal) forces needed for fast forward swimming and hovering.
Highlights
Aquatic crustaceans such as copepods, krill and mysids encompass one of the largest member groups of zooplankton [1,2,3], with tremendous diversity in sizes ranging of the orders of 0.1–100 mm.royalsocietypublishing.org/journal/rsos R
We examine flow generated by metachronal paddling under varying Reynolds number (Re), to understand how flow characteristics could change with increasing body size when stroke kinematics remain unchanged
Flow generated during power stroke (PS) by synchronous, periodic motion of two paddles consists of co-rotating vortices near the tip of each paddle
Summary
Aquatic crustaceans such as copepods, krill and mysids encompass one of the largest member groups of zooplankton [1,2,3], with tremendous diversity in sizes ranging of the orders of 0.1–100 mm.royalsocietypublishing.org/journal/rsos R. In contrast with lift-based aquatic propulsion in fishes [4], and jetting propulsion in jellyfish [5] and squids [6], drag-based metachronal swimming has received limited attention in the literature [7,8,9]. Aquatic crustaceans form a crucial connection in planktonic food webs by grazing on smaller phytoplankton and serving as prey for larger, commercially important animals such as fishes [1]. Kinematics and hydrodynamics of metachronal propulsion can improve our understanding of crustacean foraging and ecologically important behaviour such as schooling. Studies of metachronal swimming can guide the design and development of miniaturized underwater drones
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