Hydrodynamics of metachronal paddling: effects of varying Reynolds number and phase lag

• Published in: Royal Society Open Science
• Main Authors: Mitchell P. Ford, Hong Kuan Lai, Milad Samaee, Arvind Santhanakrishnan
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2019
• Subjects: metachronal paddling; rowing; crustacean swimming; drag-based propulsion; aquatic locomotion; Science; Q; Antarctic; Antarc*; Antarctic Krill
• Online Access: https://doi.org/10.1098/rsos.191387; https://doaj.org/article/4f91dee8b89e4ed7913fdc30980a5f7b

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.